Geochemical Modeling for CO2-Rock Interaction for Caprock Seal Integrity

Abstract Carbon capture and storage (CCS) is vital for mitigating greenhouse gas emissions, with caprock integrity essential to prevent CO2 leakage. This study investigates CO2–caprock interactions under conditions representative of the Kasawari Carbon Capture and Storage (CCS) project offshore Malaysia. A baseline equilibrium simulation was conducted using PHREEQC to evaluate mineral stability during CO2 injection. The simulation employed a brine composition reflective of offshore Sarawak formations, including major cations (Na+, Ca2+, Mg2+, K+) and anions (Cl-, SO42-, HCO3-), with total dissolved solids ranging from 100,000 to 200,000 mg/L. The model ran at 85°C and a CO2 pressure of 100 bars. Results show that CO2 dissolution lowered brine pH from 7.0 to 5.26, promoting mineral reactions. Calcite, initially supersaturated (SI = 1.35), dissolved completely, while dolomite shifted from highly supersaturated (SI = 2.95) to near equilibrium (SI = 0.00), suggesting potential self-sealing. Illite remained undersaturated (SI = −3.30), indicating possible clay dissolution. Quartz and kaolinite remained stable. These findings highlight the significance of carbonate dissolution and precipitation in controlling caprock integrity. Carbonate dissolution may initially increase porosity, but secondary precipitation could reduce permeability, enhancing sealing capacity. This baseline study provides insights into caprock behavior during CO2 injection and establishes a foundation for evaluating long-term integrity. Time-dependent kinetic simulations were conducted to assess long-term mineral evolution. Results show that pH gradually decreased from 7.0 to 5.54 over 10,000 years, with progressive calcite dissolution and dolomite precipitation enhancing potential self-sealing.