Reactive transport modeling of CO2 and CH4 plumes during a gas-rich water leakage in a shallow carbonate freshwater aquifer

Abstract

The recent advances in geological carbon capture and storage and the worldwide proliferation of such projects to reach net-zero emissions by 2050 highlight the importance of investigating the risks associated with CO₂ leakage from carbon storage reservoirs into shallow aquifers. A recent experiment indicates that residual CH₄ from legacy hydrocarbon fields could be a potential early indicator of CO₂ leakage. In this study, numerical reactive transport simulations are performed to examine the reasons behind the different behavior of CH₄ in regard to CO₂ as well as its potential and limitations as a monitoring parameter. The base case model was calibrated using a large data set, including a tracer test, major element concentrations, dissolved CO₂ and CH₄ concentrations, and pH monitoring.

The results show that the delay of CO₂ with respect to CH₄ is likely related to sorption, which plays a significant role in CO₂ retardation but has a minimal effect on CH₄ transport. The presence of clay minerals (montmorillonite and illite) did not significantly change the delay between the two gases. Furthermore, CO₂ retardation was found to increase in a scenario with a lower natural groundwater CO₂ concentration. The CH₄ oxidation simulation revealed that oxidation decreased the CH₄ concentration below the CH₄ concentrations from baseline water reported in the literature, suggesting that CH₄ oxidation is a critical process able to reduce the efficiency of CH₄ monitoring and should be considered if CH₄ monitoring is implemented.