## 3 Results and Discussion
### NMR Data Analysis
The amplitudes of the main peak and the secondary peak are both greater than those of single acidification or supercritical CO\({}_{2}\) treatment. This may be due to the synergistic effect of acidification and supercritical CO\({}_{2}\), which completely corrodes the filling minerals in the sandstone pores, and the number and radius of larger pore throats increase significantly. Overall, the changes in the T\({}_{2}\) spectrum curves of the sandstone samples under different treatment conditions reflect the variation patterns of the internal pore structure. The increase in the area under the T\({}_{2}\) spectrum curve is most significant for the samples subjected to the combined effect of acidizing and supercritical CO\({}_{2}\) treatment. This indicates that the acidizing-supercritical CO\({}_{2}\) composite treatment dissolves a portion of the sandstone minerals, promoting the development of internal pores within the sandstone. Considering the heterogeneous distribution of internal pores in the sandstone, during the treatment process, the mud acid always preferentially enters the larger connected pores, dissolving the rock minerals on the pore walls and facilitating the continuous development of the pore structure. This process is invariably accompanied by the gradual disappearance and re-establishment of pore throat channels between pores of different sizes. These changes are primarily manifested in the different morphological characteristics of the T\({}_{2}\) spectrum curves of the samples and the transformation of the T\({}_{2}\) spectrum from a bimodal shape to a unimodal shape with only one prominent peak. Porosity Change Analysis  
The statistical results of the porosity of the sandstone samples under different treatment conditions are presented in Table 1 and Figure 5.  
Observations reveal that the permeability of the sandstone increases to varying degrees under the three treatment conditions compared to the original sandstone core. Specifically, the porosity of the sandstone increases from 8.64 to 9.08 after supercritical CO\({}_{2}\) treatment, with no significant change. Analysis suggests that the interactions between supercritical CO\({}_{2}\) and sandstone include gas pressure, adsorption expansion, acidification reaction, and organic matter extraction. After 16 MPa CO\({}_{2}\) treatment, although the gas pressure enhances the compression effect on the matrix, adsorption causes matrix expansion, the acidification reaction has a more pronounced effect on pore dissolution, and the unique ability of supercritical CO\({}_{2}\) to extract organic matter affects the organic matter in the sandstone sample, resulting in a combined influence of the four factors that increases the porosity by 0.44. \begin{table}
\begin{tabular}{c c} \hline
**Pretreatment Conditions** & **Porosity/\%** \\ \hline Original sandstone & 8.64 \\ Sc CO\({}_{2}\) & 9.08 \\ Acidification & 11.28 \\ Acidification-Sc CO\({}_{2}\) & 12.60 \\ \hline \end{tabular} \(\;\)SC CO\({}_{2}\) here means supercritical CO\({}_{2}\) pretreatment.  
\end{table} 
Table 1: Sandstone porosity under different treatment conditions. In contrast, the porosity of the sandstone after acidification treatment is 11.28, indicating that the acid has a more significant impact on the sandstone pore structure. Compared to single supercritical CO\({}_{2}\) treatment or acidification treatment, the porosity of the sandstone after acidification-supercritical CO\({}_{2}\) combined treatment reaches 12.6, which is higher than the sum of the porosity after supercritical CO\({}_{2}\) treatment and acidification treatment, which confirms that the acidification-supercritical CO\({}_{2}\) combined treatment is not simply additive but also exhibits synergistic effects. The reason is that the acidification treatment significantly increases the pore space of the sandstone, and the reactants and products of acid treatment provide an acidic environment for supercritical CO\({}_{2}\) to act on sandstone, promoting and catalyzing its effects.