## 2 Experimental Study  
### Sample Preparation and Experimental Program  
The coal samples used in the experiments were collected from the Qin Shui Coalfield, located in Shanxi Province, China. The coal seam has an average thickness of 6 m and is relatively stable. The geological conditions are relatively simple. The gas content of the coal seam is 7.3 m\({}^{3}\)/t at a minimum, 11.6 m\({}^{3}\)/t at a maximum, and more than 8 m\({}^{3}\)/t in most areas. In order to better represent the characteristics of the original coal and come closer to the actual conditions, raw coal samples were used in the experiments [23; 24]. Samples conforming to the ISRM standard were made from chunks of the original coal. The specifications of the samples were \(\varphi 50\times 100\) mm, and the flatness of the end face was controlled to 0.05 mm. The experiments were performed using an RLW-500G triaxial experimental system, which was mainly composed of a loading module, seepage module, temperature control module, and safety protection module. The triaxial experimental system has previously been described in detail [25] and will not be described in this work. Experiments were conducted under different loading and unloading paths, and the permeabilities of the coal samples were measured synchronously. The experimental system is shown in Figure 1.  
Based on the actual stress state of coal and rock during accelerated mining at the working face, two sets of comparative paths were put forward with some appropriate simplifications. The two sets of paths were conventional triaxial compression (CTC) and accelerated triaxial compression (ATC), along with the unloading confining pressure (UCP) and accelerated unloading confining pressure (AUCP). In the experiments, CO\({}_{2}\) was used instead of CH\({}_{d}\) because of the laboratory's safety  
Figure 1: Triaxial experimental system. regulations [26; 27; 28; 29]. Five experimental groups were considered under each stress path. The initial conditions under each path were as follows: when the hydrostatic stress was 4 MPa, the gas pressure was 1 MPa, 1.5 MPa, or 2 MPa, and when the hydrostatic stress was 7 MPa or 10 MPa, the gas pressure was 2 MPa. The specific experimental schemes are described below. Path 1 (CTC):  
I: First, the axial stress and radial stress were simultaneously loaded to predetermined values. After reaching the hydrostatic stress state, the gas was injected, and the gas pressure was fixed. When the adsorption reached equilibrium, the outlet valve was opened until the gas flow was stable. II: Second, the radial stress was kept constant, and the axial stress was loaded at the rate of 50 N/s in a force-controlled mode. Meanwhile, the seepage data were recorded and stored. III: Third, after the sample was damaged, the loading rate was controlled in a displacement-controlled mode until the residual strength of the sample was stable. Path 2 (ATC):  
I: The first phase was the same as Path 1. II: Second, the radial stress was kept constant, and the axial stress was loaded to 40% of the axial peak strength of the sample under the same condition as the CTC path at a rate of 50 N/s. The seepage data were recorded and stored at the same time. Then, the loading rate was changed to 200 N/s until the sample was damaged. III: The last phase was the same as phase III under Path 1. Path 3 (UCP):  
I: The first phase was the same as Path 1. II: The radial stress was kept constant, and the axial stress was loaded to 60% of the axial peak strength of the sample under the same condition as the CTC path at a rate of 50 N/s. At the same time, the seepage data were recorded and stored. III: Then, the axial stress was kept constant, and the radial stress was unloaded at a rate of 20 N/s in the force-controlled mode until the sample was damaged. IV: The last phase was the same as phase III under Path 1. Path 4 (AUCP):  
I, II: The first and second phases were the same as Path 3.