DISSOLUTION OF CO2 IN BRINES AND MINERAL REACTIONS DURING GEOLOGICAL CARBON STORAGE: AN EOR EXPERIMENT

Abstract:

Dissolution of CO2 in formation brines is likely to be a major process which stabilises stored CO2 on longer time scales and mitigates CO2 migrating through storage complexes. However very little is known about the likely rates of CO2 dissolution as CO2 flows through natural heterogeneous brine filled reservoirs. Here we report the results of sampling fluids over 6 months after a phase of CO2 injection commenced for enhanced oil recovery coupled with injection of isotopically enriched 3He and 129Xe. Modelling of the changes in fluid chemistry has previously been interpreted to indicate significant dissolution of silicate minerals where fluids remained close to saturation with calcite. These calculations, which are based on modal decomposition of changes in cation concentrations, are supported by changes in the isotopic compositions of Sr, Li and Mg. Analysis of Sr-isotopic compositions of samples from outcrops of the Frontier Formation, which forms the reservoir sampled by the EOR experiment, reveals substantial heterogeneity. Silicate mineral compositions have 87Sr/86Sr ratios between 0.709 and 0.719 whereas carbonate cements have values around 0.7076. Calculation of CO2 dissolution based on simplified 2-D flow models shows that fluids likely sample reservoir heterogeneities present on a finer scale with CO2 fingers occupying the most permeable horizons and most water flow in the adjacent slightly less permeable zones. Smaller time scale variations in 87Sr/86Sr ratios are interpreted to reflect variations in flow paths on small length scales driven by invading CO2.