## 3 Results and Discussion  
### NMR Data Analysis  
NMR technology is widely used in the characterization of rock pore structure by examining the variation in relaxation time of hydrogen fluid in different pores [26]. According to the pore division rules of the International Union of Pure and Applied Chemistry (IUPAC), pores with a relaxation time of between 0 and 10 ms are defined as micropores, those of between 10 and 100 ms are defined as mesopores, and those greater than 100 ms are defined as macropores. As shown in Figure 4, the T\({}_{2}\) spectra of the sandstone samples after different treatments are shown. By comparison, it can be seen that the pore size distribution curve of the untreated sandstone sample is not noticeably bimodal. The main peak is approximately at T\({}_{2}\) 2.1 ms, which belongs to small pores and corresponds to a pore component of about 0.37%. The secondary peak is approximately at T\({}_{2}\) 74 ms, which belongs to mesopores and corresponds to a pore component of about 0.145%. The T\({}_{2}\) spectra before treatment of the sandstone samples under different treatment conditions show no significant difference, mainly small and medium pores with a relatively continuous distribution, indicating that the sandstone is relatively tight. As shown in Figure 3, after treatment with 14 MPa CO\({}_{2}\), the peak position of the main peak of the sandstone sample begins to move to the right, and the porosity component rapidly decreases to 0.28, while the porosity component of the secondary peak is 0.20 and the secondary peak position is about 67 ms. This is because at a lower supercritical CO\({}_{2}\) soaking pressure, the ability of sandstone to combine water and CO\({}_{2}\) to form an acidic solution is low, leading to a low degree of acidification reaction, and due to adsorption expansion, the number of small pores in the sandstone decreases and large pores evolve into mesopores. As the soaking pressure gradually increases, the ability of sandstone to combine water and CO\({}_{2}\) to form an acidic solution gradually increases, and the degree of acidification reaction also intensifies, causing the sandstone mineral matrix to be partially dissolved and corroded and the ability of supercritical CO\({}_{2}\) to extract organic matter in the sandstone also to gradually increase. Although adsorption expansion still exists, at this time, the coupling of supercritical CO\({}_{2}\) extraction, acidification, and dissolution plays a dominant role, leading to an increase in the number of small and large pores in the sandstone. After acid treatment, the T\({}_{2}\) of the sandstone evolves into a single peak shape. Compared to the original sandstone and supercritical CO\({}_{2}\) treatment, the main peak position rapidly moves in the direction of increase, and the main peak porosity component also increases significantly. After treatment with 1% HF and 12% HCl concentration soil acid (sh-1), the  
Figure 4: T\({}_{2}\) spectra of sandstone under different treatment conditions. main peak position of the sandstone T\({}_{2}\) is 5.9 ms and the porosity component is 0.5%, and the soil acid solution has a stronger effect on the dissolution of filling minerals in sandstone pores. Under the combined action of acidification and supercritical CO\({}_{2}\), the T\({}_{2}\) trend of the sandstone is similar to that of single acidification treatment. As shown in Figure 3, after the combined action of acidification and supercritical CO\({}_{2}\), the bimodal shape of the sandstone T\({}_{2}\) is not obvious, the main peak position rapidly moves in the direction of increase, and the main peak porosity component also increases significantly. After acidification-16 MPa CO\({}_{2}\) treatment, the position of the main peak of the sandstone sample and the porosity component increase to the maximum, which are 12.9 ms and 0.58%, respectively, while the porosity component of the secondary peak is 0.01 and the secondary peak position is about 890 ms.