Abstract:
Since the “13th Five-Year Plan”, significant progress has been made in the directional long-borehole staged hydraulic fracturing technology, equipment development and engineering demonstration application of underground coal mine through the technical research, equipment development and engineering test with directional long-borehole and staged hydraulic fracturing technology in combination, focusing on the two major issues of regional permeability enhancement, efficient gas drainage and rock weakening treatment of hard roof for coal seams in underground coal mine in China. It is mainly manifested in the following four aspects: (1) Two staged hydraulic fracturing technologies and tools with/without rig movement have been developed. Specifically, the borehole hydraulically fractured in stages without rig movement has exceeded 500 m in engineering application, with up to 5 stages of single-hole fracturing achieved. Besides, the borehole hydraulically fractured in stages with rig movement has exceeded 800 m in engineering application, with up to 17 stages of single-hole fracturing achieved. (2) The sand fracturing pump set for the low-pressure end and the sand fracturing device for the high-pressure end of underground coal mine were developed. Definitely, the sand fracturing pump set for the low-pressure end has the pressure up to 70 MPa, the displacement up to 90 m
3/h and the sand carrying ratio up to 20%. The sand fracturing device for the high-pressure end has the bearing pressure up to 55 MPa and the sand volume for one-time continuous sand fracturing reaching 750 kg. Besides, the sanding equipment for the low-pressure end and the high-pressure end have been applied in the field engineering, of which the results show that the equipment has good sand carrying and fracturing performance. (3) An efficient gas drainage technology model with regional permeability enhancement was established for the staged fracturing in the surrounding rock of broken-soft coal seam and in the borehole along the hard coal seam. The former was applied in Yangquan mining area in Shanxi and Hancheng mining area in Shaanxi, with the net volume of gas drained from the borehole reaching 2 811 m
3/d and 1 559 m
3/d respectively, while the latter was applied in Binchang mining area in Shaanxi, with the net volume of gas drained from the borehole up to 2 491 m
3/d. (4) A new mode of regional weakening treatment for the active leading area of staged hydraulic fracturing of directional long-borehole in the underground coal mine with hard roof and strong underground pressure has been explored. Specifically, the directional long-borehole hydraulically fractured in stages has exceeded 800 m in engineering application. After the staged hydraulic fracturing treatment of hard roof, the pressure step, dynamic load coefficient and maximum pressure value of roof have been reduced by 18.9%‒70.6%, 5.8%‒7.9% and 13.7%‒19.4% respectively compared with those in the unfractured area, effectively controlling the strong ground pressure hazard caused by the hard roof of the working face. With the improvement of staged hydraulic fracturing technology in underground coal mine and the actual requirements of the development of intelligent coal mining, the research and development requirements have been put forward for the equipment such as the large-displacement and high-pressure intelligent fracturing pump set in underground coal mine, the intelligent tools for staged hydraulic fracturing in underground long-borehole and the integrated coal mine-ground staged hydraulic fracturing technology, so as to better promote the development of hydraulic fracturing technology and equipment for underground coal mine, and thus provide technical and equipment support for the safe, efficient, green and intelligent mining in coal mine.