## 2 Materials and Equipment  
### Sample Preparation  
In this study, sandstone cores from the Xuijiehe Formation reservoir in Rongchang District, Chongqing, were used as experimental samples. The sampling depth was approximately 3100 m. The sampling location and reservoir geological section map are shown in Figures 1 and 2. Rongchang District, primarily located in the eastern Sichuan Basin and belonging to the eastern Sichuan high-steep fold belt, features reservoir sandstone lithology mainly composed of feldspar quartz sandstone and lithic quartz sandstone, followed by feldspathic lithic sandstone. Within the detrital grains, quartz content ranges from 65% to 90%, primarily consisting of monocrystalline quartz and metamorphic quartzite lithic fragments. Feldspar content varies between 2% and 21%, with plagioclase being the dominant type. Lithic fragments account for 4% to 24% of the composition, averaging 11.52%, and are mainly composed of sedimentary lithic fragments, followed by metamorphic and volcanic lithic fragments in decreasing abundance. Pore-filling materials exhibit a relatively high matrix content, averaging 3.67%. Cement content is comparatively low, with varying degrees of development observed in carbonate minerals (such as calcite), siliceous materials, and clay minerals. During the early stages of diagenesis, intergranular pores are filled with carbonate cements. Detrital grains commonly range from 0.25 to 0.5 mm in size, displaying moderate sorting, a grain-supported texture, and linear-concave-convex contacts [25].  
In accordance with the standards recommended by the International Society for Rock Mechanics and Rock Engineering, the samples were processed with a height-to-diameter ratio of 2:1, with a diameter of 25 mm and a height of 50 mm. To ensure the accuracy of the tests, all samples were drilled from the same sandstone block; anomalous samples were eliminated using longitudinal wave velocity. The parallelism of the sample end surfaces was controlled to within 0.05 mm. Ultimately, 20 samples were selected and divided into 10 groups with 2 samples each.  
The samples were first dried in a vacuum oven for 48 h at a temperature of 60 \({}^{\circ}\)C. After cooling to room temperature, they were subjected to different treatments: acid  
Figure 1: Sandstone core sampling locations. (**a**) Schematic map showing the location of the Rongchang basin in Chongqing. (**b**) Sampling location diagram.  
Figure 2: Reservoir geological section map. The yellow formations are gas-bearing sandstones.  
treatment (labeled sh-1, sh-2) with a mixture of 12% hydrochloric acid and 1% HF or 3% HF, supercritical CO2 treatment (labeled co-1, co-2, co-3, co-4), acid-supercritical CO2 composite treatment (labeled sc-1, sc-2, sc-3, sc-4), and untreated controls (y-1). The CO2 soaking pressures were 10, 12, 14, and 16 MPa, respectively, with a temperature of 80 \({}^{\circ}\)C. According to the existing fracturing construction experience in the region, the acid used for treatment was a mixture of 12% hydrochloric acid and 2% HF with a concentration ratio of 6:1. Two samples were tested for each level of supercritical CO2 soaking pressure, with control groups consisting of untreated, solely acid-treated, and supercritical CO2-treated samples.