Chemical Degradation of Cement Well Seals

Cement well seals are important features in geological storage systems for CO2. QPAC has been used to undertake fully-coupled geochemical modelling of the materials in and around the well, focusing on cement degradation.

An example of this work has been published in Wilson, Benbow, et al. (2011); a brief summary is given here.The models were developed using the QPAC Reactive Transport module. The aim was to investigate whether a simple representation of the relevant geochemical processes was capable of reproducing key measurements from experiments and field observations. The model included a solid-solution C-S-H model and a representation of the neighbouring host rock. The figure below gives an example of the QPAC calculations for the chemical evolution of the well seal, showing the calculated volume fractions of the different mineral phases over a 100 year period. It was found that reasonable agreement with the field data over 30 years was obtained, provided the Pitzer approach was employed for the thermodynamic modelling of solutions. This approach is more appropriate than other commonly-used methods when modelling high concentration saline fluids. This helps provide confidence in the calculations over longer periods, although many uncertainties remain and this continues to be an area of active research.

Volume fraction plots of solid phases for 30 and 100 year QPAC simulations of the SACROC field data