Practice of permeability enhancement through overall hydraulic fracturing of long hole in outburst-prone soft crushed coal seam with low permeability

Aim to the crushed and soft coal seam with high outburst danger in Yangquan mining area,the paper put forward a technology of whole hydraulic fracturing and permeability enhancement using long hole. The engi-neering practice test realized that the length of the coal segment of the hole underground for the whole fracturing was up to 307 m,the injected water volume of single hole was 1 510 m3 , and the maximum injection pressure was up to 26. 09 MPa. The penetration effect of fracturing was investigated and the results were the influencing range of borehole fracturing was up to 58 m,the permeability coefficient of coal seam increased by 2. 67 times,the gas flow attenuation coefficient per 100 meters of hole decreased by 55% . During a 230 days period,the extraction of pure methane was from 1 395 to 2 810 m3 / d with 2 173 m3 / d in average and 50. 86×104 m3 in cumulative volume,the concentration of extracted gas was from 49. 38% to 83. 70 with 64. 31% in average. It was analyzed that hydraulic fracturing improved the connectivity of coal seam fracture and porosity,reduced the effective stress and increased the permeability of coal seam,promoted the coal seam gas from the adsorption state to free state transformation. So the above factors were the key to the effective extraction of gas after fracturing coal seam. According to the principle of sand-filling and plug-up seam fracturing technology,the mechanism of opening and extension of the whole hydraulic fracturing seam with hy- draulic fracturing in soft and low permeability coal seam was presented. The achievements and knowledge of the engi- neering practice provides a practical experience and technical support for the whole hydraulic fracturing and highly ef- ficient gas drainage.