## 2 Geological Background  
The selected site is located in the Qinshui Basin, situated in the central and southern parts of Shanxi Province, and its tectonic structure belongs to the Neo-Caxia tectonic system. A monoclinic structure dominates the entire basin, resulting in an overall structural orientation of NNE (Figure 1A) [28; 29; 30]. Within the southern Qinshui Basin, the Indosinian tectonic movement produced a SN-trending horizontal compressive stress field. However, this movement had minimal impact, leaving behind scant tectonic evidence [29; 31]. In the Yanshanian period, near-horizontal compression of the NW-SE-trending stress field led to the formation of NNE-NE-trending secondary fold structures. These structures emerged as the primary gas-controlling configurations. Consequently, the gas-rich belt located in the southeast of the Qinshui Basin aligns in the NNE direction, following the axis of the secondary syncline [28; 32]. Marked by a NW-SE-trending near-horizontal extensional stress field, the Himalayan tectonic movement led to the formation of small-scale, near-SN-trending secondary folds. These folds often represent the local centers of CBM gas richness [30; 32].  
The Qinshui Basin's primary coal-bearing strata comprise the Taiyuan and Shanxi Formations. These formations host over 10 mineable coal seams. The maximum thickness of a single seam reaches 6.5 m, with the cumulative thickness of all seams ranging from 1.2 to 23.6 m [28]. Specifically, the Taiyuan Formation encompasses 5 to 10 coal layers, with the 15 th layer being the primary seam. In contrast, the Shanxi Formation includes three coal layers, with the No. 3 layer as its main seam (see Figure 1B). Both formations are rich in coalbed methane resources [28; 31]. This paper focuses on the Panzhuang block, a key commercial area within the Qinshui Basin [27].  
Hydrogeological drilling at the Sihe and Yonghong Mines in the Panzhuang block reveals that the No. 3 coal seam's maximum unit water inflow is 0.0122 L/(s-m), originating from the aquifer in the roof's sandstone fissures. The area's stable lithology and straightforward structure result in few faults and simplify the hydrogeological conditions [28; 31]. The top and bottom of the 3# coal seam in the Panzhuang block consist of dense and low-permeability mudstone of significant thickness. This mudstone effectively blocks the hydraulic connectivity between the Taiyuan formation, the Shanxi formation, and the groundwater. The composition of water in the Carboniferous aquifer is of a stable NACHO\({}_{3}\)-Cl type, as shown in Figure 2[31; 33]. This closed hydrological system, characterized by limited interaction with external water sources, provides favorable conditions for studying the phase change of solid-liquid fluid composition. The No. 3 coal seam of the Shanxi Formation is microscopically composed of 45-70% vitrinite, 20-36% inertinite, and 10-19% liptinite [28; 31]. The No. 3 coal seam in the Shanxi Formation comprises 45-70% vitrinite, 20-36% inertinite, and 10-19% chitin. This coal exhibits a high degree of thermal evolution, with vitrinite reflectance (R\({}_{\text{o, max}}\)) ranging from 2.79% to 3.98%. It has a gas content of 17.1 to 25.29 m\({}^{3}\)/t and a gas saturation exceeding 90%. The coalbed methane (CBM) content in the No. 3 coal seam varies from 0 to 35.57 m\({}^{3}\)/t, typically ranging between 7 and 26 m\({}^{3}\)/t. The seam thickness is generally between 5.0 and 7.0 m, and it is buried at depths of 300 to 800 m, making it a primary source of CBM production in the Panzhuang block [30,31].