This is the Generic CO₂ Geological Storage Database of Features, Events and Processes (FEPs). It provides a tool to support the assessment of long-term safety and performance of geological carbon dioxide (CO₂) storage.
The Generic CO₂ FEP database for the geological storage of carbon dioxide includes FEPs chosen for their relevance to the long-term safety and performance of the storage system after injection of carbon dioxide has been completed and the injection boreholes have been sealed.
The generic FEP database for the geological storage of carbon dioxide has been developed, with the chosen FEPs being included for their relevance to the long-term safety and performance of the storage system after injection of carbon dioxide has been completed and the injection boreholes have been sealed. Some FEPs associated with the injection phase are nevertheless considered where these can affect long-term performance. The 2000 OECD/Nuclear Energy Agency FEP database for radioactive waste provided the inspiration for this generic CO₂ database, although the aims and content of the database have been developed significantly from the original NEA model.
The database includes around 200 FEPs in a hierarchical structure, with individual FEPs grouped into eight categories. Each FEP has a text description and an associated discussion of its relevance to long-term performance and safety. Key references from the published literature are included to enable retrieval of more detailed information for each FEP. The database is internet-enabled incorporating hyperlinks to other relevant sources of information (reports, websites, maps, photographs, videos, etc.) It has the potential to provide a 'knowledge base' for the geological storage of carbon dioxide.
This is version 2.0 of the CO₂ FEP Database, which was substantially updated in December 2013 to take into account updates in understanding and evidence since the last version. A specific aim of the update was to ensure consistency with the outcomes of the project Research into Impacts and Safety for CO₂ Storage (RISCS). RISCS was a 4-year project undertaken from 2010 to 2013 with funding from the European Union's 7th Framework Programme for research, technological development and demonstration under grant agreement no. 240837 and industry partners ENEL I&I, Statoil, Vattenfall AB, E.ON and RWE. R&D partners were BGS, CERTH, IMARES, OGS, PML, SINTEF, University of Nottingham, Sapienza Università di Roma, Quintessa, CO2GeoNet, Bioforsk, BGR and ZERO. Three R&D institutes outside Europe participated in RISCS: CO2CRC from Australia, University of Regina from Canada and Montana State University from the USA.
Previous versions of the database are available: 1.1.0, 1.0.1
This database contains details of Features, Events and Processes associated with the geological storage of carbon dioxide. The database is generic, in that it is not specific to any particular storage concept or location. The FEPs included in the database have been chosen for their relevance to the long-term safety and performance of the storage system after injection of carbon dioxide has ceased, and the injection boreholes have been sealed but some FEPs associated with the injection phase are included where these can affect long-term performance and the initial status of the storage system.
For each FEP in the database a description is provided, together with a discussion of its relevance to long-term safety and performance of the system. Further information is provided in the form of relevant publications and websites. The database provides a central source of information on the geological storage of carbon dioxide, and can be used as part of systemic assessments of safety and performance.
The database has a hierarchical structure with FEPs being grouped into categories and classes with an associated numbering system. Thus FEP 1.2.3 is the 3rd FEP in the second class of category 1.
There are eight categories of FEPs as follows:
The Assessment Basis category of FEPs determines the 'boundary conditions' for any assessment, specifying what needs to be assessed and why. The Assessment Basis helps to determine which FEPs need to be considered in the analysis and which can be 'screened out' as outside the scope of the assessment.
The External Factors category of FEPs describes natural or human factors that are outside the system domain. These FEPs are most important in determining scenarios for the future evolution of the system.
The CO₂ Storage category of FEPs specifies details of the pre- and post-closure storage concept under consideration.
The CO₂ Properties, Interactions and Transport category of FEPs is concerned with those Features, Events and Processes that are relevant to the fate of the sequestered fluid. Carbon dioxide's properties can vary greatly between conditions at depth and near-surface, and a wide range of physical and chemical reactions can be important.
The Geosphere category of FEPs is concerned with the geology, hydrogeology and geochemistry of the storage system. Taken together, the FEPs in this category describe what is known about the natural system prior to storage operations commencing.
The Boreholes category of FEPs is concerned with the way that activity by humans alters the natural system. Both boreholes used in the storage operations and those drilled for other purposes are relevant to the long-term performance of the system.
The Near-Surface Environment category of FEPs is concerned with factors that can be important if sequestered carbon dioxide returns to the environment that is accessible by humans. The environment could be terrestrial or marine, and human behaviour in that environment needs to be described.
The Impacts category of FEPs is concerned with any endpoint that could be of interest in an assessment of performance and safety. Impacts could be to humans, flora and fauna or the physical environment.
The upgrade of the database to version 2.0 has been funded by the EU project Research into Impacts and Safety for CO₂ Storage (RISCS). RISCS was a 4-year project undertaken from 2010 to 2013 with funding from the European Union's 7th Framework Programme for research, technological development and demonstration under grant agreement no. 240837 and industry partners ENEL I&I, Statoil, Vattenfall AB, E.ON and RWE. R&D partners were BGS, CERTH, IMARES, OGS, PML, SINTEF, University of Nottingham, Sapienza Università di Roma, Quintessa, CO2GeoNet, Bioforsk, BGR and ZERO. Three R&D institutes outside Europe participated in RISCS: CO2CRC from Australia, University of Regina from Canada and Montana State University from the USA.
The RISCS project was specifically concerned with understanding the impacts that might occur in the unlikely event there is a leak from any storage complex. The project has produced guidance on monitoring, assessment and mitigation of leaks. Therefore, the updates made primarily reflect this aim. In addition, a more general upgrade of the database e.g. updating references and FEP descriptions has been made.
With funding from the EC project Research into Impacts and Safety for CO₂ Storage (RISCS), the database has recently been substantially updated. RISCS was a 4-year project undertaken from 2010 to 2013 with funding from the European Union’s 7th Framework Programme for research, technological development and demonstration under grant agreement no. 240837 and industry partners ENEL I&I, Statoil, Vattenfall AB, E.ON and RWE. R&D partners were BGS, CERTH, IMARES, OGS, PML, SINTEF, University of Nottingham, Sapienza Università di Roma, Quintessa, CO2GeoNet, Bioforsk, BGR and ZERO. Three R&D institutes outside Europe participated in RISCS: CO2CRC from Australia, University of Regina from Canada and Montana State University from the USA.
The RISCS project was specifically concerned with improving understanding of the impacts that might occur in the unlikely event there is a leak from any CO₂ geological storage complex. The project has produced guidance on monitoring, assessment and mitigation of leaks, A Guide to Potential Impacts of Leakage from CO₂ Storage. The updates made to the FEP database primarily reflect this focus. FEP descriptions, accompanying diagrams, references and links have been substantially updated to ensure consistency with the research undertaken and messages arising from the RISCS project. In addition, a more general upgrade of other aspects of the database has been made, to enhance FEP descriptions, references and links and to ensure they are up-to-date.
These updates build upon previous versions of the database that were funded as part of the IEA GHG Weyburn CO₂ Monitoring & Storage Project, by the EC and UK Department of Trade and Industry (DTI), the IEA, and subsequently the UK Department of Energy and Climate Change (formerly part of the DTI). The generic CO₂ FEP database has been developed over a period of more than 10 years and is an internationally recognised resource for supporting underground CO₂ storage projects, with over 1000 registered users. The database aims to capture all the key features, events and processes that influence long-term safety and performance of geological CO₂ storage complexes.
With funding from the UK Department of Energy and Climate Change, DECC, the Generic CO₂ FEP Database has recently been updated to include more information on FEPs relating to the marine environment.
Some existing CO₂ storage sites, such as Sleipner and Snøvhit (both of which are being considered in the CO2REMOVE project), are located off-shore and it is likely that many future sites will be located in marine environments. The potential long-term performance of these sites can be assessed using models audited against the Generic CO₂ FEP database.
In recognition of the importance of off-shore settings, generic FEPs relevant to the marine environment have been reviewed and updated. Version 1.1.0 of the Generic CO₂ FEP Database therefore includes more information relating to FEPs associated with the marine environment. The update also includes up-to-date references and hyperlinks and a new FEP on Impacts on Oceans.
Please contact co2@quintessa.org for more information about the updates. Quintessa gratefully acknowledges funding from the UK Department of Energy and Climate Change for the maintenance of the FEP Database System and Generic CO₂ FEP Database.
The Generic CO₂ FEP Database has been substantially updated since it was first produced in 2004, including many more up-to-date references, hyperlinks and images. The FEP Database System has also been updated and now enables influences between FEPs to be explored using additional hyperlinks and Process Influence Diagrams (PIDs), as illustrated below. Process influence diagrams are not available in the current single-web-page format.
The final report has been published for the European Union 5th framework research project CO2STORE, which develops research into the potential for large-scale storage of CO₂ in underground saline aquifer formations. The report describes several case studies, which include safety and risk assessments based on FEP analysis.
The Geological Survey of Denmark and Greenland (GEUS) have undertaken risk assessment studies for potential carbon storage projects near to the city of Kalundborg using the generic FEP database. The risk assessment involved analysis of all relevant FEPs and identification of the most important FEPs: geological features relating to the reservoir and cap rock, long term fate of injected CO₂ and impact on society and humans. Risks that could put the project on hold or eventually lead to exclusion of the potential storage site have also been considered, with several of these are relating to project costs: geological risks, low level leaks, monitoring, injectivity and well leak. Finally possible conflicts of use with geothermal energy, gas storage, hydrocarbon and drinking water have been investigated and are not expected to provide potential problems. A report on the Kalundborg case study is available here.
The Dutch research organisation TNO undertook a safety assessment for potential CO₂ sequestration in the Schweinrich structure in Genrmany. The assessment involved the development of four scenarios based on a detailed analysis of FEPs that drew on the generic database. The four scenarios covered reference containment, plus additional 'what if' scenarios that represented leaking seals, wells and faults. Simplified probabilistic calculations of CO₂ saturations in the saturated zone were then undertaken for each scenario.
The British Geological Survey (BGS) participated with a case study that considered the potential sequestration into a saline aquifer in the St George's Channel Basin in the Irish Sea. The study included an outline risk assessment based on the generic FEP database, which highlighted the following key risks:
Flow simulations indicate that a significant amount of the sequestered CO₂ could migrate to a known fault in 60 years. The effectiveness of the fault seal is not known and therefore represents a critical uncertainty.
CO₂ trapped at the top of the reservoir is likely to cause a significant departure from the hydrostatic pressure gradient. This might adversely affect the integrity of the cap rock through fracturing and/or dissolution.
The reservoir sands are probably of fluvial origin and might therefore exhibit significant spatial variation. A heterogeneous reservoir would make it difficult to target injection wells and utilise the maximum storage capacity of the target structure.
The strata above the reservoir are expected to be poorly consolidated. It may therefore be difficult to get a robust cement bond between well casing and the surrounding rock. This could allow CO₂ to escape from wells previously drilled in the storage structure. It might also allow CO₂ to escape from the injection wells
The usefulness of FEP lists to systems-based approaches to building confidence in carbon capture and storage projects was discussed at the 'Workshop on Confidence Building in the Long-Term Effectiveness of Carbon Dioxide Capture and Geological Storage', which was organised by the IEA and held in Tokyo, Japan, 24 and 25 January, 2007. A presentation by Dave Savage (Quintessa Ltd.) cited the generic FEP database and highlighted its potential use in:
aiding the development of scenarios;
improving thoroughness and comprehensiveness of assessments;
providing an audit trail for system-level models;
aiding comparisons of assessments of different sites; and
GEOTEHNOLOGIEN is undertaking a project (CO2-UGS-RISK) to establish a common performance assessment methodology for geological CO₂ storage options that will be applicable to typical sites located in Germany. The methodology involves the use of a FEP database to develop reference scenarios for performance assessment, which draws on the generic FEP database. In addition to establishing and demonstrating an assessment methodology applicable to Germany, the project also aims to identify unresolved scientific and technical questions regarding geological storage of CO₂ as an input to research and development priorities.
The Research Institute of Innovative Technology for the Earth (RITE) in Japan have developed a CO₂ FEP database, building on the generic FEP database by providing a Japanese context to the list. RITE have then used the database to define site-specific and scenario-specific FEP lists. The work was presented at the International Workshop on CO₂ Geological Storage, held in Tokyo on the 20th and 21st February 2006 and is described in a paper by Tomoda and Akimoto (RITE), which is available here.
This category of FEPs determines the 'boundary conditions' for any assessment, specifying the spatial and temporal domain of the system. The Assessment Basis determines what is being assessed and why, so that those FEPs that need to be considered in the analysis can be defined, and those which can be 'screened out' as being outside the scope of the assessment can be identified.
The purpose of the assessment of geological CO₂ storage.
Relevance to performance and safety:
The general purpose of an assessment of geological CO₂ storage is to determine the performance of the storage system. In any specific case, however, the purpose of conducting an assessment may vary from simple calculations to test initial ideas for storage concepts to support for an application for regulatory approval requiring detailed, site-specific performance assessment against relevant criteria. The level of complexity and comprehensiveness will vary according to the use to which it will be put. Additionally, the assessment endpoints of interest may not only vary in type, depending on the assessment purpose, but also in the level of rigour required for compliance demonstration.
The structure and composition of an assessment will tend to reflect the endpoints that are required to be assessed. These, in turn, will reflect the criteria that are adopted to judge the overall performance of the storage system. Thus, for example, an assessment may be constrained to considering the degree of containment within a geological feature; alternatively, it may need to address potential near-surface impacts. Invariably, a combination of endpoints will be required.
The spatial domain of interest will be dependent on the site context, which may vary from generic assessments to site specific assessments, the storage concept and the endpoints of interest. The spatial domain will contribute to determining the information requirements and modelling capabilities that may be required.
Timescales over which the assessment will be performed will constrain processes which must be considered in the assessment. Firstly, there is that over which isolation of carbon dioxide from the atmosphere is necessary to mitigate climate change. This timescale is likely to be in the order of a few hundred years at most. Another timescale of interest is potentially much longer and is that pertaining to the assessment of potential hazard to humans and the environment. This timescale could be in the order of thousands to tens of thousands of years. Other timescales of interest may include the varying times possible for gas release: blowouts can occur on the order of days whilst slow leaks may occur over tens or hundreds of years.
High level assumptions concerning the storage system(s) of relevance to the assessment. For example, the quantity of CO₂ stored, the method of injection and information concerning the assumed performance of the storage system.
Relevance to performance and safety:
Provides a background to the storage technique adopted. Note that more detailed consideration of the CO₂ storage system is provided in subsequent FEP categories.
The assumptions made in the assessment concerning general boundary conditions for assessing future human action.
Relevance to performance and safety:
For example, it can be expected that human technology and society will develop over the timescales of relevance for the assessment of CO₂ storage systems, however, this development is unpredictable. Therefore it may be necessary to make some assumptions in order to constrain the range of future human actions that are considered, such as assuming that only present-day technologies, or technologies practised in the past, will be considered.
The legal and regulatory framework within which the assessment takes place.
Relevance to performance and safety:
In undertaking an assessment it is vital to consider the appropriate regulatory framework requirements. At one extreme these may be specific, prescriptive quantitative requirements, at the other they could be non-prescriptive or may not have been fully developed.
The legal and regulatory framework can shape various aspects of an assessment, such as the required assessment endpoints, timescales of interest and assumptions concerning future human actions.
General methodological issues affecting the assessment modelling process and use of data.
Relevance to performance and safety:
Examples of general model and data issues include the treatment of uncertainty, the method for handling site specific data, and the reduction/simplification of models and data.
