The Thermodynamic Change Laws of CO2-Coupled Fractured Rock

Abstract

Under the background of the “dual carbon” target, exploring the pathway of efficient geological storage and high energy utilization of CO2 is a hot issue in CO2 emission reduction research. Under the coupling effect of high geopathic stress in deep rock layers and thermal stress generated during the geological sequestration of CO2, CO2 infiltration into coal rock changes the ambient temperature around the rock, while thermal diffusion effects cause damage to the rock and influence fracture expansion. In the present study, CO2-water–rock coupling test system and characterization of fissure surface roughness were conducted to analyze the rock’s mechanical properties, damage, and fracture evolution. Modeling of the equivalent fissure was employed to reveal the heat transfer mechanism between the rock matrix and CO2. The results obtained illustrate that the rock samples coupled with CO2 exhibited remarkable changes in mechanical properties. These changes include an increase in the number of pores, enhanced inter-pore connectivity, and a planar type of surface roughness in the fissures, ultimately resulting in an increase in conductivity. Conversely, the remaining rock samples displayed poor mechanical properties and surface fracture connectivity. As pressure decreased, the heat transfer coefficient decreased from 86.9 W/m2·K to 57.5 W/m2·K, accompanied by a temperature drop from 33.6 °C to 30.6 °C, demonstrating a proportional relationship between pressure and the heat transfer coefficient. Furthermore, the flow rate gradually increased with the rise in CO2 pressure, indicating denser flow lines with faster flow rates. At 15 MPa, CO2 exhibits enhanced mobility.