Numerical simulation study on evolution law of fracture stress field under the action of hydraulic fracturing of coal seams in kilometer-deep wells
In order to understand the evolution law of the crack stress field inside the coal body under the action of hydraulic fracturing of coal seams in kilometer-deep wells,and carry out comprehensive treatment of gas permeability enhancement more scientifically through hydraulic fracturing of coal seams with a depth of more than 1 000 m,taking the V 8-31240 working face with a depth of more than 1 000 m in Pingdingshan No.1 Coal Mine as the modeling background,the RFPA-2D software numerical simulation method was used to establish a numerical model of internal fracture development and stress field disturbance in coal bodies,and the evolution process of internal fracture stress field in deep coal bodies under hydraulic fracturing was obtained in the simulation.The research re-sults showed that under the action of hydraulic fracturing,the deep coal body was first infiltrated by water flow into the coal body,micro-cracks sprout in the area around the fracturing hole,and a"V"-shaped main-level macro was formed in the upper left and upper right areas of the hole.The cracks continued to expand and develop in a fan shape,and then the micro-cracks in the lower left and right sides of the hole connected to form"A"-shaped secondary macro-cracks.Finally,the main and secondary macro-cracks on both sides of the hole connected,showing an"X"-shaped crack expansion dominant fracturing,and generate more micro-cracks,so that the range of deep coal bodies affected by hydraulic fracturing continues to expand.At the beginning of hydraulic fracturing of deep coal bodies,internal damage occurs randomly around the fracturing holes,and as the fracturing pressure increases,the coal body damage begins to extend to the upper and lower areas of the fracturing hole,but the damage rate is much smaller than the damage rate to both sides of the fracturing hole.The coal body damage is more concentrated in the upper part of the coal seam,indicating that hydraulic fracturing will cause a greater impact on the coal seam roof.The deep coal body is subjected to hydraulic fracturing,the strip shear stress and massive tensile stress are mainly concentrated in the area near the tip of the fracture,the shear stress is released from 13.5 MPa to 1.1 MPa,the pressure relief is reduced by 91％,while the tensile stress is relieved from 11.0 MPa to 2.2 MPa,resulting in a pressure reduction of 80.0％,the pressure relief effect of tensile stress is less than that of shear stress,with the extension of fracturing fracture to the depth of coal body,fi-nally,the shear-tension double action pressure relief zone is formed in the 25 m range before and after a single fracturing hole,which relea-ses the ground stress and greatly improves the air permeability of the deep coal body.The numerical simulation research results will pro-vide a certain reference for the optimization of process parameters of hydraulic fracturing technology in kilometer-deep coal seam.
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