This category of FEPs describes natural or human factors that are outside the system domain. These FEPs are most important in determining scenarios for the future evolution of the system, and are often referred to as EFEPs (External FEPs). Three classes of FEPs are considered. Geological Factors and Climatic Factors are concerned with natural processes and events, whilst Future Human Actions is concerned with those human activities that can directly affect the storage system.
Neotectonics is the study of crustal movements that both occurred in the Earth's recent past and are continuing at the present day. These movements, which are driven directly or indirectly by global plate motions (tectonics), result in the vertical and horizontal warping, folding or faulting of the Earth's surface.
Relevance to performance and safety:
Neotectonic events have the potential to cause sudden changes in the physical properties of rocks due to stress changes and induced hydrogeological changes.
Magma is molten, mobile rock material which is generated below and within the Earth's crust and which gives rise to igneous rocks when solidified. A volcano is a vent or fissure in the Earth's surface through which molten or part-molten materials (lava) may flow, and ash and hot gases be expelled.
Relevance to performance and safety:
The high temperatures associated with volcanic and magmatic activity may result in permanent changes in the surrounding rocks, either directly, or through circulating high temperature fluids. This FEP is relevant to CO₂ storage in areas of potential magmatic activity, e.g. Japan.
Events and processes related to seismic events and also the potential for seismic events. A seismic event is caused by rapid relative movements within the Earth's crust usually along existing faults. The accompanying release of energy may result in rock movement and/or rupture, e.g. earthquakes.
Relevance to performance and safety:
Seismic events may result in changes in the physical properties of rocks due to stress changes and induced hydrogeological changes. Seismic events are most common in tectonically active or volcanically active regions at crustal plate margins.
Processes associated with high temperature groundwaters, and hydrothermal alteration of minerals in the rocks through which the high temperature groundwater flows. Hydrothermal activity may be directly associated with volcanic and magmatic activity.
Hot springs, geysers and submarine hydrothermal vents provide evidence of hydrothermal activity (see pictures below).
Relevance to performance and safety:
Can result in the hydrothermal alteration of rocks or minerals by the reaction of hot water (and other fluids) with pre-existing rocks.
Processes arising from large-scale geological changes. These processes include changes in hydrodynamic boundary conditions, the effects of changes in the physical properties of geological units (e.g., porosity, permeability etc.), and variations in potential gradients driving water flow (e.g., topographical gradients). These changes are caused by rock stress accompanied by brittle and/or plastic deformation of the rock units.
Relevance to performance and safety:
Flow of groundwater within and surrounding a CO₂ storage reservoir will influence the extent to which the CO₂ remains in one location. If the CO₂ does migrate, then groundwater flow will influence the routes taken and the extent to which the CO₂ is dissolved and diluted. Physical changes to the rock units caused by geological changes may also create or destroy potential pathways for CO₂ migration. In and below low-permeability geological formations, hydrogeological conditions may evolve very slowly and often reflect past geological conditions, i.e. be in a state of disequilibrium.
Processes related to the large scale (geological) removal of rocks and sediments, with associated changes in topography and geological/hydrogeological conditions of the system.
Relevance to performance and safety:
Potential to modify the geological and hydrogeological environment.
An extraterrestrial body in the 1-10-km size range, which impacts the earth at high velocity, explodes upon impact, and creates a large crater.
Relevance to performance and safety:
A low probability, high consequence event that has the potential to substantially disrupt the CO₂ storage system. Often screened out on the basis that the impact of the bolide will greatly exceed that of the disruption caused to the storage system.
The process of global climate change due to natural and/or anthropogenic causes. The last two million years of the Quaternary have been characterised by glacial/interglacial cycling. According to the Milankovitch Theory, the Quaternary glacial/interglacial cycles are caused by long-term changes in seasonal and latitudinal distribution of incoming solar radiation which are due to the periodic variations of the Earth's orbit about the Sun (Milankovitch cycles).
Evidence suggests that the Earth is presently in a period of global warming (see the figure below). The anthropogenic release of gases into the atmosphere may be increasing the rate of global warming by enhancing the natural 'greenhouse effect', a process by which longwave radiation emitted from the Earth is trapped in the atmosphere by 'greenhouse gases' such as CO₂.
Relevance to performance and safety:
Changes in the global climate are likely to impact the CO₂ storage system in a number of ways, such as through its effect on sea levels and the local and regional climate.
Processes related to the possible future changes, and evidence for past changes, of climate at a storage site. This is likely to occur in response to global climate change, but the changes will be specific to a particular situation, and may include short term fluctuations.
Climate is characterised by a range of factors including temperature, precipitation and pressure as well as other components of the climate system such as oceans, ice and snow, biota and the land surface. The Earth's climate varies by location and for convenience broad climate types can be distinguished, e.g. tropical, savannah, Mediterranean, temperate, boreal and tundra. Climatic changes lasting only a few decades may be referred to as climatic fluctuations. These are unpredictable at the current state of knowledge although historical evidence indicates the degree of past fluctuations.
The response of regional and local scale climate to global climate change has been considered for its implication for the geological disposal of radioactive waste in the EC's BIOCLIM programme (see link below).
Relevance to performance and safety:
Changes in the regional and local climate could affect the CO₂ storage system in a number of ways. For example, changes in groundwater recharge could affect regional hydrogeology and hence the transport of CO₂ dissolved in groundwater. It may also alter the near-surface environment to which some of the stored CO₂ may migrate. For a marine context, the near-surface environment includes both the seabed and also the overlying water column.
Processes related to changes in sea level which may occur as a result of global change, i.e. eustatic, and/or regional geological change, e.g. isostatic movements.
The component of sea-level change involving the interchange of water between land ice and the sea is referred to as eustatic change. As ice sheets melt, the ocean volume increases and sea levels rise. Sea level also changes with increased temperature due to thermal expansion and salinity changes, referred to as steric change. The sea level at a given location will also be affected by vertical movement of the land mass, e.g. depression and rebound due to glacial loading and unloading, referred to as isostatic change.
Relevance to performance and safety:
Sea level change may affect the storage system through its impact on the near surface environment and the regional or local hydrogeological regime. Potentially, sea level change could alter the disposition of on-shore and off-shore storage sites. For example, in some parts of the world, on-shore storage sites could become offshore storage sites as a result of sea level rise.
Related to the physical processes and associated landforms in cold but ice-sheet-free environments.
Relevance to performance and safety:
An important characteristic of periglacial environments is the seasonal change from winter freezing to summer thaw with large water movements and potential for erosion. Frozen sub-soils are referred to as permafrost. Meltwater from seasonal thaw is unable to percolate downwards due to permafrost and saturates the surface materials. Permafrost layers may isolate the deep hydrogeological regime from surface hydrology, or flow may be focused at "taliks" (localised unfrozen zones, e.g. under lakes, large rivers or at regions of groundwater discharge).
Processes related to the effects of glaciers and ice sheets within the region of a storage site. This is distinct from the effects of large ice masses on global and regional climate.
The ice sheet itself and the accompanying frozen ground (permafrost) beneath the majority of the ice sheet (the cold-based portion) may constitute a barrier to groundwater flow and to heat loss. If the basal transmissivity of the till and bedrock below the ice is low, water pressures may rise to levels equalling the ice pressure inducing the formation of major conduits in the subglacial material. The central parts of the ice sheet are likely to be warm-based and could permit groundwater recharge to take place, possible to great depths if high groundwater heads are generated at the base of the ice sheet. Discharge of groundwater is likely to take place close to and beyond the frontal parts of the ice sheet. Its location being determined, in part, by the presence, or otherwise, of permafrost and its thickness. Excessive recharge at the margin of the ice sheet could provide direct recharge of oxidising water to considerable depths in conductive fracture zones. If the permeability at and beyond the rim of the ice is low, e.g. due to permafrost, the water pressures may again build up resulting in hydrofracturing of the ice or the rock mass. As the ice sheet advances, these induced fractures may increase their aperture and depth due to freezing of subglacial meltwater.
Relevance to performance and safety:
Erosional processes (abrasion, overdeepening) associated with glacial action, especially advancing glaciers and ice sheets, and with glacial meltwaters beneath the ice mass and at the margins, can lead to morphological changes in the environment, e.g. U-shaped valleys, hanging valleys, fjords and drumlins. Depositional features associated with glaciers and ice sheets include moraines and eskers. The pressure of the ice mass on the landscape may result in significant hydrogeological effects and even depression of the regional crustal plate.
Processes related to warm tropical and warm desert climates, including seasonal effects, and meteorological and geomorphological effects special to these climates.
Relevance to performance and safety:
Regions with a tropical climate may experience extreme weather patterns (monsoons, hurricanes) that could result in flooding, storm surges, high winds etc. with implications for erosion and hydrogeology. The high temperatures and humidity associated with tropical climates result in rapid biological degradation and soils are generally thin. In arid climates, total rainfall, erosion and recharge may be dominated by infrequent storm events.
Run-off and erosion provide an input to the sediment supply to the terrestrial aquatic and marine environments. The sediment supply influences the sediment balance and helps to determine the flora and fauna present.
In warmer climates, both the terrestrial and marine flora and fauna are likely to be different from those found in colder climates.
Processes related to changes in hydrology and hydrogeology. These changes can be in regard to recharge, sediment load and seasonality, etc., as well as in response to climate change in a region.
Relevance to performance and safety:
The hydrology and hydrogeology of a region is closely coupled to climate. Climate controls the amount of precipitation and evaporation, seasonal ice cover, and thus the soil water balance, extent of soil saturation, surface runoff and groundwater recharge. Vegetation and human actions may modify these responses.
Processes relating to changes in ecology and human behaviour in response to climate change.
Climate affects the abundance and availability of natural resources such as water, as well as ecology, including the types of crops that can be grown by humans. Human responses, such as changes in habits, diet and the size of communities, are closely linked to climate and ecology. The more extreme a climate, the greater the extent of human control over these resources necessary to maintain agricultural productivity.
Relevance to performance and safety:
Changes in ecology and human behaviour in response to climate change will influence the relevant FEPs to be considered in the surface environment as an assessment extends into the future. These changes may also add secondary stresses to humans and ecological systems.
Processes related to human activities that could affect the change of climate either globally or in a region.
Relevance to performance and safety:
Anthropogenic emissions of 'greenhouse' gases such as CO₂ and CH₄ have been implicated as factors in global warming. One of the primary aims of CO₂ storage is to reduce the quantity of CO₂ that is discharged to the atmosphere.
Regionally, climate can be modified by human activities such as de-forestation.
Events and processes related to the degree of knowledge of the existence, location and/or nature of the storage site. These include reasons for deliberate interference with, or intrusion into, a CO₂ storage site after closure with complete or incomplete knowledge.
Knowledge of the storage site may be regained through post-closure airborne, geophysical or other surface-based non-intrusive investigation of a storage site. Such investigations might occur after information of the location of the storage system has been lost (therefore excluding monitoring of the storage system) but they could include activities such as prospecting for geological resources. The evidence of the storage, such as injection boreholes, may itself prompt investigation.
Relevance to performance and safety:
Some future human actions could directly impact upon performance of the storage system. The following could be distinguished:
- inadvertent actions, which are actions taken without knowledge or awareness of the storage site, and
- deliberate actions, which are actions that are taken with knowledge of the storage system's existence and location, e.g. deliberate attempts to retrieve any hydrocarbons associated with the CO₂, malicious intrusion and sabotage.
Intermediate cases, of intrusion with incomplete knowledge, could also occur.
Events and processes related to changes in social patterns and degree of local government, planning and regulation.
Potentially significant social and institutional developments include:
- changes in planning controls and environmental legislation;
- demographic change and urban development;
- changes in land use;
- loss of archives/records, loss/degradation of societal memory.
Relevance to performance and safety:
Social and institutional developments have the potential to affect motivation and knowledge issues, human use of the surface and sub-surface environments and the type of impacts that may be considered.
Events and processes related to future developments in human technology and changes in the capacity and motivation to implement technologies. This may include retrograde developments, e.g. loss of capacity to implement a technology.
Relevance to performance and safety:
Technologies of interest include those that might change the capacity of humans to intrude deliberately (or otherwise) into a storage site or that may cause changes that would affect the movement of CO₂ and associated contaminants. Technologies that may otherwise affect the performance and safety of the storage system should also be considered. Technological developments are likely but may not be predictable especially at longer times into the future.
Events related to any type of drilling activity in the vicinity of the CO₂ storage system. These may be taken with or without knowledge of the storage and may include activities such as:
- exploratory and/or exploitation drilling for natural resources;
- attempted recovery of residual hydrocarbon resources;
- drilling for water resources;
- drilling for site characterisation or research;
- drilling for further storage; and
- drilling for hydrothermal resources.
Relevance to performance and safety:
Has the potential to disrupt geological features that provide a barrier to CO₂ migration and provide a relatively quick migration pathway to the near-surface.
Events related to any type of mining or excavation activity carried out in the vicinity of the storage site. These may be taken with or without knowledge of the site.
Mining and other excavation activities include:
- resource mining;
- excavation for industry;
- excavation for storage or disposal;
- excavation for military purposes;
- geothermal energy production;
- injection of liquid wastes and other fluids;
- scientific or archaeological investigation;
- shaft construction, underground construction and tunnelling;
- underground nuclear testing;
- malicious intrusion, sabotage or war;
- recovery of materials associated with CO₂ injection (e.g. hydrocarbons).
Relevance to performance and safety:
Mining and other underground activities have the potential to disrupt the geosphere (storage reservoir, surrounding and overlying rock) and near-surface environment. They therefore have the potential to significantly affect the migration and distribution of stored CO₂.
Events and processes related to any type of human activities that may be carried out in the surface environment that can potentially affect the performance of the storage system, or leakage pathways, excepting those FEPs related to water management which are described elsewhere.
Examples include:
- quarrying, trenching (e.g. for pipelines);
- boreholes;
- excavation for construction;
- residential, industrial, transport and road construction;
- pollution of surface environment and groundwater;
- land reclamation projects (e.g. offshore island construction, etc.);
- dredging of the seabed.
Relevance to performance and safety:
Human activities in the surface environment have the potential to affect CO₂ release processes, should leakage occur. They may also determine the types of impact to be considered.
Events and processes related to groundwater and surface water management including water extraction and treatment, reservoirs, dams, and river management.
Water is a valuable resource and water extraction and management schemes provide increased control over its distribution and availability through construction of dams, barrages, canals, pumping stations and pipelines. Groundwater and surface water may be extracted for human domestic use (e.g. drinking water, washing), agricultural uses (e.g. irrigation, animal consumption) and industrial uses. Saline water may be extracted and desalinated for similar purposes.
Relevance to performance and safety:
Extraction and management of water may affect the movement of CO₂ or associated contaminants to and in the surface environment.
The presence of injected CO₂ may hinder future extractive operations by obscuring seismic traces or by making the drilling process more difficult. Conversely, the presence of CO₂ locally, might allow for more economic future enhanced oil recovery operations. This FEP assumes that future technical advances might identify useable resources in the vicinity of previous CO₂ injection.
Relevance to performance and safety:
Drilling through a formation filled with supercritical CO₂ might cause 'blowouts' or loss of CO₂ along the wellbore. A CO₂ 'bubble' will change the velocity of seismic waves, distorting the 'image' of the underlying formations, and reducing confidence in the understanding of this structure.
Events related to deliberate or accidental explosions and crashes such as might have some impact on a closed storage site, e.g. underground nuclear testing, an aircraft crash on the site, acts of war, marine collisions or trawler damage to exposed sea-bed structures.
Relevance to performance and safety:
Explosions and crashes are likely to be low probability events that could have a significant impact on the performance of the storage system by disrupting the expected evolution of the system.
This category of FEPs specifies details of the storage concept under consideration. It is split into two classes for the pre- and post-closure periods.
