A Two-Phase Layered Model for Simulating Leakage Along Faults Considering the Blocking Effect of CO2 Exsolution

Abstract

CO₂ geological storage is associated with the risk of faults activation and CO₂ leakage along the activated faults. During the leakage, pressure decline along the fault causes CO₂ to exsolve from formation water, forming a lower aqueous phase with dissolved CO₂ and an upper two-phase zone with CO₂ bubbles dispersed in water. The CO₂ bubbles in this zone significantly reduce fluid mobility, lowering leakage rates. Existing leakage models neglecting such CO₂ exsolution effect overestimate leakage rates and risks. To address this limitation, this study improves the calculation method for CO₂ gas-phase exsolution location by establishing a pressure distribution equation within faults and deriving an analytical solution for the exsolution location using monitorable fault inlet pressure; a two-phase layered flow model is then developed by incorporating two-phase relative permeability which can reflect the flow resistance effects after CO₂ exsolution. The application of the developed model to a CO₂ leakage case in an oil field located at Ordos Basin, China, demonstrates that the model can reflect the effect of gas-phase CO₂ exsolution along faults. Quantitative analysis shows a 67%–75% reduction in CO₂ leakage rate simulated by the layered model compared to non-layered models, indicating that the developed model can address the influence of CO₂ exsolution on blocking leakage fluid flow and thus calculate the leakage amount more accurately. 2025 Society of Chemical Industry and John Wiley & Sons, Ltd.