Cooling Effects on Capillary Sealing in CO2-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.