Details of the storage concept, the fluids injected, and factors for the design, construction, operation and decommissioning phases.
Relevance to performance and safety:
The details of the storage concept are fundamental to determining which FEPs in other categories need to be considered in a given assessment. Some details of storage operations may affect the post-closure performance.
Features related to the concept of storage, such as whether a closure exists (storage in an abandoned oil or gas field) or whether isolation of CO₂ is dependent upon slow diffusion rates through an extensive open structure (saline aquifer storage).
Relevance to performance and safety:
Different processes will be relevant to different storage concepts. For example, the rate of CO₂ migration in an open aquifer will be relevant to safety assessment of saline aquifer storage, but less relevant to storage in a closed geological structure.
Features related to the amounts of CO₂ injected and the rate of injection into the storage aquifer/reservoir.
Relevance to performance and safety:
High rates of CO₂ injection could have adverse affects, such as hydrofracturing.
For fast injection rates, displacement of oil/water will not be efficient during enhanced oil recovery; instead one will recover CO₂; premature breakthrough is undesirable from an economic perspective.
The composition and physical state (liquid, supercritical fluid etc.) of injected CO₂, with contents of impurities etc. Temperature and pressure of injected fluid are also relevant.
During CO₂ storage operations, the principal injected gas is CO₂ captured and concentrated from human activity sources. However, the gas that is injected into a reservoir may not be 100% CO₂, especially if there is some recycling of gas (in the case of enhanced oil recovery). Impurities can include: H₂S, CH₄, N₂, NOₓ, SO₂ and mercaptans. These may be present either intentionally or because it could be particularly difficult or superfluous to separate them from CO₂.
Relevance to performance and safety:
The presence of even small amounts of other gases has a strong effect on the phase behaviour of CO₂-dominated gases. High-pressure equations of state for CO₂-dominated gas mixtures are required to take into account changes in critical pressures and temperatures caused by the presence of other gases. Impurities will reduce the critical temperature which, in turn, has effects on interfacial tension.
Amongst the possible companion gases, NOₓ and SO₂ are particularly relevant because:
- NOₓ and SO₂ are polluting gases that are generated by the same power plants that generate large amounts of CO₂ and attract emission taxes in certain countries (e.g. Italy). Their injection, in smaller amounts, with CO₂ could therefore help the economics of storage.
Impurities may affect pore water chemistry (pH and redox conditions, for example) depending on the impurities involved. Special care is needed when considering corrosive gases, such as H₂S.
Microbiological contamination of injected fluid. Contamination of supercritical CO₂ is considered unlikely as it is a very good solvent. However, other fluids may be injected into the storage site that may be contaminated with microbes.
Relevance to performance and safety:
The introduction of microbes into the storage system may affect both the performance of the storage system and the endpoints considered.
Features related to the sequence of events and activities occurring during storage system design, CO₂ injection and sealing. This could include enhanced oil recovery processes.
Relevance to performance and safety:
Relevant events may include phased drilling of wells and emplacement of CO₂, sealing of boreholes, and monitoring activities to provide data on the transient behaviour of the system or to provide input to the final assessment. The sequence of events and time between events may have implications for long term performance, e.g. chemical and hydraulic changes during a prolonged injection phase.
Features related to measures to control events at or around the storage site during the design, construction, operation and decommissioning phases. The type of administrative control may vary depending on the stage in the storage system lifetime.
Relevance to performance and safety:
The pre-closure administrative control will influence the quantity and quality of information about the storage project that is available post-closure, therefore helping to determine societal memory. The better the amount and quality of information available, the lower the possibility of inadvertent intrusion.
Processes related to any monitoring undertaken during the operational and closure phase. The extent and requirement for such monitoring activities may be determined by issues such as storage concept, geological setting, regulations, or public pressure.
A number of monitoring techniques exist including seismic data, electrical resistance, soil gas and isotopic characteristics.
Relevance to performance and safety:
Monitoring during the operational phase contributes towards the amount and quality of information initially available after closure concerning the behaviour and distribution of stored CO₂.
Features related to quality control procedures and tests during the design and operation of the storage system.
It could be expected that a range of quality control measures would be applied during operation of the storage system and supply of CO₂ to be stored. There may be specific regulations governing quality control procedures, objectives and criteria.
Relevance to performance and safety:
The degree of quality control during the design, construction, operation and decommissioning of a storage system can affect the post-closure performance by influencing the integrity of the engineered parts of the system (borehole seals, for example) and by influencing the quantity, quality and accuracy of records.
Events related to accidents and unplanned events during site investigation, CO₂ emplacement and closure which might have an impact on long-term performance or safety.
Accidents are events that are outside the range of normal operations although the possibility that certain types of accident may occur should be anticipated in operational planning. Unplanned events include accidents but could also include deliberate deviations from operational plans, e.g. in response to an accident, unexpected geological events or unexpected aspects of CO₂ quality and injection arising during operations.
Relevance to performance and safety:
Accidents and unplanned events may affect the post-closure performance of the storage system. One example may be the incomplete sealing of an injection borehole that may subsequently provide a pathway for CO₂ migration.
The CO₂ injection process is influenced greatly by the target reservoir formation pressure (pore pressure).
The formation pressure is considered overpressure if it is above the normal hydrostatic pressure for the given depth, i.e. above the pressure at the bottom of a water column equal to the reservoir depth below surface. Overpressuring may occur at any depth, naturally or artificially. Man-made overpressure could be accidental or deliberate.
An example of deliberate overpressure is injecting gaseous CO₂ in a shallow aquifer at a faster rate than water can drain from the reservoir zone. As more and more CO₂ is injected, the CO₂ gas column below the seal at the top of the reservoir increases, as does the formation pressure at the top of the gas column. In such a case an artificial overpressure is created intentionally. The maximum tolerable overpressure is calculated as a function of the desired storage volume and of a chosen safety factor below the critical fracturing gradient of the top seal rock.
An example of accidental overpressure is given by the unforeseen depressurisation (water production, CO₂ leakage) of a storage reservoir where the CO₂ is in liquid form, below but near the critical phase change pressure. The change of phase to gas creates a gas column that could exercise an unforeseen overpressure at the top of the gas column.
Relevance to performance and safety:
Deliberate or accidental overpressuring during the operational phase will affect the initial geosphere conditions for a post-closure assessment. It has the potential to cause fractures in the sealing formation and hence provide migration pathways for the stored CO₂.
Administrative control of the storage site after closure of the project.
The administrative control of the post-closure site may differ from that of the pre-closure site with subsequent implications for the resources available for administrative control, the degree of access and the availability of information etc.
Relevance to performance and safety:
There may be potential for loss of information in any transfer of administrative control. A lack of awareness about the details of a CO₂ storage project could result in inadvertent disruption in the future.
FEPs related to any monitoring undertaken during the post-closure phase. This includes monitoring of parameters related to the long-term safety and performance. The extent and requirement for such monitoring activities may be determined by issues such as storage concept, geological setting, regulations, or public pressure.
A number of monitoring techniques exist including:
- gathering of seismic data;
- analysis of electrical resistance;
- analysis soil gas and;
- investigation of isotopic characteristics.
Relevance to performance and safety:
Post-closure monitoring will provide information regarding the performance of the storage project and may trigger post-closure remedial actions, if necessary.
Features related to the retention of records of the content and nature of a CO₂ storage site after closure and also the placing of permanent markers at or near the site.
Relevance to performance and safety:
It is expected that records will be kept to allow future generations to recall the existence and nature of the storage reservoir/aquifer following closure.
The degree to which the stored CO₂ could be deliberately removed, if required. Either as an extreme remedial action, because the CO₂ is required as a resource, or because its presence is impeding access to other geological resources.
Relevance to performance and safety:
The degree to which reversibility is considered in the design of a storage system may influence its long-term performance, such as by leaving viable boreholes in-situ after closure, for example.
Events and processes related to actions that might be taken following closure of a storage site to remedy problems that may be associated with its not performing to the standards required, may result from disruption by some natural event or process, or may result from inadvertent or deliberate damage by human actions.
Relevance to performance and safety:
The aim of possible future remedial actions will be to modify the performance and safety of the CO₂ storage system.
This category of FEPs is concerned with those Features, Events and Processes that are relevant to the fate of the stored fluid. Carbon dioxide's properties can vary greatly between conditions at depth and near surface, and a wide range of physical and chemical reactions can be important. The category is divided into three classes for the properties, interactions and transport of carbon dioxide.
The fundamental physical and chemical properties of carbon dioxide, taking into account impurities.
Relevance to performance and safety:
Carbon dioxide's properties can vary greatly with pressure, temperature and impurities, and an understanding of these properties is essential before the fate of stored fluid can be assessed.
Physical properties of CO₂ including density, viscosity, interfacial tension and thermal conductivity and their dependence on pressure and temperature.
Relevance to performance and safety:
The physical properties of CO₂ determine the way in which it will behave in the environment once injected, including its transport behaviour (ability to flow and diffuse), its phase relations (e.g. transition between supercritical and gaseous fluids), and its partitioning among different phases (e.g. miscibility with gaseous hydrocarbons).
FEPs related to the phase behaviour (gas, liquid, supercritical fluid) of CO₂. The presence of contaminants in the injected CO₂ (e.g. N₂) and gas and hydrocarbons in the reservoir will affect the phase behaviour and partition of CO₂ between different physical states.
Relevance to performance and safety:
CO₂ phase behaviour is a primary consideration for modelling CO₂ migration.
CO₂ solubility is the amount of CO₂ that can dissolve in water for given conditions. The solubility can vary as a function of temperature, pressure, and precise composition of the water (e.g. salinity, dissolved species/complexes, presence of hydrocarbons). Changing temperature and/or pressure and/or water composition accompanying migration of CO₂ can therefore influence CO₂ solubility potentially leading to gas exsolution.
CO₂ is present in the aqueous phase predominantly as: aqueous CO₂; carbonic acid (H₂CO₃); bicarbonate (HCO₃-); and carbonate (CO₃--). These dissolved species may also form aqueous complexes with other dissolved constituents (e.g. Na+, Mg++, Ca++, etc.). Coupled reversible reactions occur among these dissolved species in response to changing environmental conditions.
Relevance to performance and safety:
CO₂ solubility influences the migration of CO₂ and coexisting fluids and may affect the impacts of CO₂ on sensitive domains (i.e. regions within which there are potentially impacted receptors such as organisms). CO₂ dissolution and/or exsolution will affect the pressure and density distribution of formation fluids and the chemical compositions of these fluids. Dissolution of CO₂ in water will cause the water to become more dense, thereby causing it to become more reactive with respect to solid phases, for example, enhancing their tendency to dissolve and/or causing sorbed chemical species to desorb. These effects may increase the tendency for contaminants such as heavy metals to be mobilised and transported. The decrease in pH and/or the increased transport of contaminants may increase the potential for adverse impacts on biota.
Potential interactions of carbon dioxide with solid, liquid or gaseous media.
Relevance to performance and safety:
A wide range of physical and chemical reactions can be important, and an understanding of these interactions is essential for the assessment of potential impacts.
This FEP concerns the response of the caprock to increasing pressurization caused by CO₂ injection. The nature of this response will depend partly upon the history of pressure variations prior to CO₂ injection. Although the pressure within a storage reservoir will generally increase during CO₂ injection, if CO₂ is injected into a depleted hydrocarbon reservoir, the initial pressure upon commencement of CO₂ injection may be lower than the undisturbed (pre-hydrocarbon extraction) pressure. Depending upon the circumstances, the final pressure attained during CO₂ injection may or may not exceed this undisturbed natural pressure. When CO₂ is injected into a saline aquifer that has not been exploited, generally reservoir pressures will increase above the undisturbed natural values during injection.
Relevance to performance and safety:
The pressure evolution during CO₂ injection may cause pathways within the cap rock to be opened allowing CO₂ to enter the cap rock. It is necessary to consider whether such a phenomenon could result in the CO₂ travelling entirely through the cap rock (i.e. cap rock failure). Such effects could be the most pronounced if the pressures attained exceeded the hydrostatic fluid pressure in the reservoir (i.e. if over-pressurization occurs).
Increased pressurisation caused by the injection of CO₂ will lead to modification of pressure gradients in the pre-existing fluids (formation water, liquid and gaseous hydrocarbons, etc.) within the reservoir. These pressure gradients may drive flow of the fluids and/or modify the patterns and fluxes of pre-injection flows.
Relevance to performance and safety:
Injected CO₂ must displace pre-existing fluids within the reservoir and this displacement depends in large part upon the pressurisation that occurs. The degree to which reservoir fluids will flow in response to pressurisation caused by CO₂ injection will influence the rate at which CO₂ can be injected and ultimately the volume of the CO₂ that can be stored.
Hydrocarbons could be mobilised by CO₂, such as by miscible displacement, and transported to the near-surface. This is of particular relevance if enhanced oil recovery is an additional aim of the storage concept. Kolak and Burruss (2003) demonstrate that polyaromatic hydrocarbons (PAHs) can be mobilised by storage in deep coal beds.
Stored CO₂ can also precipitate asphaltenes from crude oil under certain conditions of composition, temperature and pressure. Such precipitation in the vicinity of injection wells can lead to loss of injectivity and even plugging of the wells.
Relevance to performance and safety:
Mobilised hydrocarbons may migrate to the near-surface environment.
Precipitated asphaltenes can clog pores, reducing permeability and affecting fluid flow paths.
Injection of CO₂ into a geologic formation may result in displacement of saline formation fluids into potable water supplies. Limitations on the pressure in a formation (for seal integrity) will mean that existing fluids are displaced/replaced. Displaced fluids are highly likely to be saline. Because the pressure wave created by injection travels much further than the physical CO₂ front, displacement of saline formation fluids can occur at locations outside the CO₂ storage area. Inter-connection of aquifer systems may enable saline fluids to enter potable water formations.
Relevance to performance and safety:
Displaced saline formation fluids may contaminate near-surface aquifers with subsequent impacts, such as contamination of potable water supplies.
Features and processes related to the mechanical processes and conditions resulting from the injection of CO₂ that affect the rock, boreholes and other engineered features, and the overall mechanical evolution with time. This includes the effects of hydraulic, mechanical and thermal loads imposed on the rock by the injected CO₂. Injection of CO₂ into a reservoir can cause (directly or indirectly) changes of the geomechanical properties of the reservoir rock. Direct changes can be due to change of reservoir pressure and temperature (PVT system). Indirect changes (of rock properties) might result from geochemical and mineralogical changes after storage of CO₂.
Relevance to performance and safety:
Mechanical changes of the reservoir resulting from CO₂ injection (such as generation of fractures, reactivation of fractures/faults, changes of bulk elastic properties and effective reservoir) could lead to subsidence/uplift (at surface), induced seismicity, changes in migration pathways, even burst/leakage of the seal. Examples of other relevant processes include borehole lining collapse and rock volume changes, which lead to cracking.
Injection of CO₂ may cause and trigger seismic events and earthquake hazards through processes such as reducing friction at existing faults. This may occur both in seismically active areas and in areas characterised by a low background seismicity (reactivation of ancient fault planes, changes in the orientation, fluid-pockets occurrence). This FEP includes microseismicity.
Relevance to performance and safety:
Seismicity can introduce sudden physical changes to the storage system and may expose any local population to earthquake hazards.
Injecting the CO₂ may cause acidification of formation water, leading to mineral dissolution and subsidence. This is of particular relevance to shallow storage sites.
Injection of large quantities of CO₂ into a confined aquifer may increase pore pressure and 'lift' the overlying rocks upwards.
Relevance to performance and safety:
Deformation may affect geological processes and may result in impacts of concern at the surface.
