Innovative engineering practice of soft coal seam permeability enhancement by controllable shock wave for mine gas extraction:A case of Zhongjing Mine, Shuicheng,Guizhou Province,China

Low cost and high efficiency gas extraction from soft coal seam is one of the worldwide technical problems. The effect of controlled shock wave (CSW) permeability enhancement of coal seam in Zhongjing coal mine provides a successful demonstration to solve this problem. Through the CSW permeability enhancement of coal seam in borehole,the average daily gas extraction volume of single borehole is increased by 3. 37 times,with the maximum of 6. 54 times,the methane concentration is increased by 2. 31 times,and the effective radius of CSW operation is up to 40-60 m. The original difficult-to-extract coal seam is transformed into easy-to-extract coal seam,and the required bore- hole engineering work is only 11% -18% of the traditional borehole engineering work. When borehole diameter and impact energy are the same, the optimum CWS density of permeability enhancement in the demonstration project should be between 1. 0 and 1. 1 times / m,and the length of CSW operation section should be more than 120 m. Under the optimum CSW density condition,the enhanced-permeability effect of extraction hole in the mining face is mainly related to the starting time of extraction and extraction environment around a borehole. The later the exposure of exca- vation along coal seam roadway and the more obvious the mutual interference of CSW boreholes in mining face,the better the extraction effect. Comparing with the gas flow curve pattern from a traditional borehole with the monotonous attenuation of power exponent,the gas flow curve from a CSW borehole occurs in a “two-stage” pattern,in which the flow curve of most boreholes in first stage has a flow “peak” similar to that of surface CBM well for drainage and de- pressurization,and the flow attenuation coefficient in second stage is much smaller than that of the comparison bench- mark. It is suggested that the mechanism of above effects lies in two aspects. Firstly,taking a CSW borehole as the cen- ter and outward,the flow field in coal seam transits from mining flow field to CWS flow field,which greatly expands the effective flow radius. The CSW flow field is similar to the flow field of coal seam in surface well,and coal seam gas can be produced through drainage and depressurization. Secondly,the flow field reformed by CSW provides more abundant gas source for the flow field of mine and roadway,effectively retards the rapid reduction of gas flow attenuation coeffi- cient,and significantly improves the extraction effect.