Study On Gas Drainage Technology Of Long Borehole Staged Hydraulic Fracturing In Underground Roof Of Broken And Soft Coal Seam

Hydraulic fracturing is one of the key technologies to achieve efficient gas drainage in low permeability coal seams.In this paper,aiming at the problems of difficult bedding drilling,difficult pore forming and poor gas extraction in broken soft and low permeability coal seams,the research on hydraulic sand fracturing technology of directional long borehole in roof of broken soft and low permeability coal seam in underground coal mine is carried out,which relies on the science and technology innovation fund project(No.2017XAYZD06)of CCTEG Xi’an Research Institute(Group)Co.,Ltd.The paper applies multidisciplinary knowledge,including coal fields geology,gas geology,rock mechanics,etc.It mainly studies the technical parameters of roof fracturing fracture extension characteristics and roof staged fracturing construction technology of broken soft coal seam in coal mine by integrating a variety of methods for research,including theoretical analysis,numerical simulation,and engineering tests.In addition,the engineering application test was carried out in the typical broken soft and low permeability coal seam in Yangquan,Shanxi Province to verify the conclusion.The main conclusions and understandings are as follows:(1)The rock mechanical parameters and in-situ stress distribution characteristics of the broken and soft coal seam and its roof are analyzed.revealing the mechanism of high-strength fracture transformation of broken and soft coal seams and realizing gas-efficient pumping.The results show that the elastic modulus and Poisson’s ratio of the roof are 2.56-4.41 times and 0.68-0.84 times that of the coal seam respectively.The minimum horizontal principal stress of the coal seam roof is 1-3 MPa higher than that of the coal seam,indicating that fracturing in the coal seam roof is conducive to the formation of complex fracture network,and the longitudinal cracks are more likely to extend to the coal seam until into the coal seam.The hydraulic fracture length and width of the roof are 4.48 and 2.91 times that of the broken and soft coal seam respectively.(2)The directional drilling hydraulic sand blasting perforation,double sealing single card and open hole dragging sand carrying staged fracturing technology for roof of broken and soft coal seam in underground coal mine was formed.The construction technology parameters of hydraulic sand staged fracturing for roof of broken and soft coal seam of Yangquan mining area were optimized.The staged fracturing spacing of broken and soft coal seam roof is 40-50 m,which can effectively reduce the inter-fracture interference caused by the induced stress field between the main fractures between adjacent fracturing sections.The angleβbetween the direction of directional sand blasting perforation and the vertical stress is less than 20°,which can greatly reduce the difficulty of crack initiation and propagation,and is conducive to the better penetration of the lower coal seam.The single-stage water injection volume is greater than 150 m~3,which can effectively ensure that the fracture half-length of each fracturing section is more than 25 m,and can better extend into the lower coal seam.(3)In the broken and soft coal seam of Yangquan mining area,two roof directional long borehole open hole sand staged fracturing gas drainage engineering application tests were carried out.The length of directional long borehole in broken and soft coal seam roof is 609 m,and the open hole sand fracturing is carried out according to 6 and 10 sections respectively.The effect test shows that the influence radius of two-hole fracturing is 25-35 m,and the average value of gas drainage pure volume in 100days of two fracturing boreholes is 1025.1 m~3/d and 2810.6 m~3/d.The average value of gas extraction per 100 meters of the two boreholes was 7.42-23.65 times and 22.13-66.71 times higher than that of the directional drilling and bedding gas drainage holes in the same area.Broken and soft coal seams are widely developed in Carboniferous-Permian coalfields in North China and Southwest China,and coal seam gas disasters are serious.The popularization and application of the research results of this paper is of great significance and engineering application value for the efficient drainage and treatment of gas in broken soft low permeability and high outburst coal seam in China.