Water phase geochemistry of stored CO₂. This includes the solubility trapping of CO₂ in water (H₂O) to form carbonic acid (H₂CO₃). Subsequent ionic trapping of carbonic acid with hydroxide ions (OH-) forms bicarbonate ions (HCO₃-), which can react in turn with further hydroxide ions to form carbonate (CO₃).
Relevance to performance and safety:
Modification of the water phase geochemistry can disturb the equilibria between the water and solid phase of the reservoir and result in further geochemical (for example, solid phase geochemistry) and physical changes with resulting implications for the long-term performance of the storage system.
Chemical barriers (pH, Eh-pH, ion exchange) may exist in aquifers to retard the migration of CO₂ from depth. The precipitation of CO₂ bearing solids may result from such interactions.
Relevance to performance and safety:
Such barriers will affect the rate of migration of CO₂ from depth.
The sorption and desorption of CO₂ on geological materials. Sorption onto coal and the displacement of methane (CH₄) is the primary mechanism behind the enhanced coalbed methane recovery (ECBM) method for geological CO₂ storage.
Relevance to performance and safety:
The rate of sorption and desorption of CO₂ on geological materials affects its mobility and therefore the performance of the storage system.
Within rock formations heavy metals may occur within solid mineral phases and / or could be sorbed onto mineral surfaces. The coexisting formation fluids, which may be aqueous or non-aqueous, may also contain heavy metals. Complexation may occur between dissolved carbon species derived from CO₂ and heavy metals dissolved in formation fluids, thereby increasing the solubility of the heavy metals. The influence of dissolved CO₂ on pore water chemistry can also reduce the pH of formation water. This decrease may also cause an increase in the solubility of metal-bearing solid phases. Additionally, the pH decrease may change the equilibrium between sorption/desorption of metals, thereby resulting in significant release of these metals to the fluid.
By these processes CO₂ could potentially cause heavy metals to be released to the fluid phase. If this fluid phase is a water resource, or if the fluid phase is transported to a water resource, there could be an adverse impact from the heavy metals upon the quality of the resource. These impacted water resources are most likely in sub-surface aquifers, but possibly also could be water bodies at the earth's surface.
Relevance to performance and safety:
This process has the potential to release heavy metals, which may then migrate to water resources with potential impacts of concern.
Geochemistry of the mineral phase relevant to stored CO₂, including ion exchange and mineral dissolution.
Relevance to performance and safety:
Geochemical reactions between stored CO₂ and the mineral phase of the storage system will affect the evolution of the system and the sorption (and therefore mobility) of the CO₂.
The dissolution of minerals due to the addition of CO₂ (an 'acid gas') to the geochemistry and precipitation. For example, the dissolution of albite and precipitation of calcite modelled for the Sleipner site by Gaus et al. (2003).
Relevance to performance and safety:
CO₂ reaction with the host rock will modify the porosity and permeability of the reservoir, fluid flow (direction or velocity), mechanical properties (e.g. strength), and CO₂ storage capacity.
The process of exchanging one ion in the liquid phase for another ion on a charged, solid substrate.
Injected CO₂ may perturb ion exchange equilibria between relevant minerals (such as sheet silicates) and the pore fluid. Some cations may be released to the pore fluid and others fixed as a consequence.
Relevance to performance and safety:
Disturbance of the rock-pore fluid equilibria may affect the capacity of the rock to store CO₂.
CO₂ is likely to be dried to prevent corrosion during transport. Injection of dry CO₂ will cause it to take up water from the pores of the host formation and overlying rocks. It has the potential to 'suck' water out of an overlying clay.
Relevance to performance and safety:
If clay dehydrates, it will shrink and crack. This might aid CO₂ migration upwards.
Gases such as CO₂, methane, and H₂S will occur naturally in the geosphere, either sorbed onto minerals, dissolved in formation fluids or as a free gas phase. Gas solubility will depend upon pressure, temperature and the salinity of the formation fluid.
Relevance to performance and safety:
Gases naturally present in the geosphere could affect the behaviour of CO₂ injected into a storage reservoir and could accompany CO₂ along potential migration paths.
CO₂ migration through the reservoir and into the overlying barrier sequence could result in the CO₂ stripping other gases entrained within the sediments. These gases could include radon, methane (CH₄) and hydrogen sulphide (H₂S).
Relevance to performance and safety:
The presence of other gases in a leaking CO₂ gas stream is important in deciding the level of CO₂ leakage that can be tolerated and may constitute an important hazard.
Gas hydrates are 'ice-like' solids that form at low temperatures and high pressures. They are formed of 'cages' of water molecules surrounding a gas molecule.
Relevance to performance and safety:
Cooling of the reservoir (e.g. by injecting cold CO₂ or through adiabatic expansion) well below normal in-situ temperatures might stabilize gas hydrates. Their growth might seal fluid flow pathways (at least temporarily).
If CO₂ is injected below deep water or permafrost then rising CO₂ might hit the hydrate stability zone before escaping to the ocean or air, so hydrates could act as a secondary chemical barrier. Similarly, storage could be focused on actively forming CO₂ hydrate as a stable, immobile phase to lock up the CO₂ (Koide et al., 1997).
Features and processes related to the biological/biochemical processes that affect the CO₂, borehole seals and rock/pore fluid, and the overall biological/biochemical evolution with time. This includes the effects of biological/biochemical influences on the CO₂ and engineered components by the surrounding geology. Microbes exist in the subsurface and are used in hydrocarbon operations to improve hydrocarbon recovery. Microbes can also catalyse geochemical reactions, including methanogenesis, but the latter reaction is thermodynamically unfavourable and is unlikely.
Relevance to performance and safety:
Examples of relevant processes are:
- microbial growth;
- microbially/biologically mediated processes; and
- microbial/biological effects of evolution of redox (Eh) and acidity/alkalinity (pH) , etc.
This FEP has probably low relevance to the safety/fate of CO₂. However, CO₂ releases may affect/impact microbe populations being used in independent hydrocarbon-recovery enhancement projects.
Microbes can metabolise CO₂, for example, methanogenic microbes use H₂ to reduce CO₂ to methane (CH₄), a process called methanogenesis. These microbes need anaerobic conditions.
Relevance to performance and safety:
Methanogenesis, if it occurs, could affect the pressure distribution of CO₂. The fate and impact of the CH₄ produced may be an endpoint of interest in itself.
Transport processes that may affect stored carbon dioxide and associated impurities.
Relevance to performance and safety:
An understanding of those processes that could transport carbon dioxide, and associated impurities, within the geosphere, near-surface and surface environments is fundamental to the assessment of long-term performance and safety.
Advection of free CO₂ occurs in response to differences in pressure. The pressure difference may be due to differences in the pressure of injected CO₂ and formation pressures.
The rate and direction of advection is affected by the physical properties of the rock, such as porosity and permeability.
Advection may also occur though fractures. Fracture flow will be episodic with high transport efficiencies. Resealing of fractures (for example by cementation) will reduce and ultimately block fluid flow.
Relevance to performance and safety:
Advective flow is a key transport process for migrating CO₂, and associated contaminants, in the geosphere (reservoir, surrounding, and overlying rock), and in near-surface and surface environments. Patterns of advective flow may be variable, depending upon the nature of the pathway through which flow occurs (e.g. through the entire thickness of a permeable rock formation or through locally permeable sections of fault planes). If such a pathway allows CO₂ to leave a storage complex (i.e. to leak) the nature and spatial extent of impacts upon sensitive domains (i.e. regions within which there are potentially impacted receptors such as organisms) will depend upon the pattern of advection and the flux.
Fault valving is a process resulting from gradual build up of pore pressure due to fluid generation, causing the subsequent opening of a fault along with fluid escape towards surface. This mechanism has been recognised as causing earthquakes in many parts of the world, as a result of hydrocarbon generation or infiltration of other fluids.
Relevance to performance and safety:
Large transient releases of pore fluids may occur during fault valving episodes. The patterns of these releases will be controlled by the properties of the particular fault that shows valving behaviour, including its size, geometry, the nature of its intersection with the CO₂ accumulation, and the mechanical properties of the rocks that it cuts. If fault valving allows CO₂ to leave a storage complex (i.e. to leak), the nature and spatial extent of impacts upon sensitive domains (i.e. regions within which there are potentially impacted receptors such as organisms) will depend upon the pattern of advection and the flux.
Different relative densities of fluids in a geological system will result in buoyancy-driven flows as less dense fluids will have a tendency to flow upwards and more dense fluids will tend to flow downwards. CO₂ that is emitted as a free phase at the ground surface will be more dense than the surrounding atmosphere and, if atmospheric conditions are sufficiently quiet, the CO₂ will tend to accumulate near the ground surface. Similarly, any emissions of CO₂-charged water at a seabed or a lakebed will tend to be more dense than the overlying water. If this overlying water is sufficiently still, the CO₂-charged water will tend to accumulate at the base of the free water column, near the seabed or lakebed.
The density of a fluid will depend on its temperature, pressure, and composition. Interactions among different coexisting fluids may influence their densities. For example, dissolution of CO₂ within coexisting formation water will increase the density of the formation water. Fluids of relevance include supercritical and gaseous CO₂, formation water, and liquid and/or gaseous hydrocarbons.
Relevance to performance and safety:
Density-driven flows may help to transport CO₂ either towards sensitive domains (regions which there are receptors such as organisms that potentially may be impacted) or away from such domains. Density-driven flow of CO₂ from a storage reservoir will probably be broadly upwards and hence is more likely to be towards sensitive domains than away from them. Conversely, density-driven flow of CO₂-charged formation water from a storage reservoir will probably be broadly downwards and is more likely to be away from sensitive domains than towards them. Density-driven flow of CO₂ emitted at the ground surface in terrestrial environments may result in impacts to surface/near-surface ecosystems if the CO₂ concentrations reach sufficiently high levels (implying that CO₂ is emitted at a sufficient rate and in sufficient quantities and is not dispersed sufficiently by atmospheric processes). Similarly, density-driven flows of CO₂-charged water emitted a sea bed or lakebed may cause impacts to sea/lakebed ecosystems if the CO₂ concentrations reach sufficiently high levels (implying that the CO₂-charged water is emitted at a sufficient rate and in sufficient quantities and is not dispersed sufficiently by hydrodynamic processes).
This depends on interfacial tension and capillary pressure.
Capillary pressure is the pressure difference existing across the interface separating two immiscible fluids due to interfacial tension. The interfacial tension itself is caused by the imbalance in the molecular forces of attraction experienced by the molecules at the surface and is a function of temperature and pressure.
At a given pressure, increased interfacial tension values between water and CO₂ will make larger pores accessible to CO₂ (this is only valid for water-wet systems). The change from a water-wet system to a CO₂-wet system has an effect on capillary forces (i.e. displacement of water by enhanced pressure versus CO₂ injection with less capillary pressure) and the displacement capacity (i.e. as a non-wetting fluid, CO₂ will have less displacement capacity). If the injection velocity is high, effects of capillary forces are small.
Relevance to performance and safety:
Interfacial tension and capillary pressure determine the location of CO₂ within the pore spaces of the reservoir and the displacement capacity of the reservoir. Injection of CO₂ will displace pre-existing fluids within the reservoir. If these fluids are intersected by permeable pathways (e.g. faults or open boreholes), or if pressurization of these fluids causes such pathways to open, the displaced fluids may leave the containment complex (i.e. leak). Alternatively, the displaced fluids within the reservoir may in turn displace fluids that lie outside the reservoir if the reservoir is connected to surrounding rock formations by a suitable permeable pathway. Consideration needs to be given to the possibility that formation fluids that are displace by injected CO₂ may impact upon sensitive domains (i.e. regions within which there are potentially impacted receptors such as organisms).
The process of dissolution of CO₂ in formation fluids. The rate of dissolution depends on factors such as the interfacial area between the CO₂ and the formation fluids and temperature.
Relevance to performance and safety:
Dissolution in formation fluids can be an important process in determining the period that free CO₂ remains in the reservoir. Additionally, the formation fluids that dissolve the CO₂ will become acidified and hence more reactive with respect to other phases, notably minerals within the rock. As a result of this reactivity, the CO₂-bearing water may dissolve formation minerals and/or leach contaminants such as heavy metals from the rock. As a consequence of its greater reactivity due to dissolving CO₂, the CO₂-charged water may potentially impact upon any sensitive domains within which it comes into contact (i.e. regions within which there are potentially impacted receptors such as organisms). For example, low-pH CO₂-charged water that leaks from a storage reservoir and is emitted at the seabed may cause the carbonate shells of calcifying marine organisms to dissolve.
Processes related to the transport of CO₂, and associated contaminants, in formation water / groundwater and surface water, including advection, dispersion and molecular diffusion.
Advection is the process by which contaminants, such as CO₂ and its associated contaminants, are transported by the bulk movement of the water in which they are dissolved. Advective water movement can occur along connected porous regions, such as fractures and faults. Processes that affect the movement of formation water, such as fault valving, will also affect the migration of dissolved CO₂, and associated contaminants.
As water migrates upwards through rock formations, the pressure decreases and dissolved CO₂ may come out of solution forming a free gas phase. This can result in “co-migration” and “co-discharge” of CO₂ and deep formation fluids. Similarly, there may be co-migration of free phase CO₂ gas and water from a reservoir (i.e. multi-phase flow).
Dispersion is the collective name for the consequences of a number of processes that cause 'spreading-out' of contaminants in all directions (such as CO₂ and associated its contaminants) dissolved in water, superimposed on the bulk movement predicted by a simple advection model. It results in a spatially distributed contaminant plume. Spreading of the solute plume can occur in the direction of advection, in which case it is known as longitudinal dispersion, or it can occur perpendicular to the direction of advection, in which case it is known as transverse dispersion.
Diffusion is the process whereby chemical species move under the influence of a chemical potential gradient (typically a concentration gradient). In the storage domain, diffusion of CO₂ might be the most significant CO₂ transport medium in the cap rock and low permeability sedimentary host rock environments where advective transport does not occur or is limited, and diffusion of other chemical species may give rise to chemical regimes in parts of the system that inhibit or enhance the transport of CO₂. Diffusion can occur in any direction, and is not related to the direction of flow within the system.
Relevance to performance and safety:
The transport of CO₂, and associated contaminants, within formation water / groundwater, is likely to be a key migration process and therefore an important consideration in determining performance and safety. Impacts on sensitive domains (i.e. regions within which there are potentially impacted receptors such as organisms) may arise if such water leaves the CO₂ storage complex, or indeed forms outside the storage complex due to interaction between leaking CO₂ and the water within the formations through which it migrates. The patterns of migration and the fluxes of the water will influence the magnitudes of the impacts and their spatial distribution. At a secondary level, dispersion within surface water environments (both freshwater and marine) will influence environmental concentrations of CO₂ and associated contaminants and consequently the potential for any associated impacts.
Processes by which CO₂ is lost from the storage system. Once in the near-surface, changes in pressure and temperature result in the potential for phase changes and degassing, with resulting changes in the transport properties of stored CO₂. Examples of CO₂ release processes include:
- surface and undersea blowouts;
- CO₂ geysers (such as Crystal Geyser in Utah, see picture below);
- submarine gas release (see picture below).
Natural gas seepages from hydrothermal vents provide an example of seepages in the marine environment (see picture below). Seepage of CO₂ into the marine environment will reduce pH as dissolved CO₂ will form bicarbonate and carbonate ions.
Relevance to performance and safety:
The potential for CO₂ to be lost from a storage system will determine the performance of that system. The area over which CO₂ is lost, as well as the CO₂ flux, will influence the area and volume of potentially impacted sensitive domains (i.e. regions within which there are potentially impacted receptors such as organisms) and the nature/severity of the impacts.
The rapid turnover and degassing of CO₂ from a surface water body.
