## 3 Results and Discussion
### Mechanical Property Analysis
#### 3.2.2 Determination and Analysis of Crack Initiation Stress and Damage Stress
Under the combined action of acidization and supercritical CO\({}_{2}\), the reduction in sandstone initial stress is more significant compared to solely acidization or supercritical CO\({}_{2}\) treatment, with a similar pattern to that of supercritical CO\({}_{2}\) soaking, averaging a 20% decrease and reaching maximum and  
Figure 7: Determination of crack initiation stress in the damage stage of brittle rock. minimum values of 12.5 MPa and 11.2 MPa at 8 MPa and 16 MPa, respectively. The reason for this phenomenon is that the combined effect of mud acid and supercritical CO\({}_{2}\) dissolves and extracts some sandstone minerals. According to the research of [PERSON] [28; 29], the reservoir in this area is dominated by calcareous and siliceous cementation, and the reservoir pores are dominated by primary intergranular pores between clastic particles and dissolution pores produced by dissolution, which exacerbates the damage to the sandstone and reduces the initial stress of the sandstone. Figure 8: Stress curve of sandstone cracking under different pretreatment conditions. Figure 9: Damage stress curve of sandstone under different pretreatment conditions. As shown in Figure 8, the trend of damage stress variation for specimens with different crack inclinations is similar to that of peak strength, both showing a monotonic decrease. Under supercritical CO\({}_{2}\) soaking conditions, as the soaking pressure increases, the damage stress of the sandstone gradually decreases, with a maximum value of 32.7 MPa at a soaking pressure of 8 MPa, which is close to the original sandstone damage stress of 35.3 MPa. When the soaking pressure reaches 16 MPa, the damage stress decreases to a minimum value of 23.3 MPa. Under the conditions of sandstone acidization treatment, the damage stresses of the sandstone soaked in low- and high-concentration hydrochloric acid are 27.3 MPa and 25 MPa, respectively, both of which are lower than the original sandstone damage stress. It is evident that acidization treatment also significantly impacts the damage stress of the sandstone, and within a certain concentration range, the higher the concentration of hydrochloric acid, the more significant the deterioration of the sandstone. Under the combined effect of acidization and supercritical CO\({}_{2}\), the reduction in sandstone damage stress is more significant compared to purely acidization or supercritical CO\({}_{2}\) treatment. The pattern of damage stress variation is similar to that of supercritical CO\({}_{2}\) soaking, with the maximum and minimum values being 12.5 MPa and 11.2 MPa at 8 MPa and 16 MPa, respectively, and a reduction range from 38.8% to 49.58%. The reason is that the combined action of hydrochloric acid and supercritical CO\({}_{2}\) dissolves part of the sandstone matrix, leaving large pores and making the pore network more complex. At the same time, the combined action of acidization and supercritical CO\({}_{2}\) leads to the expansion of existing cracks and the formation of new cracks. The chemical interaction between supercritical carbon dioxide and acidic substances can damage or destroy the mineral lattice structure in the sandstone, causing changes in its internal structure. Some minerals may be dissolved or partially dissolved, leading to damage to their lattice structures and thereby exacerbating the damage to the sandstone and affecting its overall stability and strength. This results in a reduction in damage stress to the sandstone.