## 3 Results and Discussion
### Mechanical Property Analysis
#### 3.2.3 Analysis of Sandstone Strain Variation  
As shown in Figure 10, according to the definition, rock strain contraction is positive, and strain expansion is negative. It can be understood that the axial strain of the uniaxial compressed rock sample is always positive, while the lateral strain is always negative. In the failure stage, the lateral strain of the rock continuously increases. At this time, the volumetric strain of the rock sample changes from positive to negative, indicating a transition of the rock sample from compression to expansion. The trends of the stress-strain curves of the sandstone under different treatment conditions are generally consistent, but there are certain differences between different pre-treatment conditions. Compared to the original sandstone, the pure acidization treatment and supercritical CO\({}_{2}\) treatment does not change the peak axial strain of the sandstone, while the axial deformation of the samples treated with both acidization and supercritical CO\({}_{2}\) decreases by 38.57%. The combined action of acidization and supercritical CO\({}_{2}\) leads to changes in the internal pore structure and physical properties of the samples, thereby altering their compressive performance and deformation characteristics. During the compression failure process of the sandstone samples under different treatment conditions, although the peaks are different, they all show a pattern of first increasing and then decreasing. The volumetric strain (\(\varepsilon v\)) of the samples first contracts and becomes smaller and then gradually expands; at the same time, \(\varepsilon v\) maintains positive values during the initial compaction stage, elastic deformation stage, stable crack expansion stage, and unstable crack expansion stage. As the volumetric strain reaches its peak, the peak point is the boundary where the sample transitions from compressive deformation to expansion and dilation. In the post-peak softening stage, as the expansion continues, \(\varepsilon v\) will show negative values, resulting in volumetric expansion. The evolution of crack volume strain during uniaxial compression of rock is related to axial strain, radial strain, the Poisson ratio during the elastic stage, and the elastic modulus. By substituting the experimental parameters into Equation (6), the evolution pattern of crack volume strain under different pretreatment conditions can be obtained, as shown in Figure 11.  
Figure 11: Characteristics of crack volume evolution in sandstone under different treatment conditions. Figure 10: Stress–strain curves of sandstone under different treatment conditions. From Figures 3-10, it is evident that the combined action of acidization and supercritical CO\({}_{2}\) accelerates the rate of increase in crack volumetric strain in the sandstone, manifesting as a rapid expansion of internal crack volume in the sandstone. However, at the initial stage of loading, the crack volumetric strain of each group of rock samples changes from negative to nearly 0, indicating that the initial cracks inside the sandstone are compressed and closed during this phase. Additionally, it is found that, with the exception of the original sandstone samples, as the soaking pressure of supercritical CO\({}_{2}\) increases, the growth rates of crack volumetric strain in the sandstone increases to 3.26, 4.24, and 5.29, respectively, and the range of strain growth also gradually expands. This indicates that the action of acidization and supercritical CO\({}_{2}\) enhances the ductility of the rock samples, causing the overall deterioration of the samples, which leads to frequent interconnection of internal cracks and continuous aggravation of structural damage. From the T\({}_{2}\) spectrum, it is known that the internal pores in the original sandstone are unevenly developed, with significant anisotropy.