Due to the high solubility of CO₂ in water, a lake can dissolve a volume of CO₂ that, in gaseous form, is more than five times its volume. CO₂ rich water is denser that pure water which can result in an unstable stratification. A drop in temperature will reduce the solubility of CO₂ in water. If the water reaches its solubility limit as a result, bubbles will nucleate. As the bubbles rise and grow, a chain reaction occurs where a sudden ex-solution of CO₂ can result in a rapid degassing of the water body with an eruption of rising, expanding bubbles.
Limnic eruptions can be triggered by events such as landslides that disturb the unstable stratification.
Relevance to performance and safety:
Limnic eruption provides a release mechanism for CO₂, migrating from a storage system to the atmosphere.
Natural limnic eruption events can be catastrophic. On the 29th of August 1986, a massive limnic eruption from Lake Nyos in Cameroon resulted in 1800 deaths as the CO₂ smothered local villages. The CO₂, which is volcanic in origin, seeps through the lake bed sediments and builds up in the lower strata of the water column.
Sub-marine discharges to the sea floor will typically occur at localised points or as diffuse emanations but over areas that are fairly restricted (most likely a few metres to tens of metres in length). These points or areas may be either isolated (e.g. as in a leaking borehole) or occur in groups, either distributed over a wide area, or aligned along faults. The spatial distribution of discharges over a wide area, and between individual points may be very heterogeneous. CO₂ rich waters discharging to the water column will be diluted and dispersed to an extent and at a rate that depends upon the occurrence and vigour of currents. For some marine water bodies, currents may be very weak for sufficiently long periods to enable stratification to develop. Potentially, in the absence of sufficient dispersion, and if they are sufficiently dense, CO₂ rich waters can spread laterally over a wide area of the seabed.
Relevance to performance and safety:
Stratification in marine systems can increase the potential impacts of CO₂ on benthic marine flora and fauna by enabling higher CO₂ concentrations to develop over a wider area of the seabed than in the absence of stratification.
Surface seepages of CO₂ may contain significant amounts of other gases due to co-migration, such as hydrogen sulphide (H₂S). This is especially the case where the CO₂ reservoir is a reducing chemical environment. H₂S is derived from the hydration of sulphide minerals (e.g. FeS) or from the chemical reduction of aqueous sulphate species. H₂S is highly toxic and therefore potentially harmful to the biosphere. Even in low concentrations it is deleterious to long-term health (human/animal), in addition to being extremely unpleasant.
Relevance to performance and safety:
The co-migration of other gases to the surface environment may cause areas of leakage to become uninhabitable - at least temporarily. The detection of H₂S provides a quick (non-analytical/aesthetic) test for CO₂ escape and thus can be used as a 'marker' gas.
This category of FEPs is concerned with the geology, hydrogeology and geochemistry of the geosphere. The category covers the reservoir, overburden and surrounding rock up to the near-surface which is considered in a separate FEP category.
Taken together, the FEPs in this category describe what is known about the natural system prior to storage operations commencing. The category is divided into three classes: Geology, Fluids, and Geochemistry.
The geographic location of a CO₂ storage reservoir will influence the type of impacts to consider, e.g. continental or sub-marine, in the vicinity of a volcano, or tectonic activity, etc. In addition, proximity to human populations will increase importance of any release to the surface.
Relevance to performance and safety:
Proximity to natural hazards will increase their importance in being considered in the assessment. Proximity to human populations places more emphasis on the significance of near-surface releases.
Natural resources within the geosphere including solid mineralogical resources, such as coal or minerals, fluid and gaseous resources, such as hydrocarbons or water, and other resources such as geothermal or microbial resources.
Relevance to performance and safety:
The presence of natural resources may mean that future human exploitation of the system cannot be ignored in assessing long-term performance since they may increase the possibility of future human intrusion.
The generic type of reservoir being considered for storage of CO₂. For example:
- oil reservoir (such as the Weyburn project);
- gas reservoir (such as the Coal-Seq project);
- aquifer (such as at Sleipner); and
- coal beds (such as the Coal-Seq and RECOPOL projects).
Relevance to performance and safety:
The generic reservoir type will provide a high-level indication of the geological characteristics of the storage location. It will also contribute towards the extent and type of historical exploitation of any geological resources.
Geometry of the CO₂ storage reservoir including the spatial distribution, depth and the topography of the top.
Relevance to performance and safety:
The geometry of the storage reservoir helps to determine the capacity of the geosphere.
The geometry of the top is particularly important because supercritical CO₂ is buoyant and will therefore migrate to the top of a reservoir. Once at the top of the reservoir, it will migrate according to the precise topography of the top. Local 'highs' could produce small-scale traps within the overall aquifer; bigger structures would produce bigger traps.
Spill points are determined by the lowest point that can retain the stored CO₂.
Degree to which geological resources (such as oil and gas) have been exploited prior to the injection of CO₂.
Relevance to performance and safety:
The extent of previous exploitation will help to determine the initial state of a storage reservoir. For example:
- the existence and nature of boreholes; and
- the presence of geological resources, such as oil and gas.
Previous exploitation of the storage reservoir area will improve the amount of historical information available concerning the reservoir characteristics.
The nature of the relatively impermeable layer of rock overlying the storage reservoir that forms a barrier to the upward migration of buoyant fluids, such as stored CO₂.
Relevance to performance and safety:
The cap rock or sealing formation plays a key roll in preventing the stored CO₂ from migrating to the surface environment.
This concerns the concept of successive lithological, hydraulic and/or chemical barriers acting successively to prevent fluid escape to surface environments. From a geological point of view the permeability barrier is probably the most important, in comparison with other types of traps. However, it may be necessary to consider a sequence of traps in CO₂ migration models in addition to the conventional low permeability barriers.
For example, within the Weyburn storage project, the primary cap rock is the Watrous formation; however, low permeability formations at higher stratigraphical layers provide potential additional seals, preventing upward migration of stored CO₂.
Relevance to performance and safety:
Features with the potential to retard or prevent CO₂ migration in the geosphere are important considerations when determining the performance of a storage project.
The systematic description of rocks in terms of their mineral assemblage and texture.
The lithology of the geosphere (including both the reservoir and the caprock) determines the reservoir physical and transport properties (including porosity and permeability). It concerns the mineralogical composition, texture (grain size, sorting) and fabric (sedimentary structures, vertical and horizontal heterogeneities). Potential reservoir lithologies include sandstones and limestones.
Relevance to performance and safety:
The physical, chemical, and mineralogical properties of the reservoir rocks affect the capacity to store CO₂, as well as fluid flow and CO₂ migration. These factors determine which water-rock reactions can take place and also influence the rock strength and elastic properties (such as compressibility, shear strength, Poisson's ratio etc).
The slow physical, chemical and/or biological processes by which unconsolidated sediments become sedimentary rock. These processes can result in changes to the original mineralogy.
Relevance to performance and safety:
The state of lithification/diagenesis contributes to determining the physical and chemical characteristics of sedimentary rock. For example, porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitisation.
Structure and density of discrete voids within the rock (pores).
Relevance to performance and safety:
The pore architecture determines the porosity and permeability of the rock, which are key features when considering the mobility of fluids and gases within the rock.
Unconformities are geological surfaces separating older from younger rocks which represent a gap in the geologic record. Such a surface might result from a hiatus in deposition of sediments, possibly in combination with erosion, or deformation such as faulting. An angular unconformity separates younger strata from eroded, dipping older strata. A disconformity represents a time of nondeposition, possibly combined with erosion, and can be difficult to distinguish within a series of parallel strata. A nonconformity separates overlying strata from eroded, older igneous, or metamorphic rocks.
Relevance to performance and safety:
Unconformities can act both as potential seals or lateral migration pathways for fluids. For example, the impermeable barrier resulting from the widespread development of diagenic anhydritised carbonate associated with the unconformity between the Mississipian beds and overlying Triassic Watrous Formation in the vicinity of the Weyburn pool.
Heterogeneities are variations in the rock properties of a geological formation.
Relevance to performance and safety:
Heterogeneities can result in directional variations in permeability, which affects the mobility of fluids and gases in the rock. For example, experience from the Saline Aquifer CO₂ Storage project (SACS) has shown that both stratigraphical and structural local permeability heterogeneities have the potential to affect CO₂ distribution and migration profoundly (Chadwick et al., 2003).
The image below shows seismic sections through the Sleipner injection site from Zweigel et al. (2001). The strong amplitudes are taken to be CO₂ accumulations.
Fractures are cracks or breaks in rock. Fractures along which displacement has occurred are called faults. Fractures and faults can occur over a wide range of scales.
Relevance to performance and safety:
Fractures can enhance conductivity, for example, by a conductive fracture connecting permeable regions together. They can also act as seals, by bringing a relatively permeable region into contact with low conductivity rock, for example, or because they contain impermeable fills, such as clays.
CO₂ leaks in fractures and faults are most likely to reach the atmosphere where the faults or fractures intersect with the ground surface; leakage in this manner will result in localised CO₂ releases rather than diffuse releases over a larger area.
Often a fault will not be seen at the land surface or the seabed because it will be covered in sediment / soil. In these circumstances, CO₂ leaking through the fault may disperse as it is transported through the sediment or soil before being emitted to the atmosphere or water column. In such a case the CO₂ emission at the surface or seabed may appear to have no vent source and indeed such emissions have been described in the literature as being “diffuse”, even though they might be localised (e.g. covering an area of only a few square metres).
Natural or man-made features within the geological environment that may not be detected during site investigations.
Examples of possible undetected features are fracture/fault zones, the presence of brines or old mine workings. Some physical features of the storage environment may remain undetected during site surveys and even during preliminary borehole drilling. The nature of the geological environment will indicate the likelihood that certain types of undetected features may be present and the site investigation may be able to place bounds on the maximum size or minimum proximity to such features.
Relevance to performance and safety:
Undetected features could significantly affect the performance of a storage system. For example, local permeability hererogeneities with the potential to affect the distribution and migration of CO₂ profoundly at Sleipner (within the Saline Aquifer CO₂ Storage project, SACS) were only discovered after effectively being 'illuminated' by the stored CO₂ (Chadwick et al., 2003).
Temperature profile in the geosphere prior to the injection of CO₂.
The critical temperature for CO₂ is 31.1 degrees centigrade. The average geothermal gradient is approximately 25 degrees centigrade per kilometre. If the surface temperature is 10 degrees centigrade, the critical temperature will be reached at a depth of 840 metres. However, a considerable variation in geothermal gradients and sub-surface temperatures can be expected at a depth of 1000 metres. For example, in Europe temperatures at 1000 m range from 20 to 75 degrees centigrade, with local temperatures of more than 200 degrees centigrade in volcanic areas.
Relevance to performance and safety:
Relevant to temperature dependent physical/chemical/biological/hydraulic processes, such as CO₂ phase behaviour.
The pressure of fluids within the pores of a formation, normally hydrostatic pressure, or the pressure exerted by a column of water from the formations depth to the sea level prior to the injection of CO₂.
The critical pressure of CO₂ is 7.38 mega-Pascals. The average underground hydrostatic pressure increases with depth by approximately 10.5 mega-Pascals per kilometre for aquifers that are in open communication with surface water. Applying this average gradient, the critical pressure of CO₂ will be reached at a depth of around 690 metres. However, aquifers or hydrocarbon reservoirs that are sealed off from the rest of the sub-surface may be under- or overpressured.
Relevance to performance and safety:
Contributes towards determining the mobility of stored CO₂.
Petrophysical properties of the geosphere prior to the injection of CO₂. These include features such as permeability, porosity, residual saturation, capillary pressure and wettability.
Relevance to performance and safety:
Petrophysical properties influence how injected CO₂ will migrate in the geosphere. For example, permeability influences the direction and rate of CO₂ movement, and porosity and residual saturation influence the dimensions of the CO₂ plume.
Details of fluids in the geosphere, which comprises of the reservoir, overburden and surrounding rock prior to the injection of CO₂. Water will generally be present, but other fluids, particularly hydrocarbons, may be important, dependent on the storage concept.
Relevance to performance and safety:
Water and other fluids in the storage system will affect the transport and interactions of injected carbon dioxide.
Natural formation water flow pathways (directions, velocities) in the geosphere will be important in determining the long-term migration paths for CO₂. This depends on factors including: hydraulic heads, permeability and porosity distribution, the existence of fracture networks, connection between aquifers, position of the recharge and discharge areas.
Relevance to performance and safety:
This will affect the migration of dissolved CO₂ in the reservoir and the geosphere (direction, timing), the position of the interface between supercritical CO₂ and aquifer water (inclined interface). There may also be a possible effect on overlying aquifers used for drinking water.
The presence and distribution of hydrocarbons, such as oil and/or gas, within the storage system.
Relevance to performance and safety:
Hydrocarbons have potentially important implications for a storage system, by both influencing the likelihood of previous geological exploitation of the area, and by being an important component of the system with which stored CO₂ can interact.
This category of FEPs is concerned with the way that activity by humans alters the natural system. Both boreholes used in the storage operations and those drilled for other purposes are relevant to the long-term performance of the system. The category is divided into two classes: one on the drilling process, and the other on sealing and abandonment.
Alteration of the far-field or virgin characteristics of a formation, usually by exposure to drilling fluids. Fracturing associated with formation damage can increase porosity, whereas the water or solid particles in the drilling fluids, or both, can decrease the pore volume and effective permeability of the formation in the near-wellbore region.
A number of mechanisms can result in a decrease in porosity, including:
- solid particles from the drilling fluid physically plug or bridge across flowpaths in the porous formation;
- water contacting certain clay minerals in the formation; this typically causes the clay to swell, increasing in volume and decreasing the pore volume; and
- chemical reactions between the drilling fluid and the formation rock and fluids can precipitate solids or semisolids that plug pore spaces.
Relevance to performance and safety:
Formation damage has a number of potential implications for assessing CO₂ storage:
- it can make information from affected boreholes non-representative of the true characteristics of the damaged formations; and
- damaged regions themselves may provide flowpaths for CO₂ migration, particularly if damage results in fracturing.
At the time of drilling, boreholes are lined with a metal casing. Cement is pumped downhole inside the casing string, and it is pushed upward under and outside the casing lower end, between the casing and the rock wall. In multi-stage cement jobs, cement is squeezed between the casing and the rock wall though purpose made perforations. The cement could be pushed behind casing from the bottom hole to the surface, or to a predetermined depth. In such a case, the height of cement outside the casing is checked with the Cement Bond Log (CBL).
Curing of cement is the process of maintaining the proper temperature and moisture conditions to promote optimum cement hydration immediately after placement. Proper moisture conditions are critical because water is necessary for the hydration of cementitious materials. As cement hydrates, strength increases and permeability decreases. When hydration stops, strength gain ceases. Therefore, proper hydration of the cement is important in the fabrication of strong, durable concrete. The degree of curing of cement used in boreholes determines their properties, including strength, permeability and durability.
Alteration of borehole linings (metal and cement/concrete) will occur with time, depending on the natural fluid composition of the deep reservoir and the input of high concentrations of CO₂ carrying natural H₂S, which may accelerate corrosion.
Relevance to performance and safety:
Borehole lining and completion will contribute to determining the performance of a borehole both during its operational and post-closure phases. This is important from the perspective of CO₂ storage, since boreholes may provide preferential short circuits to the surface with potential release of CO₂ and contamination of upper aquifers.
The process of performing major maintenance or remedial treatments on a borehole often associated with the re-use of existing boreholes. Workover techniques include flushing of the formation and the removal and replacement of the borehole lining.
Relevance to performance and safety:
Workover will result in modified borehole properties that may need consideration within an assessment.
