# Double Unloading Gas Control Technology for Fracturing Soft Coal Seams in Overlying Key Strata
## 1 Introduction  
About 50% of mines in China are classified as high gas or coal and gas outburst mines, with soft and low-permeability coal seams accounting for as much as 82% of these mines [1, 2]. As mining operations go deeper, the in situ stress, gas content, and gas pressure gradually increase, while the permeability further decreases. The permeability of most outburst mines is only \(1\times 10^{-4}\) to \(1\times 10^{-3}\) md [3, 4, 5]. The poor gas permeability of coal seams makes it challenging to pre-extract gas, resulting in a high amount of residual gas. During the mining process, frequent abnormal gas outflow leads to the gas concentration in the working surface exceeding the limit, posing an increasingly serious threat of gas disasters [6, 7, 8]. Conventional drilling has limited effectiveness in dealing with this type of coal seam, as it has a small extraction radius, low drainage efficiency, and poor pressure relief effect on the original coal seam, making gas drainage more difficult [9, 10]. Therefore, the main approach for gas control is to utilize external force measures to release surrounding rock stress, modify coal seam gas migration characteristics, and enhance drainage efficiency [11, 12, 13]. At present, various technologies are used to address high gas and low permeability coal seams, including mining protective layers [14, 15], hydraulic slotting [16, 17], hydraulicfracturing [18; 19; 20; 21], hydraulic punching [22; 23], and intensive gas drainage drilling [24]. Protective layer mining is currently the most effective method to prevent coal and gas outbursts, which has been successful in field practice [15; 25]. [PERSON] et al. [26] conducted industrial experiments on pulse hydraulic fracturing in both structural and non-structural zones, analyzing the extension direction of coal seam cracks during the pulse hydraulic fracturing process. [PERSON] et al. [27] conducted experiments using large-diameter boreholes in soft coal seams in geological structures, reducing the risks of collapse and coal and gas outbursts. However, conventional pressure relief, permeability enhancement, and drainage methods face new challenges in deep coal seam gas control. Mining protective layers requires favorable geological conditions and may not be feasible when the adjacent protective layer is too thin or the distance between coal seams is inappropriate. Additionally, cracks formed via hydraulic measures have limitations, as they have a short maintenance time and can be re-compacted under high stress conditions [28; 29; 30]. The'soft coal' undergoes plastic deformation internally when subjected to high-pressure water, resulting in blocked cracks and reduced gas permeability [31]. In the soft coal seam, ordinary extraction drill holes are prone to problems such as jamming and plugging during the construction process, making gas extraction more difficult. This is mainly due to the large ground stress in the deep soft coal seam [32], and the current technology and process methods cannot realize effective pressure relief on the soft coal seam continuously, and it is difficult to fundamentally prevent the occurrence of coal and gas protrusion accidents. In addition, the above-mentioned technologies also have environmental impacts. During hydraulic fracturing, a certain amount of fracturing fluid is used and, when it enters underground drinking water sources through rock fractures, it may pose a threat to human health [33]. This paper aims to address the above issues by adopting a method that combines hydraulic fracturing in the overlying key layer with large-diameter borehole in the coal seam for comprehensive penetration enhancement management. The study involves hydraulic fracturing and decompression in the overlying key strata while simultaneously conducting large-diameter borehole drilling within the coal seam.