# The Thermodynamic Change Laws of CO2-Coupled Fractured Rock
## 3 Results
### Characteristics of Fracture Evolution of Rock Samples before and after CO\({}_{2}\) Coupling  
The coupling conditions were established using a CO\({}_{2}\) mass fraction of 8% in the fracturing fluid, a temperature of 25 \({}^{\circ}\)C, a pore pressure of 7 MPa, and a processing time of 10 h. Subsequently, scanning electron microscope (SEM) images of \(\times\)500 and \(\times\)2000 were captured after conducting the MTS procedure on the sandstone. These images were then compared with SEM images of rock samples in their natural state.  
The results show that rock samples in their natural state have less distribution of fractures on the surface, and are primarily dominated by a single fracture. They also exhibit small pore diameters (red circles in the figure), low pore development, poor connectivity, a flatter surface, and mineral filling in both pores and fractures, as shown in Figure 7a,b.  
After CO\({}_{2}\) coupling, the rock samples were divided into uneven sheet-like structures through pores and cracks. There was a notable increase in the number of pores, and they became larger in size (red circles in the figure). The development degree of the pores significantly improved, with locally visible pores emerging and most of the pores being interconnected with each other. Figure 8a,b reveals that original cracks exhibit irregular changes, and the formation of new cracks resulted in interconnected patterns.  
The observed changes in the fracture structure of rock samples before and after the CO\({}_{2}\) coupling effect may be attributed to the thermal and physical properties of CO\({}_{2}\) itself. When the CO\({}_{2}\)-coupled fracturing fluid contacts the rock, it reduces the level of free energy in the system and strengthens the wettability of the rock. The presence of CO\({}_{2}\) fracturing fluid enhances the permeability of the rock mass. Furthermore, the convection heat transfer slightly increases the temperature of the rock mass, thereby reducing its density. Secondly, CO\({}_{2}\) transitions into the supercritical state after surpassing the critical temperature. In this supercritical state, CO\({}_{2}\) exhibits low surface tension and weakened  
Figure 6: The distribution of tensile strength against temperature.  
Figure 7: SEM images of the rock sample in their natural state (the red circle indicates the hole). (a) \(\times\)500; (b) \(\times\)2000.  
intermolecular force. Finally, the combination of CO2 and water results in the formation of carbonic acid, an acidic fluid that causes the erosion of the coal rocks in the reservoir and its cement. This process is accompanied by the deposition of new minerals and the transportation of debris particles, which changes the pore structure of coal rocks. The enhancement in CO2 mobility within the fissure facilitates self-pressurization during the fissure expansion. This phenomenon accelerates rock seepage, promoting fissure expansion and effectively increasing the seepage capacity of low-permeability coal rock. It is worth noting that seepage is an affecting parameter during the phase transition of liquid CO2 to a gaseous state.