Often, monitoring wells are needed to monitor the physical conditions (pressure, temperature, etc.) of the storage reservoir, both inside and/or outside the area immediately affected by the storage operations or above the storage reservoir.
Monitoring wells could be 'adopted,' by using existing wells to host the appropriate instrumentation (piezometers, pressure gauges, thermometers, etc.), or they could be purpose drilled anew.
Relevance to performance and safety:
Observation or monitoring wells may provide an accidental leakage route for the stored CO₂, particularly wells drilled inside the area of storage.
The drilling of boreholes (wells) for site investigation, resource exploitation and/or CO₂ injection will result in many documents being generated in paper or digital form. Typical well records include location co-ordinates, depth, electric logs, mud logs, drilling parameter logs, composite log, testing reports (if applicable), coring report (if applicable), and a final report. Physical records from cutting samples, cores and fluid samples will also be documented.
The principal tool to gain knowledge about the sequence of drilled rock formations is Borehole Logging. This is achieved by lowering an electro-mechanical tool to the bottom of the hole at the end of a steel rope (with connecting electrical cables) and by measuring petrophysical parameters on the way up. Common measures include hole diameter, natural gamma ray response, spontaneous electrical potential, rock resistivity, velocity of acoustic waves through rock, neutron susceptibility, etc. Borehole logging can be extended to include focussed fluid testing over short intervals or at specific points along the borehole.
The documents originating at the time of drilling are often the most accurate records of the succession of events associated with the drilling of the well and represent (particularly after years and decades) a precious source of information. The curation of such unique records is an invaluable tool to pass knowledge to future generations.
Relevance to performance and safety:
Well records provide a key source of baseline information regarding the storage site. They provide information concerning the nature of the rocks drilled by the well and their petrophysical characteristics in terms of sealing potential and reservoir potential. Additionally, records from wells drilled over a period of time can give a picture of how the system is evolving either naturally, or as a result of the exploitation of geological resources. This baseline information is an important input into the initial conditions relevant to the system to be assessed, as well as for providing an indication of the likely importance of other FEPs.
Incomplete well records equate to a potential gap in understanding of the storage system and may result in potentially important features being overlooked in the assessment process.
FEPs relevant to the closure of boreholes drilled within the system domain.
Relevance to performance and safety:
The way that boreholes are closed and sealed is directly relevant to the likelihood that they could act as 'short circuits' for carbon dioxide transport.
Features related to the cessation of CO₂ injection operations at a site and the sealing of injection and monitoring wells.
When a borehole is drilled to the potential storage reservoir, it creates communication with possible overlying reservoirs and with the surface. Cementing and abandonment procedures are designed to permanently plug such communication channel. At the time of well abandonment, cement plugs tens to hundreds of metres thick are placed at intervals inside the well casing.
The cement plugs are commonly located across potential problem spots (e.g. perforations, casing shoes, top of liner, etc.), to minimise leaking risks. Particular attention should be paid to the quality of the original cement job behind the casing string. If uncemented space is detected, known or suspected behind casing, depending on the lithology across such interval, it may be important to squeeze extra cement between the rock face and the casing to complement the final abandonment plugs inside the casing.
Individual boreholes may be closed in sequence, but closure refers to final closure of the whole system, and may include removal of surface installations. The schedule and procedure for sealing and closure may need to be considered in the assessment.
Relevance to performance and safety:
The intention of borehole sealing is to prevent human access to the stored CO₂ and to prevent the borehole from providing a migration pathway for the CO₂. Correct cementing and abandonment operations are essential to achieve restoration of pristine sealing above the designed storage reservoir formation.
Borehole linings and seals (metal and cement) will evolve with time and may degrade. The nature of the evolution, and whether it will be detrimental to seal performance, will depend on the temperature and stress conditions and natural fluid compositions in the deep reservoir and overlying rocks. This evolution may be influenced by the input of high concentrations of CO₂. Any H₂S present may accelerate corrosion of metal linings. Cement will react with CO₂ at high partial pressures and may undergo a range of reactions in the presence of fluid with low pH and appreciable concentrations of sulphate, chloride, and magnesium ions. Seal failure will occur if liners have degraded and corroded.
Borehole seals will be designed to minimise the likelihood of failure. Monitoring will typically be undertaken to ensure performance and to mitigate any observed performance defects. The main risk therefore will typically be associated with the longer-term post-monitoring period.
Relevance to performance and safety:
Seal failure may provide preferential short circuits to the surface with potential release of CO₂ and associated contaminants to the surface or near-surface environment. Should it occur, failure may provide a preferential pathway either through the borehole annulus or around the outside of the casing. The cause and mechanism of any seal failure will influence the nature of the release. For example, the failure of all cement plugs within a wellbore could potentially cause CO₂ to leak to the surface. In contrast failure of the cement between the casing and rock in the section of borehole that crosses the caprock may cause CO₂ to leak from the reservoir into shallower rock formations, but not directly to the surface through the well.
Uncontrolled flow of fluid (liquid, gas or supercritical fluid) into the wellbore, followed by transport of the fluid to a shallower geological formation (underground blowout) or to the land surface or seabed. Underground blowouts may cause contamination of aquifers or other underground resources whereas blowouts to the land surface could lead to fluid erupting into either terrestrial or marine ecosystems.
Relevance to performance and safety:
A blowout would provide a rapid pathway for CO₂ to reach a subsurface aquifer or other underground resource, or to arrive at the land surface or seabed. The arrival of CO₂ at one or more of these receptors could contaminate resources and / or have adverse impacts upon ecosystems. The cause and mechanism of borehole seal failure, and its timing (e.g. during borehole drilling or post-borehole drilling) will influence the nature of the release.
CO₂ storage projects may be part of an Enhanced Oil Recovery (EOR) project or stand-alone deep saline aquifer projects. Either way, it is likely that the target geological structure has been the object of past exploration efforts, possibly involving drilling wells.
The existence of old wells could be obvious if the wells are still active, but could be overlooked if the old (orphan) wells have been long cemented and abandoned. Technical details of such abandoned vintage wells may in fact not be readily available, or may be lost. In such a case, the old cementing (sealing) job could be substandard.
Relevance to performance and safety:
Old substandard plugged wells could provide a potential CO₂ leakage route to the surface or to possible reservoirs above the designed CO₂ storage reservoir.
There is little chance of detecting a substandard well abandonment before the beginning of CO₂ injection to the designed reservoir, particularly if the existence of an old well has been overlooked. If old abandoned wells are known in the area of the CO₂ injection operations, the risk is minimised by carefully checking any potential CO₂ leak to the surface at the old well head location.
If old wells are unknown and not suspected, it is good practice to run a baseline soil gas survey (if applicable) and successive soil gas surveys at intervals after the beginning of CO₂ injection.
The slow downward gravitational movement of soil around boreholes.
Relevance to performance and safety:
This process results in changing properties of the soil around borehole casings after abandonment. It may either increase or decrease the degree of sealing and therefore the potential for the region immediately around the borehole to act as a migration pathway for CO₂ and/or associated contaminants.
This category of FEPs is concerned with factors that can be important if stored carbon dioxide returns to the accessible environment. The environment could be terrestrial or marine, and human behaviour in that environment needs to be described. The category is divided into three classes: Terrestrial Environment; Marine Environment; and Human Behaviour.
This class of FEPs is concerned with factors that can be important if stored carbon dioxide returns to the accessible terrestrial environment.
Relevance to performance and safety:
The near-surface environment is where most potential impacts would be incurred. The FEPs in this class are relevant if that environment is terrestrial.
Features related to the relief and shape of the surface environment and its evolution.
Relevance to performance and safety:
This FEP refers to local land form and land form changes with implications for the surface environment, e.g. plains, hills, valleys, and effects of river and glacial erosion thereon. In the long term, such changes may occur as a response to other geological changes.
Features related to the characteristics of the soils and sediments and their evolution.
Different soil and sediment types, e.g. characterised by mineralogy, particle-size distribution and organic content, will have different physical and chemical properties that may influence erosion/deposition, solid phase-water interactions, transport of water and CO₂ etc. Soil cracks, such as may be caused by dessication or the action of plant roots, have been identified as potentially important features that can provide focussed, preferential transport routes for release of CO₂ from soils. The solid-phase composition of the soil or sediment will determine its pH buffering capacity and the extent to which it can react with CO₂-aqueous solutions.
Relevance to performance and safety:
The characteristics of soils and sediments will strongly influence the behaviour of CO₂ within them. In particular, these characteristics will largely determine the ability of CO₂ to interact with the soil / sediment ecosystem and is therefore an important factor when considering exposure of local environmental receptors, such as flora and fauna.
Soil and sediment characteristics will influence the type of vegetation and land use. They will also determine relevant processes to consider should CO₂ and/or associated contaminants migrate to the terrestrial surface environment.
FEPs related to all the erosional and depositional processes that operate in the surface environment, and their evolution with time. Relevant processes may include fluvial and glacial erosion and deposition, denudation, aeolian erosion and deposition. These processes will be controlled by factors such as the climate, vegetation, topography and geomorphology.
The picture below illustrates the relationship between fluvial stream flow velocity and particle erosion, transport, and deposition.
Relevance to performance and safety:
Erosional and depositional processes will influence the way in which the surface environment has evolved and will evolve over the time-scale of interest.
FEPs related to the characteristics of the atmosphere, weather and climate, and their evolution with time. In case of CO₂ leakage to the surface, the weather is a relevant factor in determining the subsequent dispersion of displacement of the gas, which is influenced by factors including wind, dissolution in atmospheric moisture, associated evolution of gas concentrations, and gas accumulations in landscape depressions.
Atmospheric processes include physical transport of gas, aerosols and dust in the atmosphere, and chemical and photochemical reactions.
Meteorology is the investigation of the properties of the atmosphere and the processes that affect them, including relationships among evaporation, atmospheric precipitation, temperature, pressure, wind speed and direction, and insolation. Atmospheric characteristics and the processes affecting them vary temporally over different timescales. For example, short-term weather events (e.g storms) may range from hours to days in length. In contrast, long-term climate changes, such as glaciation and deglaciation, may span centuries to millennia.
Relevance to performance and safety:
This FEP will determine the behaviour of CO₂ should it reach the atmosphere and is therefore an important factor when considering exposure of local environmental receptors, such as flora and fauna.
Meteorological characteristics influence the near-surface hydrological regime with its subsequent consequences for CO₂ migration; for example soil moisture levels affect egress of gas. Atmospheric pressure changes will drive gaseous exchange between the atmosphere and soil gas; this process is termed “atmospheric pumping”. This process will affect release of CO₂ from soils, and hence CO₂ concentrations in soil gas.
The variability in meteorological characteristics should be considered by any assessment of the consequences of CO₂ emissions from the earth's solid surface or surface water bodies.
Processes related to near-surface hydrology at a catchment scale and also soil water balance, and their evolution with time.
The hydrological regime is a description of the movement of water through the surface and near-surface environment. It includes the movement of materials associated with the water such as gas or particulates and extremes such as drought, flooding, storms and snow melt.
Relevance to performance and safety:
The hydrological regime and water balance will influence the way in which CO₂ migrates should it reach the near-surface environment.
Features related to the characteristics of aquifers and water-bearing features and their evolution.
Relevance to performance and safety:
Shallow Aquifers
Aquifers may yield significant amounts of water to wells or surface springs and may thus be a flowpath for CO₂ to the surface environment. The presence of aquifers and other water-bearing features will be determined by the geological, hydrological and climatic factors.
Shallow aquifers will be able to dissolve CO₂, reducing further upward migration. The amount of CO₂ that can dissolve will depend on factors such as the mineralogy of the aquifer, location of the water table, the chemical composition of pore waters, CO₂ flux rates, hydrogeology, and the physical characteristics of the leakage path (e.g. whether of small cross-sectional area, as in a leaking well, or with a much larger effective cross-sectional area, as in some leaking faults that contain multiple discrete leakage paths). The water table location is important in determining the water-body thickness available to interact with any CO₂ rising from depth. The lowering of shallow aquifer piezometric levels reduces the thickness of shallow CO₂-buffering water-bodies. The consequence is a reduced capacity to buffer CO₂ migrating from depth.
Surface Water Bodies
Streams, rivers and lakes often act as boundaries of hydrogeological systems. They may represent a significant source of dilution for CO₂ and contaminants entering these systems, or act as reservoirs of CO₂. In hot dry environments, where evaporation dominates, concentration or exsolution of gas is possible. Water with dissolved CO₂ is denser than non-saline waters without significant dissolved CO₂, which can result in stratification and the potential for limnic eruption. Differences in water body impacts can be seen depending upon the nature of the body, such as whether or not a body of water is stratified or homogeneous, the volume of the water body and its depth / surface area.
Features and processes related to the characteristics of terrestrial and aquatic (predominantly fresh-water) flora and fauna, and their evolution. This FEP includes plants, animals, fungi, algae and microbes.
Relevance to performance and safety:
Flora and fauna may be affected by concentrations of CO₂ in the near-surface environment and may be indicators of CO₂ leakage. Flora and fauna may also become stressed as a result of increased CO₂ concentrations, causing an increased susceptibility to climate change, pollution, etc.
Features and processes related to interactions between terrestrial and freshwater populations of animals, plants, algae, fungi, microbes and their evolution.
Characteristics of the ecological system include the vegetation regime and natural cycles such as forest fires or flash floods that influence the development of the ecology. The plant, animal, algal, fungal and microbial populations occupying the surface environment are an intrinsic component of its ecology. Their behaviour and population dynamics are regulated by the wide range of processes that define the ecological system. Human activities have significantly altered the natural ecology of most environments.
Relevance to performance and safety:
The ecology of the terrestrial near-surface environment determines the types of organisms present and their inter-dependencies. These can influence the types of impact of interest and can provide a mechanism for monitoring CO₂ leakage. CO₂ leakage may stress ecological systems, which could result in higher susceptibility to other stresses, such as drought, pollution etc. Similarly, CO₂ leakage could occur to systems that are already stressed by such factors, leading to an enhanced likelihood of observable impacts. Similarly, CO₂ leakage could occur to systems that are already stressed by such factors, leading to an enhanced likelihood of observable impacts.
Features and processes related to the characteristics of coasts and the near shore, and their evolution. Coastal features include headlands, bays, beaches, spits, cliffs and estuaries.
Relevance to performance and safety:
The processes operating on these features, e.g. active erosion, deposition, longshore transport, may affect mechanisms for the migration of CO₂, and associated contaminants, entering the surface environment.
Features and processes related to the characteristics of seas and oceans and their evolution. This includes the topography and morphology of the seabed, thermal stratification and salinity gradients, hydrodynamic mixing, and marine currents. Other features may include water column depth, temperature stratification, and salinity. Ocean biogeochemistry (the interactions of biology, chemistry and geology in the ocean), in particular the cycling of materials and nutrients through the different parts of the system, is also included.
Relevance to performance and safety:
The local oceanographic features and processes determine the potential for dilution or accumulation of CO₂, or associated contaminants in the marine environment.
Any influx of CO₂ to the marine environment, whether from the sea bed or sea surface, has the potential to modify the ocean carbon and other nutrient cycles. Increased concentrations of CO₂ in the deeper marine environment may lead to similar effects on biogeochemistry as have been observed as a result of sea surface acidification.
Features and processes associated with sediments in the present marine environment. This includes both the physical and chemical characteristics of the sediments, along with sedimentation and resuspension processes.
Relevance to performance and safety:
Marine bed sediment characteristics will influence the ecology of the marine environment. They will also influence relevant processes to consider should CO₂ and/or associated contaminants migrate to the marine environment.
Features and processes related to the characteristics of marine flora and fauna, and their evolution. These flora and fauna include plants, animals, fungi, algae and microbes.
