## 1 Introduction  
Coalbed methane is a type of clean energy that has a high calorific value and clean combustion products [1; 2; 3]. Many countries, including the United States, China, and Australia, are producing gas to optimize their energy structure. The gas production in the United States is declining because of shale gas. However, the production in other countries is increasing, and these countries will continue to focus on developing gas resources in the future [4; 5; 6]. Additionally, coal mines that contain gas have the possibility of coal and gas outburst (CGO) accidents in the mining process, and gas extraction can solve this problem [7; 8]. Therefore, efficiently extracting gas and eliminating the potential for CGOs is an urgent problem that needs to be solved. To promote gas extraction, scholars from different countries have proposed various methods to increase coal seam permeability. For most coal mines, dense borehole gas extraction technology is initially used [9; 10; 11]. The effectiveness of this technology in low permeability coal seams is poor. Additionally, this technology requires a substantial amount of work and long extraction times, which affect the subsequent production process. Some researchers have proposed a series of pressure relief technologies, such as hydraulic flushing technology and hydraulic slotting technology [12; 13], to improve the effectiveness of gas extraction in cross-measure boreholes and shorten the extraction time by approximately 20% to 30%. To some extent, these technologies play a role in promoting gas extraction. However, the influence radius of a single cross-measure borehole is limited (usually 3 m to 5 m), and a large amount of work is required for extraction. Additionally, cross-measure boreholes easily collapse and become blocked, which reduces the final extraction efficiency. Some researchers have proposed using hydraulic fracturing technology (HFT) in coal mines. After years of improvement, traditional HFT in coal mines has developed into directional HFT, pulse HFT, tree-type HFT, etc. [14; 15; 16; 17; 18]. Additionally, some new types of fracturing fluid have been proposed [19]. The effect of these HFTs is significantly improved compared with the pressure relief technologies mentioned above. These HFTs can significantly increase the influence radius (generally 30 m to 60 m) and shorten the extraction time by approximately 50%. However, these HFTs have certain application limitations. For example, the roof and floor must be intact because coal seams with geological structures cannot hold pressure and cannot be fractured. Additionally, in many multi seam coal mines, protective seam mining technology is used as an auxiliary technology to ensure extraction effectiveness [20; 21]. Protective seam mining technology applies to only the seams below the first seam in multi seam areas with appropriate coal seam intervals. For the first seam in multi seam or for single coal seams, one of the abovementioned permeability enhancing technologies (usually HFT) is generally used to promote gas extraction. In fact, for many coal mines, the local conditions will not be fully considered when implementing HFT. When HFT is used in a tectonic region, an unfractured area will be produced around geological structures; otherwise, CGO accidents or water inrush accidents will occur. For example, a hydraulic fracturing project in an area with secondary faults triggered a CGO accident in the Hongyan coal mine (Chongqing, China) on 31 August 2017. For some coal mines with multiple geological structures, HFT is not used for fear of causing accidents. These mines use only dense borehole gas extraction technology or pressure relief technology, which results in poor extraction efficiency. Additionally, imperfect permeability enhancement leads to a long extraction time, which affects the coal mine production.