## 2 Background and Method
### Experiment on the Pore Structure Characteristics  
The water storage space of sandstone aquifer contains various pores of different origins. The primary pores are gradually compacted and filled during deposition, burial, and diagenesis [37; 38; 39]. Simultaneously, the particles are destroyed by dissolution, resulting in secondary pores and a change in the primary pore structure. The number, size, and connectivity between primary and secondary pores are critical factors affecting strata aquifers.  
The microscopic observations of casting sheets can be used to effectively study the size, distribution, type, connectivity, characteristics, and geometric morphology of rock pores; average pore throat ratio; average pore radius; throat; coordination number; fracture length and width; and fracture rate [40; 41]. These parameters determine the pore structure characteristics of the rock. Therefore, after the collection of the rock samples from the coal mine, cast thin sections were made with a rock pore casting instrument, and microscopic analyses were performed using an Axio Scope A1 Pol optical microscope. The microscopic pore characteristics of rocks were studied using the fractal theory [42].  
Figure 1: Location of the study area in Shaanxi Province, China and the geological structure of the Guojiahe coal mine.  
The macro pore characteristics of sandstone can be investigated using porosity and NWA. Sandstone aquifer is a rock mass formed during geological evolution and is formed through the diagenesis of clastic materials. The water-bearing capacity of sandstone aquifer is closely related to porosity and NWA [43]. The Poropdp-200 porosity measuring instrument was used to measure the porosity of the samples (Figure 3) [44; 45]. The equipment constitutes a data control and analysis module, pulse attenuation control module, pressure pump, porosity measurement module, and high-precision pressure sensors with a pressure range of 0-200 psi, and the accuracy of \(\pm\)0.1% of the full range was used in the study. Helium was used as the test medium. According to [PERSON]'s law, the sample's pore volume was calculated using the helium pressure difference before and after the test to calculate its porosity.  
Figure 3: Poropdp-200 porosity measuring instrument.  
Figure 2: Stratigraphic column of Guojiahe coalmine.  
The NWA of rock is a critical physical and mechanical property and refers to the ratio between the maximum weight of water absorbed by a rock sample under atmospheric pressure and the specimen's drying weight (Equation (1)). The NWA size mainly depends on the number, size and opening degree of pores and fissures in the rock, which can effectively reflect the water-holding capacity of rock and the connectivity between rock pores.  
\[W_{m}=\frac{m_{0}-m_{s}}{m_{s}}*100\% \tag{1}\]  
\(W_{m}-Nature\)\(water\)\(absorption(\%)\); \(m_{0}-Mass\)\(of\)\(rock\)\(saturated\)\(with\)\(water(g)\);  
\(m_{s}-Quality\)\(of\)\(rock\)\(after\)\(drying(\%)\)  
Before measurement, the rock samples to be tested were placed in a vacuum oven and dried at 110 \({}^{\circ}\)C for 24 h. Next, the samples were weighed to the accuracy of 0.1 g after cooling to room temperature. The weighed test pieces were then placed in the water tank. Subsequently, water was injected to 1/4 of the test pieces' height and filled to 1/2 and 3/4 places every 2 h. After 8 h, all the core samples were immersed. Next, the sample quality was measured every 8 h until no change was observed in the sample quality. Finally, Equation (1) was used to calculate the NWA of the sample.