# Simulation of Gas Fracturing in Reservoirs Based on a Coupled Thermo-Hydro-Mechanical-Damage Model
## 4 Analysis of Numerical Simulation Results  
### Variation of Rock Mass Physical Parameters under Pneumatic Fracturing  
The progressive nature of surrounding rock mass damage around the borehole can be discerned from the observations in Figure 3, where the extent of damage steadily increases over time. Moreover, under the influence of a higher temperature field, the degree of rock damage is significantly greater compared to the case where the temperature field is not considered. This is because of the principle of thermal expansion and contraction. When the temperature is higher, the gas inside the rock mass expands, making the rock mass more susceptible to external gas pressure-induced failure. Under higher temperatures, the gas molecules become more active, and after the rock mass is damaged, these active gas molecules further contribute to the internal damage of the rock mass, promoting the development of internal fractures. From Figure 4, it can be observed that as time progresses, the tensile stress near the borehole increases gradually, while the compressive stress decreases. Furthermore, considering the temperature field, the first principal stress is larger. This phenomenon arises from the gas fracturing procedure, where a considerable volume of gas is injected into the vicinity surrounding the borehole. Consequently, tensile stress increases while compressive stress decreases. As the rock mass experiences failure, the injected gas facilitates the propagation of internal fractures within the rock mass. As these fractures extend, the stress exerted on the rock mass transforms into tensile stress, resulting in an expansion of the tensile stress range. Under the influence of a higher temperature field, gas molecules become more active, resulting in greater gas pressure for the same gas volume. Consequently, the first principal stress experienced by the rock mass under the temperature field is larger. Figure 1: Schematic diagram of shale gas reservoir exploitation. Figure 2: Establishment of a numerical simulation model for gas pressure-induced fracturing in rock specimens. It can be observed from Figure 5 that under the influence of the temperature field, the maximum absolute value of the third principal stress is significantly higher compared to the case without considering the temperature field. This is because the third principal stress, being negative or compressive stress, results in strong gas pressure around the borehole in the rock mass. Although the rock experiences tensile stress during the rock failure process, there is still considerable compressive stress exerted by the gas on the rock. Under the influence of the temperature field, gas molecules become more active, and at the same gas volume, higher temperature leads to greater gas pressure. Therefore, under the temperature field, the maximum absolute value of the third principal stress is higher compared to the case without considering temperature. Additionally, as the gas pressure increases, the internal pressure also increases, resulting in a higher minimum absolute value of the third principal stress within the rock under the temperature field compared to the case without considering temperature. From the graph, it can be observed that the compressive stress around the borehole gradually decreases. This is because as time passes, the surrounding rock mass around the borehole undergoes damage, causing the range of maximum compressive stress on the sample to move inward and decrease in magnitude. Figure 4: Graph of the variation of the first principal stress with time during the damage process. Figure 3: Graph of crack damage development in specimens. It can be observed from Figure 6 that with the passage of time, the permeability around the borehole is gradually increasing, and under the influence of the temperature field, the permeability around the borehole is even greater, with a broader range of permeability increase.