Cooling Effects on Capillary Sealing in CO ₂ –Plume Geothermal Systems: Capillary Pressure Modeling and Leakage Assessment

The CO₂-Plume Geothermal (CPG) technology is an emerging approach that utilizes CO₂ as a working fluid to simultaneously extract geothermal energy and achieve long-term underground carbon sequestration. The circulation of cold CO₂ significantly lowers reservoir temperature and alters rock wettability, which threatens the long-term security of CO₂ sequestration by weakening the capillary sealing capacity of caprock. To address this concern, we investigate the mechanisms by which cooling-induced wettability alteration affects the capillary sealing capacity of CPG caprock and quantitatively evaluate the associated leakage risks. First, we propose a temperature-saturation dependent capillary pressure (TSPC) model to quantify the impact of temperature variations on caprock capillary sealing capacity. Subsequently, the TSPC model is integrated into our in-house numerical reservoir simulator to quantitatively assess CO₂ leakage for a field-scale CPG operation. Simulation results show that temperature reduction during CPG operation decreases capillary pressure by 45%-87%, which substantially undermines the ability of the caprock to prevent CO₂ upward breakthrough. The decline in capillary pressure weakens the sealing capacity and leads to nearly 50,000 tons of CO₂ leakage in a representative CPG reservoir simulation. The above results demonstrate a previously unrecognized CO₂ leakage mechanism in CPG systems caused by cooling-induced wettability alteration. In addition, we propose practical mitigation strategies to address this leakage risk and enhance the long-term security of CO₂ sequestration after CPG operations.