Relevance to performance and safety:
Flora and fauna may be affected by concentrations of CO₂ in the marine environment and may be indicators of CO₂ leakage. Descriptions of the marine flora and fauna therefore provide a baseline against which assessments can be made.
Features and processes related to interactions between populations of algae, animals, microbes and their evolution.
The algal, animal and microbial populations occupying the marine environment are an intrinsic component of its ecology. Their behaviour and population dynamics are regulated by the wide range of processes that define the ecological system.
Relevance to performance and safety:
The ecology of the marine environment determines the types of organisms present and their inter-dependencies. These can influence the types of impact of interest (e.g. nutrient fertilisation) and can provide a mechanism for monitoring CO₂ leakage.
Features and processes related to characteristics, e.g. physiology, metabolism, of individual humans. This FEP includes considerations of variability of physiology and metabolism in individual humans due to age, health, and other features.
Physiology refers to body and organ form and function. Metabolism refers to the chemical and biochemical reactions which occur within an organism, or part of an organism, in connection with the production and use of energy.
Children and infants, although similar to adults, often have characteristic differences, e.g. metabolism and respiratory rates, which may lead to different characteristics of exposure to CO₂ or contaminants.
Relevance to performance and safety:
Human physiology and metabolism determine the affect of exposure to CO₂ and associated contaminants.
Features related to intake of food and water by individual humans and the compositions and origin of intake. This includes considerations of how diets, of individual humans, may vary with age and other variations (ingestion of soil by infants, for example).
This FEP also includes processes related to the treatment of foodstuffs and water between origin and consumption. For example, once a crop is harvested it may be subject to a variety of storage, processing and preparational activities prior to human or livestock consumption. Water sources may be treated prior to human or livestock consumption, e.g. chemical treatment and/or filtration.
Relevance to performance and safety:
The human diet provides a potentially important exposure pathway to contaminants released into the foodchain as a result of the CO₂ storage system.
Food preparation processes may change the distribution and content of CO₂ and/or associated contaminants in the product.
Features related to non-diet related behaviour of individual humans, including time spent in various environments, pursuit of activities and uses of materials. This includes consideration of variability of the habits of individuals due to age and other factors.
The human habits refer to the time spent in different environments in pursuit of different activities and other uses of materials. The diet and habits will be influenced by agricultural practices and human factors such as culture, religion, economics and technology.
Relevance to performance and safety:
Human habits will determine the exposure pathways of interest in an assessment. Smoking, ploughing, fishing, and swimming are examples of behaviour that might give rise to particular modes of exposure to CO₂ and/or contaminants mobilised as a result of the CO₂ storage system.
FEPs related to land and water use by humans in the near-surface environment and the resulting implications for CO₂ leakage and contaminant transport and exposure pathways. This includes consideration of:
- the use of natural or semi-natural tracts of land and water such as forest, brush, rivers, lakes and the sea;
- rural and agricultural land and water use (including freshwater and marine fisheries);
- urban and industrial land and water use related to developments, including transport, and their effects on hydrology; and
- leisure and other uses of environment.
Relevance to performance and safety:
These FEPs can influence the potential transport and exposure pathways for CO₂ and its associated contaminants as well as the potential evolution of the system during the timescales of interest. Particular considerations are relevant for each type of land use addressed, for example:
- special foodstuffs and resources may be gathered from natural land and water which may lead to significant modes of exposure to CO₂ or contaminants;
- an important set of processes are those related to agricultural practices, their effects on land form, hydrology and natural ecology, and also their impact in determining contaminant uptake through food chains and other exposure paths;
- human populations are concentrated in urban areas in modern societies. Significant areas of land may be devoted to industrial activities. Water resources may be diverted over considerable distances to serve urban and/or industrial requirements;
- human populations currently use freshwater resources for agriculture, drinking, etc., but saline water should also be considered as future generations may desalinate and use it for similar purposes.
- significant areas of land, water, and coastal areas may be devoted to leisure activities. e.g. water bodies for recreational uses, mountains/wilderness areas for hiking and camping activities.
Features related to characteristics, behaviour and lifestyle of groups of humans that might be considered as target groups in an assessment.
Relevant characteristics might be the size of a group and degree of self-sufficiency in food stuffs/diet. Associated with this is a consideration of the amount of resources required to meet the needs of the community.
Relevance to performance and safety:
This FEP involves a consideration of aggregated human behaviour in order to consider their dependency on and interactions with their environment. It therefore provides input to a consideration of potential human interference with the CO₂ storage system as well as input to considering potential exposure pathways.
Features related to houses, or other structures or shelters, in which humans spend time.
Relevance to performance and safety:
The structure or materials used in building construction can be significant factors for determining potential exposure pathways to CO₂ or contaminants. For example, given that CO₂ is denser than air, it may accumulate in the basements/cellars of dwellings. Continued accumulation of CO₂ in buildings may lead to adverse health effects due to elevated concentrations.
This category of FEPs is concerned with any endpoint that could be of interest in an assessment of performance and safety. The classes of impact considered are: Impacts to Humans, Impacts to Flora and Fauna, and Impacts to the Physical Environment. Note that:
- financial impacts are assumed to be implicitly considered within each of the impact FEPs; and
- unless stated, the FEPs refer to both CO₂ and mobilised contaminants (minerals, heavy metals, hydrocarbons, gases).
Loss of stored CO₂ from the intended storage reservoir. Loss includes both consideration of loss to other parts of the geosphere and to the near-surface and surface environments, such as loss to marine water and surface water bodies, where CO₂ may result in stratification or pooling.
Relevance to performance and safety:
Loss of containment may be an endpoint of interest to the assessment. For example, the assessment context may dictate that near-surface or surface processes are outside the scope of the assessment.
FEPs relevant to adverse impacts on the physical environment. Note that these may be endpoints of interest in themselves, but may also cause other impacts of interest.
Relevance to performance and safety:
Adverse impacts on the environment can be postulated as a result of storage operations, even if there are no associated impacts to humans or on flora and fauna.
The existence of water aquifers may be important if they are subject to CO₂-induced chemical changes or CO₂-induced saline intrusion. The migration of CO₂ into an aquifer will result in the acidification of the water. Depending on the mineralogical composition of the aquifer and the chemical composition of the water, chemical reactions may occur which release heavy metals from the solid phase. The mechanisms which may cause this release include dissolution of metal oxides or oxyhydroxides, the reaction/diagenesis of clay minerals, and the desorption of metals that are adsorbed on clay surfaces or organic complexes.
Relevance to performance and safety:
Contamination of groundwater resources may result in impacts on flora, fauna and/or humans if the water is abstracted or flows to the surface environment. These potential impacts are considered in the subsequent FEP classes, however, contamination of groundwater may be an endpoint of interest in itself.
Soils and sediments may have elevated concentrations of CO₂, should it leak from the storage system, and/or other contaminants, such as heavy metals, hydrocarbons or even increased salinity resulting from CO₂ storage.
For example, natural CO₂ leaking from a trapped reservoir near Mammoth Mountain, in California, has resulted in soil gas concentrations of 20 to 90%.
Relevance to performance and safety:
Increased CO₂ concentrations and/or contamination of soils and sediments with associated substances may be sufficient to modify the ecology and/or use of the impacted area by humans.
Release of CO₂ to the atmosphere from the storage system or other contaminants, such as radon or methane, which are mobilised as a result of the storage.
Relevance to performance and safety:
Release of stored CO₂ or mobilised methane to the atmosphere will reduce the effectiveness of the storage system at preventing greenhouse gases from being emitted to the atmosphere.
Atmospheric contamination could also lead to health impacts on humans and wildlife.
The impact of CO₂ storage on the exploitation of natural resources such as oil and gas.
Enhanced oil recovery (EOR) and enhanced coal bed methane (ECBM) recovery projects involving the injection of CO₂ are based on improved recovery of oil and methane respectively resulting from CO₂ storage. However, CO₂ storage may result in the contamination of geological resources (such as hydrocarbons and minerals) or inhibit their recovery or future exploitation.
Note that the potential impact on groundwater resources is considered in 'Impacts on groundwater'.
Relevance to performance and safety:
Impacts on the exploitation of natural resources can be positive (such EOR and ECBM) and/or negative (such as inhibited recovery). These may result in other (for example, financial) impacts, or may be endpoints of interest in themselves.
Release of CO₂ or other contaminants from its storage system into a marine environment. The nature of the release could determine the affected areas of the marine environment, and whether or not a plume of CO₂-charged water is formed. A plume may affect different organisms depending upon its neutral buoyancy height.
Relevance to performance and safety:
Release of stored CO₂ will alter the marine environment, leading to marine water acidification and potential negative impacts on marine flora and fauna.
The injection of CO₂ may result in modifications to both the deep hydrogeology and near-surface hydrology.
Relevance to performance and safety:
Changes in the deep hydrogeology or near-surface hydrology may affect aquifer abstraction or even surface hydrology for groundwater driven features. These impacts may be either positive or negative.
The injection of CO₂ will modify the geochemistry of the storage system. This may be confined to the immediate vicinity of the storage location, or, through leakage from the reservoir, may affect other locations.
Relevance to performance and safety:
The extent of geochemical modifications may be an endpoint of interest, due to resulting changes to geological processes. For example, the acidification of the geochemical regime may cause minerals to be dissolved, with potential implications for the porosity and stability of the geological formations.
Injection of CO₂ into a geological formation may induce seismic events and processes.
Relevance to performance and safety:
Induced seismicity may be an endpoint of interest in itself, or it can result in other impacts, such as physical disruption of the surface environment.
The gradual or sudden sinking (subsidence) or elevation (uplift) of the topography of the terrestrial surface or marine sea-bed.
Relevance to performance and safety:
Deformation of the terrestrial surface or sea-bed may be an endpoint of interest in itself, or may result in other impacts, such as damage to property.
Addition of CO₂ in a limestone or carbonate-rich aquifer could result in dissolution of the rock matrix and the enlargement of voids. If this process takes place at relatively shallow depth collapse may result in subsidence at the surface and sinkhole formation.
For example, CO₂ leakage around a borehole drilled to extract natural CO₂ from a reservoir in Florina, Greece, resulted in subsidence around the borehole that filled up with water (see image below).
Relevance to performance and safety:
Large scale collapse structures may cause significant change to surface topography and possible CO₂ migration paths. Sinkholes can provide locations where leaking CO₂ can accumulate.
The surface waters of the oceans are slightly alkaline, with a pH range of 7.8-8.5. The variation is due to local, regional and seasonal effects. At present, a portion of the higher levels of atmospheric CO₂ is already being taken up by the oceans. The effects of the dissolution of some of the additional atmospheric carbon dioxide into the surface ocean have been an increase in the concentrations of dissolved carbon dioxide and bicarbonate (smaller relative, but greater absolute, effect on bicarbonate than on dissolved carbon dioxide), and a decrease in the carbonate concentration and pH.
Relevance to performance and safety:
The potential for ocean acidification may be an assessment endpoint in itself. It also has the potential to impact on marine flora and fauna.
Oxygen is an essential requirement for respiration in animals and is therefore needed to sustain life. High concentrations of CO₂ in air or water will lead to suffocation of terrestrial and aquatic animals due to a lack of oxygen reaching the blood stream. High CO₂ concentrations can also lead to blood acidification, which can cause animals to die.
If meteorological conditions do not disperse CO₂ released to the atmosphere, it can accumulate close to the surface and remain in depressions, such as natural hollows, allowing CO₂ to reach high concentrations if released in sufficient quantities under particular atmospheric conditions.
In a similar way to gaseous CO₂ being denser than air, water containing high concentrations of dissolved CO₂ will be denser than pure water and may be denser than marine water or saline natural formation waters. Dense CO₂-charged water may accumulate at the bed of a sea or lake and may under certain circumstances contribute to the limnic eruption phenomenon, as observed at Lake Nyos. In addition to the possibility of dissolved CO₂ causing asphyxiation in aquatic organisms, if gaseous CO₂ forms a layer at the surface of water bodies, it can prevent the oxygenation of the water and lead to a reduction of the O₂ concentration thereby contributing to asphyxiation of aquatic organisms.
Benthic and deep sea organisms might be adversely affected by increased levels of CO₂ and changes in pH. There is some evidence for the modification of fish behaviour and changes in mortality as a result of sudden decreases in pH. Research has indicated that for some systems, changes of the order 1 pH may have an impact in terms of dissolution of calciferous structures affecting biota such as crustaceans and molluscs. Changes of more than 2 pH units are more likely to be significant, placing an energy burden on animals to maintain their shells. Substantial changes in pH are likely to be very localised even if a leak does occur.
Secondary effects of high CO₂ concentrations, such as impacts on fauna food sources and habitats, may also result in increased stress on fauna populations. The impacts of any increased stresses will depend on the prior condition of the fauna before exposure, and may lead to higher susceptibility to other phenomena, such as climate change and pollution.
Relevance to performance and safety:
Asphyxiation and/or other impacts on fauna could be endpoints relevant to performance and safety. In the marine environment, this includes potential effects on calcification of marine fauna, including corals.
Plants and algae (in both terrestrial and marine environments) use energy in sunlight to photosynthesize carbohydrates from CO₂ and water (H₂O). Increasing concentrations of CO₂ around the photosynthetic tissues increases the rate of photosynthesis and therefore growth and productivity in terrestrial and aquatic plants and algae. At low elevated CO₂ concentrations biodiversity and total ecosystem productivity / biomass may not be significantly changed, however the dominant species may change because some species benefit from elevated CO₂ more than others.
The roots of most terrestrial plants need oxygen to break down carbohydrates to provide energy for root growth and healthy metabolism, a process called aerobic respiration. High concentrations of CO₂ in the soil reduces the availability of O₂ and can cause roots and therefore plants to die. High concentrations of CO₂ in the soil may also have direct toxicological effects, even when O₂ is not depleted.
At Mammoth Mountain, in California, CO₂ has accounted for up to 95% of the gas concentration in soil at the edge of Horseshoe Lake due to release from natural geological CO₂ reservoirs caused by volcanic activity. These high soil concentrations have resulted in areas of forest being killed (see picture below).
In the marine environment, some phytoplankton are calciferous, i.e. bearing, producing or containing polymorphs of calcium carbonate. The carbonate ion can be at supersaturated concentrations in sea water. As the pH in the ocean drops, so the concentration of the carbonate ion drops until solid carbonate phases become undersaturated. In this situation, calcium carbonate structures may become more vulnerable to dissolution resulting in reduced calcification of phytoplankton. Increased CO₂ can also disrupt the marine nitrification cycle.
The response to, and impacts of, elevated CO₂ will also depend on the condition of the plants and algae before exposure (e.g. whether already stressed due to other factors) and the sensitivity of individual species may depend on the stage of their lifecycle at the time of exposure to CO₂.
Relevance to performance and safety:
Elevated levels of CO₂ in the soil, atmosphere and/or water may impact on the growth and/or calcification of plants and algae.
Contaminants other than CO₂ may be introduced to the biosphere as a result of geological CO₂ storage due to:
- impurities associated with the storage fluid, such as hydrogen sulphide (H₂S), methane (CH₄), nitrogen oxides (NOx) and mercaptans;
- the mobilisation of substances in the geological environment due to the storage of CO₂, such as hydrocarbons, brine, CH₄ and heavy metals;
- the mobilisation of heavy metals in near-surface sediments.
Relevance to performance and safety:
Should they migrate to the biosphere, contaminants associated with and/or mobilised by CO₂ storage may have a toxic effect on organisms.
