## 3 Results and Discussion
### Mechanism of Sandstone Changes after Acidification-Sc CO\({}_{2}\) Treatment  
Rock is generally composed of matrix, detritus, and pores, exhibiting strong heterogeneity and anisotropy. The matrix grains, minerals, and cementing materials primarily determine the rock's strength, brittleness, and deformation characteristics, while pores and microcracks are the primary storage locations for water and hydrocarbon resources. The XRD diffraction pattern of the sandstone before and after acidification-supercritical CO2 composite action is shown in Figure 12. According to the X-ray diffraction pattern, the mineral composition of tight sandstone is mainly quartz, feldspar, and chlorite. The diffraction peak intensity of quartz, chlorite, and feldspar in the sandstone minerals decreases to varying degrees after acidification-supercritical combined action. Table 2 shows the XRD (X-ray diffraction) test results, which are a comparison of the initial sample (y-1), the sample treated with acid for 24 h (sh-1), the sample treated with supercritical CO\({}_{2}\) (co-1), and the sample treated with acid-supercritical CO\({}_{2}\) (sc-1). Figure 12: XRD diffraction patterns of y-1 **(a)** and SC-4 **(b)**. Figure 13 and 14 present enlarged images of the surface morphology of the sandstone samples under different treatment conditions. Observing the local crack on the surface of the untreated sandstone sample under 600\(\times\) magnification (a), it can be seen that the sample surface is generally intact and dense, with barely visible microcracks between the matrix. Quartz, green clay minerals, and other minerals are scattered on the sample surface, and some mineral particles have rigid grain edge pores, with a clear boundary between mineral particles and a tight cementing structure. After treatment at 16 MPa CO\({}_{2}\), the mineral bonding water is extracted and captured by supercritical CO\({}_{2}\), generating HCO\({}_{3}\), which undergoes secondary ionization to produce H\({}^{+}\) and carbonate salts, resulting in full dissolution. The sample surface morphology is partially destroyed and can be seen by the naked eye, with a significant increase in the number of pores and microcracks, even forming collapse pores. Mineral particle edges show clear boundaries, indicating that the tight cementing structure between mineral particles has been partially destroyed, and under fluid action, it is possible to \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Pretreatment Conditions**} & \multicolumn{4}{c}{**Mineral Composition**} \\ \cline{2-5}  & **Quartz (\%)** & **Feldspar (\%)** & **Clay Minerals (\%)** & **Magnesite (\%)** \\ \hline Original sandstone & 42.1 & 2.4 & 51.6 & 3.9 \\ Sc CO\({}_{2}\) & 45.7 & 2.3 & 48.1 & 3.9 \\ Acid treatment & 54.8 & 1.3 & 41.2 & 2.7 \\ Acidification–Sc CO\({}_{2}\) & 62.5 & 0.7 & 34.2 & 2.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Statistical results of XRD testing. Figure 13: Images magnified 600\(\times\) of local microcracks in sandstone under different treatment conditions. observe mineral particles being peeled off. After acid treatment, the sandstone sample surface becomes rough and changes from gray to light yellow (Figure 15). Under SEM observation, it is found that surface minerals are dissolved, generating numerous irregular intergranular corrosion pores, and the morphology is significantly destroyed. The number of pores and microcracks increases significantly, and acid treatment further enlarges the initial microcrack width, providing a favorable space for subsequent supercritical CO\({}_{2}\) action on the sandstone. Compared with untreated sandstone, single acidization, or supercritical CO\({}_{2}\) treatment, acidization-supercritical CO\({}_{2}\) combined processing leads to more significant degradation of the sandstone. The pre-treatment of acid significantly increases the porosity of the sandstone, thereby greatly increasing the reaction volume between supercritical CO\({}_{2}\) and the sandstone.