Semi‐analytical approach to reactive transport of CO₂ leakage into aquifers at carbon sequestration sites

Reactive transport modeling plays a critical role in predicting and quantifying impacts of potential CO 2 leakage into aquifers at geological carbon sequestration sites. However, a numerical approach generally requires significant computation. This study presents a semi‐analytical approach to reactive transport of CO 2 leakage into an aquifer through decoupling transport equations of reactive aqueous species with algebraic manipulation and then solving the algebraic equation sets representing the mass action law of geochemical reactions with the Newton‐Raphson method. The semi‐analytical approach was implemented in a simulation tool, SASCO 2 M, and verified against a numerical approach for 2‐D synthetic cases. Verification shows that the semi‐analytical approach matched reasonably well the results simulated with the numerical approach. The semi‐analytical approach was applied to simulate pulse‐like CO 2 release tests which were used to demonstrate CO 2 leakage detection in a shallow aquifer. The semi‐analytical approach reproduced the overall trends of groundwater pH, dissolved inorganic carbon, alkalinity, and concentrations of Ca and Br observed in the testing well. The semi‐analytical approach was further applied to assess the efficiency of a groundwater monitoring network for CO 2 leakage detection in a shallow aquifer at a CO 2 ‐enhanced oil recovery site. This study demonstrates that the semi‐analytical approach is simple and efficient and can be followed as a strategic procedure for assessing risks of CO 2 leakage on groundwater quality and efficiency of groundwater monitoring networks for leakage detection at geological CO 2 sequestration sites.