Geological storage of CO₂ may have an impact on the biosphere at a community, population and/or ecosystem level, with subsequent implications for biodiversity. At low elevated CO₂ concentrations biodiversity and total ecosystem productivity / biomass may not be significantly changed, however the dominant species may change because some species benefit from elevated CO₂ more than others. Such changes will only occur when CO₂ is elevated for prolonged periods, comparable with and greater than the lifecycle of the key fauna and flora.
High concentrations of CO₂ and associated or mobilised contaminants for a short period of time may not significantly affect communities and biodiversity, although the flora and fauna will be stressed and will therefore be more vulnerable to impacts from other sources. The magnitude of the impacts will also depend on the condition of the ecosystem before exposure and the sensitivity of individual species may depend on the stage of their lifecycle at the time of exposure to CO₂.
If concentrations are high enough to be toxic, or to cause asphyxiation, then many species may die, resulting in reduced biodiversity, modified populations, and reduced total biomass.
For example, the sudden release of natural CO₂ due to the limnic eruption at Lake Nyos, Cameroon, resulted in the sudden death of wildlife, but longer-term impacts on the ecology were minimal. However, the continued gradual release of natural CO₂ into soil near Mammoth Mountain, California, has been sufficient to kill trees and damage the local ecosystem since 1996 until the present day.
Release of CO₂ to aquatic environments would increase pCO₂ concentrations and decrease pH. Marine and freshwater environments are the most widespread aquatic environments that could be so affected. However, brackish water environments, or water bodies that are less saline than fully marine water (e.g. the Baltic Sea) or more saline than seawater (e.g. saline lakes) may be present near some future CO₂ storage sites. Similar to terrestrial ecosystems, such perturbations have the potential to impact upon aquatic ecology, and the magnitude of impacts will be sensitive to the magnitude and duration of CO₂ release. For example, long term releases of CO₂ have been found to stimulate the growth of marine phytoplankon. This resulted in increased growth of mussels because of the increased food supply. However the reproductive cycle of the mussels was disrupted which would affect the ecosystem in the longer-term.
Relevance to performance and safety:
The degree of potential ecological disruption resulting from geological CO₂ storage may be an endpoint of interest, especially if the ecosystem affected is considered valuable and/or sensitive to perturbations.
Microbes will be present at depth as well as in the near-surface and surface environments. CO₂ storage may disrupt microbial aerobic respiration but may enhance anaerobic respiration, with subsequent implications for the processes in which the microbes are involved. Field tests have shown that low elevated CO₂ can increase microbe numbers, although overall activity may be lower. At higher CO₂ levels microbial populations and activity may both be reduced. Alternatively, overall populations may be sustained but the balance of microbial species may change, depending upon factors such as the adaptability of different organisms and their responses to CO₂.
Microbes play an important role in all terrestrial and marine ecosystems, including those associated with extreme environments, such as deep sea hydrothermal vents.
Primary producers of biomass (also known as autotrophs) are the organisms at the base of the food web, from which the other organisms obtain their energy. Whereas in terrestrial ecosystems plants are the primary producers of the system, in the marine environment it is microbes (phytoplankton and cyanobacteria) which are the primary producers.
Relevance to performance and safety:
The potential impact of CO₂ storage on aerobic and anaerobic microbial respiration in the geosphere, terrestrial and marine biosphere may be an endpoint of interest. A low-pH, high CO₂ environment may favour some species and harm others.
Elevated atmospheric concentrations of CO₂ can result in both acute and chronic health effects in humans. If meteorological conditions do not disperse CO₂ released to the atmosphere, it can gather close to the surface and accumulate in depressions, such as valleys or at the surface of lakes. In these accumulations, CO₂ may reach high concentrations if released in sufficient quantities and at a sufficient rate under certain atmospheric conditions.
The primary health effect of concern is asphyxiation. Oxygen is an essential requirement for respiration in humans and is therefore needed to sustain life. High concentrations of CO₂ in air will lead to suffocation of humans due to a lack of oxygen reaching the blood stream. Asphyxiation can occur once atmospheric concentrations reach approximately 10% CO₂. Release of CO₂ into dwellings and buildings may be of concern if sufficient ventilation is not available.
Other health effects include those directly associated with elevated concentrations of CO₂ in the blood stream, such as acidosis (acidification), and physiological responses to the elevated blood CO₂, such as stimulation of the sympathetic nervous system and the release of catecholamines (such as adrenaline).
The 1986 Lake Nyos disaster in Cameroon provides a graphic example of the potential effects of high atmospheric concentrations of CO₂. A large limnic eruption resulted in the death of approximately 1800 people in the surrounding area and up to 27 km away.
Relevance to performance and safety:
The potential for CO₂ to be released to the atmosphere in sufficient quantities to cause health effects in humans will be an endpoint of interest in assessing the geological storage of CO₂.
Contaminants other than CO₂ may be introduced to the biosphere as a result of geological CO₂ storage due to:
- impurities associated with the storage fluid, such as hydrogen sulphide (H₂S), methane (CH₄), nitrogen oxides (NOₓ) and mercaptans;
- the mobilisation of substances in the geological environment due to the storage of CO₂, such as hydrocarbons, CH₄ and heavy metals.
Such contaminants may be toxic to humans and could cause harm if exposure pathways exist.
The toxicity will depend on:
- the form of exposure, e.g. ingestion or inhalation, leading to internal exposure or proximity to concentrations of contaminants leading to external exposure and;
- the metabolism of the contaminant and physico-chemical form if inhaled or ingested, which will determine the extent to which the element/species may be taken up and retained in body tissues.
Relevance to performance and safety:
The potential for contaminants associated with an/or mobilised by CO₂ storage to cause harm to humans is an endpoint of interest when assessing the geological storage of CO₂.
Impacts on humans due to physical disruption of the environment caused by geological CO₂ storage. For example, damage to buildings due to induced seismicity, damage to farmland due to subsidence or uplift.
Relevance to performance and safety:
Physical disruption of the environment caused by CO₂ storage may have a detrimental impact on humans.
Impacts on humans due to ecological modification. These may be negative (for example, reduced timber yields due to damage caused to trees by CO₂ in the soil) or positive (for example, increased crop yields due to higher atmospheric CO₂).
Relevance to performance and safety:
Ecological modification caused by CO₂ storage may have a positive or negative impact on humans.
Features, Events and Processes (FEPs) for Geologic Disposal of Radioactive Waste
Date
2000-08-01
Publisher
OECD
Description
Safety assessments of disposal sites for radioactive waste involve analyses of potential releases of radionuclides from the disposed waste and subsequent transport to the human environment. An important stage of assessment is the identification and documentation of all the features, events and processes (FEPs) that may be relevant to long-term safety. This report provides an international compilation of FEPs as well as a basis for selecting the FEPs that should be included in safety analyses.
In search of evidence of deep fluid discharges and pore pressure evolution in the crust to explain the seismicity style of the Umbria-Marche 1997-1998 seismic sequence (Central Italy)
Legal Aspects of Underground CO₂ Storage: Summary of Developments under the London Convention and North Sea Conference
Date
2001-12-14
Publisher
The Fridtjof Nansen Institute
Description
A report by The Fridtjof Nansen Institute on behalf of Statoil evaluating the position of CO₂ storage in light of the institutional framework of the London and OSPAR Conventions.
Houghton J T, Ding Y, Griggs D J, Noguer M, van der Linden P J and Xiaosu D (Eds.)
Title
Climate Change 2001: The Scientific Basis
Date
2001
Publisher
Cambridge University Press
Description
Part of the Third Assessment Report (TAR), has been produced by Working Group I of the Intergovernmental Panel on Climate Change and focuses on the science of climate change. It covers the physical climate system, the factors that drive climate change, analyses of past climate and projections of future climate change, and detection and attribution of human influences on recent climate.
Optimal Geological Environments for Carbon Dioxide Disposal in Brine Formations (Saline Aquifers) in the United States - Pilot Experiment in the Frio Formation, Houston Area
Date
2003-04
Publisher
Bureau of Economic Geology, The University of Texas at Austin
Description
for U.S. Department of Energy, National Energy Technology Laboratory
Monitoring Carbon Dioxide Sequestration using Electrical Resistance Tomography (ERT): A Minimally Invasive Method
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
1
Pages
353--538
Description
A paper describing numerical simulations and laboratory experiments to determine the potential of ERT methods to detect and monitor CO₂ in the subsurface.
Hoversten G M, Gritto R, Daley T M, Majer E L and Myer L R
Title
Crosswell Seismic and Electromagnetic Monitoring of CO₂ Sequestration
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
1
Pages
371--376
Description
Demonstrating a methodology for jointly interpreting crosswell seismic and electromagnetic data, in conjunction with detailed constitutive relations between geophysical and reservoir parameters.
Sensitivity and Cost of Monitoring Geologic Sequestration using Geophysics
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
1
Pages
2003
Description
Rock physics models were used to calculate anticipated contrasts in seismic velocity and impedance in brine saturated rock when CO₂ is introduced. Results are presented from a model based on Texas Gulf coast geology. Results indicate monitoring costs may be only a small percentage of overall geologic sequestration costs.
Strutt M H, Beaubien S E, Beaubron J C, Brach M, Cardellini C, Granieri R, Jones D G, Lombardi S, Penner L, Quattrocchi F and Voltatorni N
Title
Soil Gas as a Monitoring Tool of Deep Geological Sequestration of Carbon Dioxide: Preliminary Results from the EnCana EOR Project in Weyburn, Saskatchewan (Canada)
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
1
Pages
391--396
Description
Natural background levels and concentration distributions have been established for measured soil gases for comparison with future data sets to estimate CO₂ storage integrity for the reservoir rocks.
Issues Related to Seismic Activity Induced by the Injection of CO₂ in Deep Saline Aquifers
Publication
Journal of Energy and Environmental Research
Date
2002-02
Publisher
National Energy Technology Laboratory, US Department of Energy
Volume number
2
Pages
32--47
Description
Case studies, theory, regulation, and special considerations regarding the disposal of carbon
dioxide (CO₂) into deep saline aquifers are investigated to assess the potential for induced
seismic activity.
Geological Sequestration of CO₂ in Coalseams: Reservoir Mechanisms, Field Performance and Economics
Publication
Greenhouse Gas Control Technologies: Proceedings of the 5th International Conference on Greenhouse Gas Control Technologies
Date
2001
Publisher
CSIRO Publishing
Volume number
1
Pages
593--598
Description
Describes a joint U.S. Department of Energy and industry project to study the reservoir mechanisms, field performance and economics of CO₂ sequestration in coalseams.
Effects of Supercritical CO₂ on the Integrity of Cap Rock
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
1
Pages
483--488
Description
Paper concerning an investigation of the effects of supercritical CO₂ on the integrity of cap rock by using samples of siltstone cap rock from an injection site in Japan.
CO₂ Solubility in Water and Brine under Reservoir Conditions
Publication
Chem. Eng. Comm.
Date
1990
Publisher
Gordon and Breach Science Publishers S.A.
Volume number
90
Pages
23--33
Description
Paper describing the determination of the reference Henry's constant from the literature, along with a correlation for the A parameter from the Krichevsky-Ilinskaya equation.
A cold geyser appears to be a contradiction in terms. But a combination of carbon dioxide, effervescing groundwater and a fortuitous oil exploration well can create a very spectacular water fountain.
Bruant R G, Giammar D E, Myneni S C B and Peters C A
Title
Effects of Pressure, Temperature and Aqueous Carbon Dioxide Concentration on Mineral Weathering as applied to Geologic Storage of Carbon Dioxide
Publication
Greenhouse Gas Control Technologies: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies
Date
2003
Publisher
Pergamon, Oxford, UK
Volume number
2
Pages
1609--1612
Description
Describes experiments to investigate the effects of pressure, temperature and aqueous solution composition on rates and mechanisms of silicate mineral dissolution and carbonate precipitation
Experimental Geochemical Studies Relevant to Carbon Sequestration
Publication
Online paper presented at the First National Conference on Carbon Sequestration
Date
2001
Publisher
National Energy and Technology Laboratory, US Department of Energy
Description
Describes 3 on-going CO₂ sequestration research studies at the Oak Ridge National Laboratory: 1) isotopic and tracer studies in support of the GEO-SEQ project; 2) isotopic partitioning in carbonate-brine systems under subsurface conditions; and 3) volumetric properties and phase relations of CO₂-CH₄-H₂0 fluids.
Paper SPE-66537, presented at SPE/EPA/DOE Exploration and Production Environmental Conference, San Antonio, Texas
Date
2001-02
Publisher
GEO-SEQ, a National Energy Technology Laboratory (NETL) sponsored project
Description
This paper presents scoping studies of the amounts of CO₂ that can be trapped into the various phases (gas, aqueous, and solid) for a range of conditions that may be encountered in typical disposal aquifers.
Raistrick M, Shevalier M, Mayer B, Durocher K, Perez R, Hutcheon I, Perkins E and Gunter B
Title
Using carbon isotope ratios and chemical data to trace the fate of injected CO₂ in a hydrocarbon reservoir at the IEA Weyburn Greenhouse Gas Monitoring and Storage Project, Saskatchewan, Canada
Publication
8th International Conference on Greenhouse Gas Control Technologies
Date
2006-06-22
Publisher
Elsevier
Description
Proceedings from GHGT6 Trondheim, Norway, 19-22 June 2006
Vomvoris S, Scholtis A, Waber H N, Pearson F J, Voborny O, Schindler M and Vinard P
Title
Lessons learned from the use of hydrochemical data for the evaluation of the groundwater flow models developed within the Swiss L/ILW programme
Publication
Use of hydrogeochemical information in testing groundwater flow models - technical summary and proceedings of a workshop organized by the NEA Coordinating Group on Site Evaluation and Design of Experiments for Radioactive Waste Disposal (SEDE), Sweden
Beaubien S E, Ciotoli G, Coombs P, Dictor M C, Krüger M, Lombardi S, Pearce J M and West J M
Title
The impact of a naturally occurring CO₂ gas vent on the shallow ecosystem and soil chemistry of a Mediterranean pasture (Latera, Italy)
Publication
International Journal of Greenhouse Gas Control
Date
2008-07
Publisher
Elsevier
Volume number
2
Pages
373--387
Description
The link below will take you to the International Journal for Greenhouse Gas Control website, from where you can navigate to Volume 2, Issue 3, which is where this article is located.
Iglesias-Rodriguez M D, Halloran P R, Rickaby R E M, Hall I R, Colmenero-Hidalgo E, Gittins J R, Green D R H, Tyrrell T, Gibbs S J, von Dassow P, Rehm E, Armbrust E V and Boessenkool K P
Mitchell A C, Phillips A J, Hamilton M A, Gerlach R, Hollis W K, Kaszuba J P and Cunningham A B
Title
Resilience of planktonic and biofilm cultures to supercritical CO₂
Publication
The Journal of Supercritical Fluids
Date
2008-12
Publisher
Elsevier
Volume number
47
Pages
318--325
Description
The link below takes you to The Journal of Supercritical Fluids website, from where you can navigate to Volume 47, Issue 2, where this article is located.
Mitchell A C, Phillips A J, Hiebert R, Gerlach R, Spangler L H and Cunningham A B
Title
Biofilm enhanced geologic sequestration of supercritical CO₂
Publication
International Journal of Greenhouse Gas Control
Date
2009-01
Publisher
Elsevier
Volume number
3
Pages
90--99
Description
The link below will take you to the International Journal of Greenhouse Gas Control website, from where you can navigate to Volume 3, Issue 1, where this article is located.
Natural analogue of the rise and dissolution of liquid CO₂ in the ocean
Publication
International Journal of Greenhouse Gas Control
Date
2008-01
Publisher
Elsevier
Volume number
2
Pages
95--104
Description
The link below will take you to International Journal of Greenhouse Gas Control website, from which you can navigate to Volume 2, Issue 1, which is where this article is located.