# 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 \({}^{\circ}\)C to 30.6 \({}^{\circ}\)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. CO2 geological storage; crack extension; heat transfer; CO2 fracturing; thermal damage  
the triaxial compressive and tensile strength of rocks decreases gradually with the increased adsorption time of supercritical CO2. The elasticity modulus and Poisson's ratio of rocks increased gradually with increased CO2 pressure and temperature [11]. [PERSON] et al. [12] believed that supercritical CO2 has different penetration enhancement effects on coal under different pressures. CO2 pressure affects the fluidity and modification of the fluid in the coal matrix. Researchers have found that supercritical CO2 significantly reduces the mechanical properties of coal rocks. Microcracks and fissures develop in rock mass under the interaction of CO2 and rocks [13; 14]. [PERSON] et al. [15] studied the distribution characteristics of cracks in coal after CO2 fracturing, revealing the extension mechanism of sandstone cracks during the fracturing process. Since water is one of the main factors affecting the mechanical properties of materials [16], the decreased fracture width or increased surface roughness makes the fluid more prone to decreased overpressure [17]. It weakens the discharge capacity of fractures. The mechanical mechanism of hydraulic fracturing for coal structure destruction is investigated, revealing that hydraulic fracturing can enhance the effectiveness of top coal pre-cracking [18]. [PERSON] et al. [19; 20; 21] investigated the mechanism of fracture morphology evolution in fractured coal under water and supercritical CO2 fracturing conditions. CO2 significantly affects coal rock wettability and fracture expansion. The above studies mainly focus on rock tensile strength. CO2 plays an important role in weakening rock mechanical properties and microcrack extension. However, the process of injecting CO2 into the deep earth involves solid mechanics and physical fields as well as temperature and flow field changes triggered by the phase change process of CO2. Additionally, the analysis of coal-rock structural modification should consider multi-field coupling characteristics such as heat-hydraulic-solid [22; 23]. CO2 exchanges heat with the rock matrix during fissure seepage, which changes the temperature field distribution of the fissured rock and affects the expansion of the fissure.