Formation Process And Mechanism Of Multi-branch Hydraulic Fracture In Fractured Coal Rock Mass

The formation process and propagation characteristics of multi-branch hydraulic fractures in fractured coal rock mass play significant roles in permeability enhancement of coal measures gas reservoirs and the direction of hydraulic fracturing.In order to study the formation process and mechanism of multi-branch hydraulic fractures in fractured coal rock mass,the control effect of multi-scale structural characteristics of fractured coal rock mass on hydraulic fractures was studied by field investigation,laboratory tests and analogy analysis,and a simple global and local embedding algorithm of cohesive element was proposed,as well as a pore pressure node merging method for simulating fracture network was given.A hydraulic fracture propagation damage model was established,including the mixed damage model of tension and shear and the natural fracture fracture compression-shear damage model based on Barton model,which was also compiled by user subroutine USDFLD.The numerical model generation methods of discontinuous conjugate joint network,orthogonal cleat network and Voronoi fracture network were proposed.The stress disturbance range of hydraulic fracture was studied,and the numerical or similar model boundary effect of multi-fracture fracturing was discussed.The formation process of multi-stage hydraulic fracture in fractured coal rock mass was modeled,and the influence of natural fracture network characteristics such as bedding,discontinuous conjugate joint network and cleat system on the formation process of hydraulic fracture was studied,and the effects of natural fracture roughness,direction and ground stress on fracture network shape,influence width,the process of fracture bifurcation,the ratio of tension and shear cracks,the characteristics of secondary cracks,etc were analyzed.Finally,the formation mechanism of multi-branch hydraulic fractures is studied,and a theoretical model for identifying the bifurcation of a single hydraulic fracture when it encounters a natural crack is established.The roughness of natural crack,fracture flow pressure,compressive strength of natural fracture surface,effective stress field and direction of natural fracture were studied.The induced stress field characteristics of conventional fracturing and repeated fracturing were compared,and the bifurcation characteristics of dynamic hydraulic fractures under large flow fracturing was analyzed.Results are the following.(1)When the difference in principal stress is constant,increasing the minimum principal stress is not beneficial to open secondary fractures,and promotes the transition of main fractures from tension-type failure to shear-type failure.In the same geostress environment,the possibility of generating tension-type hydraulic fractures in shallow coal seams is higher than that in deep coal seams.The control effect of natural fractures on the failure type of main hydraulic fractures is gradually enhanced with the increase of the minimum principal stress.(2)There are not only tension fractures but also a considerable proportion of shear fractures in hydraulic fracture network of fractured coal rock mass.During the formation process of hydraulic fracture network,tension-type hydraulic fracture and shear-type hydraulic fracture can alternately occur,resulting in a multi-branch structure of tension-type primary fractures,second-stage shear-type secondary fractures and third-stage tension-type secondary fractures.However,in the natural fracture network with low roughness,the main fractures are shear-type,and the second-stage fractures are tension-type.The length and the number of secondary shear cracks significantly affect the number of next stage fracture branches,thus affecting the effectiveness of hydraulic fracturing macroscopically.The ratio of secondary fracture length to main fracture length is generally more than 4.34,up to 14.20.Increasing the proportion of secondary fractures is very important to improve permeability or conductivity of coal seams.(3)The concept of hydraulic fracture tortuosity was defined,which is the reciprocal of the number of periodic tortuosity of hydraulic fracture in a certain length in the direction of maximum principal stress.The hydraulic fracture tortuosity was used to evaluate the control effects of maximum principal stress and natural fractures on the direction of hydraulic fractures.With the increase of stress difference,the tortuosity of hydraulic fracture decreases,and the reduction of tortuosity is beneficial to produce more fracture branches in discontinuous conjugate joint strata and increase the effectiveness of hydraulic fracturing.(4)When the sharp angle bisector of conjugate joints is nearly perpendicular to the maximum principal stress,the tortuosity of the main hydraulic fractures and the influence range of the hydraulic fracture network are larger,the larger shear-type fractures can be formed.When the sharp angle bisector of conjugate joints is approximately parallel to the maximum principal stress,the number of fracture branches is larger,but the influence range in the width direction of the hydraulic fracture work is narrower.When the direction of one joint group is closed to the direction of maximum principal stress,the control effect of joint on the direction of hydraulic fracture is stronger than that of in-situ stress field.(5)Multi-scale natural fracture,repeated fracturing and large flow dynamic fracturing are the main mechanisms of multi-branch hydraulic fracture formation in fractured coal rock mass.Repeated fracturing is better than conventional fracturing in fracture network reformation,especially in the aspect of hydraulic fracture network influencing width.The direction of local stress field near hydraulic fractures can be changed by 90 degreee under repeated fracturing,which provides stress environment for multi-stage bifurcation in subsequent fracturing process.