Study On The Law Of Hydraulic Fracture Morphology In Hard Roof Under The Influence Of Mining Activities

Hard roof is an important factor that causes strong mine pressure on the working face and rock burst.It has the characteristics of high strength,strong integrity,undeveloped cracks,and strong self-supporting ability.Hydraulic fracturing is a commonly used pressure relief solution.In the past,the researches on hydraulic fracturing of hard roof were mostly focused on the effects of pressure relief under field test conditions.However,there was little theoretical research on the law of hydraulic crack propagation in hard roof,especially the propagation law under inhomogeneous stress field.In fact,the geometric parameters of hydraulic fracture are of great significance for the pressure relief control of hard roof.Therefore,the hydraulic crack propagation law of hard roof under the influence of mining stress and remaining coal pillar was studied,and the related theories were adopted to obtain the propagation shape in different stress environment.Besides,the law of hydraulic fracture propagation under different stope structure parameters and ground stress conditions was obtained through numerical simulation and laboratory experiment,which will also provide scientific references for the design and construction of hydraulic fracturing under similar conditions.The main research results are as follows:(1)Based on arch stress theory,a mining stress arch model was established and the features of arch shape and frame were analyzed.Through analysis of the simulation of RFPA2D_FLOW,the transformation laws of stress arch were obtained and showed as follows: The mining arch is the maximum principal stress bundle area above stope,and its shape is an “elliptical ring” shell structure with a certain thickness.The height and thickness of stress arch increase with the increase of mining width.The direction of the long axis of the stress arch coincides with the far field maximum principal stress direction.The height and thickness of arch increases with the increase of stress.(2)Based on Boussinesq problem and combined with superposition theory,a shape model of the maximum principal stress under coal pillar was established.The maximum principal stress spreads to the deep coal pillar floor,whose trace is part of the conic curve.The value of maximum principal stress is related to the width of pillar,the load of the pillar on the floor,and point coordinate.The maximum principal stress direction has nothing to do with pillar load,but only with the pillar width and the point coordinate.(3)Based on RFPA2D_FLOW software,the hydraulic fracture propagation law influenced by mining stress was analyzed.Affected by mining stress,the hydraulic fracture will gradually deflect to form the arc-shaped crack finally.As the width of mining increases,the mining stress arch shape gradually evolves,and the height of arch increases.As a result,the high position roof gradually enters the mining stress arch area.The crack cumulative deflection angle decreases gradually as the distance between hydraulic position and stope increases.The single-step and cumulative deflection angle increase with the decrease of lateral pressure coefficient.(4)Through the physics experiment,the hydraulic fracture propagation pattern of hard roof under the influence of coal pillar stress was verified.When the fracturing position is located on the center extension cord of the coal pillar,the hydraulic fracture starts to crack,forming a "groove"-type hydraulic fracture.When the fracturing position is on the side below coal pillar,hydraulic fracture will eventually form a curved and irregular "S"-type crack.Affected by the remaining coal pillar,the expansion of hydraulic surface is ever-changing and a curve surface is formed finally.