Study On The Distribution Of Mining-induced Fracture-stress Field And Its Influence On Hydraulic Fracturing Pattern In Hard Roof

Hard roof is an important factor of the occurrence of strong ground pressure,which directly affects the safe production of working face.Hydraulic fracturing can be adopted to weaken hard roof with high-pressure liquid.Then,the integrity of hard roof is broken by stratification and partition,the effect of which is positively correlated to hydraulic fracturing pattern.The hydraulic fracturing pattern of tigh rock stratum such as hard roof is strongly affected by ground stress.During the mining process,complex fracture network is formed in the overburden rock strata leading to the variation of the original ground stress and haydraulic fracturing pattern.Therefore,it is necessary to study the mechanical behaviors of fractured rock,mining-induced fracture-stress field and the influence of mining-induced stress on hydraulic fracturing pattern,which can contribute to the design and optimization of hydraulic fracturing in hard roof.Therefore,with the theme of“study on the distribution of mining-induced fracture-stress field and its effect on the hydraulic fracturing pattern in hard roof”,the mechanical behaviors of rock and fractured rock.Based on the equivalent continuum theory,the constitutive model of fractured rock is proposed.The fracture mode of overburden rock and the distribution of mining-induced fracture-stress are calculated with the proposed model.Furthermore,based on the distribution of mining-induced stress and the new hydro-mechanical model,the influence of mining sress and geological conditions on hydraulic fracturing in hard roof is investigated.The main obtained results are as follows:(1)Based on equivalent continuum theory,the constitutive model of fractured rock is established.Through the analysis of rock strength characteristics for true 3D tests,it is found that₁ promotes₂ within 15%;the constitutive model can be used to describe the evolution of tensile and shear fractures as well as discontinuous mechanics behaviors for contact and non-contact state of fractures.The simulations(three-point bending,uniaxial compression,true 3D)are performed to confirm its validity,which show that it can provide a promising method to calculate the mining-induced fracture-stress field.(2)The distribution of mining-induced fracture-stress field is obtained.The fracture of overburen rock is dominated by shear mode.Shear fractures have a 3D annular shape above the overlying strata whereas a ribbon shape is observed for tensile fractures.The upper of goaf is the destressed zone showing an asymmetrical elliptic shape,which is consistent with the distribution of mining-induced fractures.At the front of the working face,stress-increasing and stability zones are formed parallel to the mining direction while the stress distribution is bell-shaped perpendicular to the mining direction.The results can provide theoretical support for simulations of hydraulic fracturing pattern in hard roof.(3)The influence of mining-induced stress on hydraulic fracturing pattern in hard roof is investigated.Through hydraulic fracture seepage tests,a nonlinear seepage model considering Reynolds number and equivalent hydraulic aperture is established to perfect the hydro-mechanical model of hydraulic fracturing.Through simulating the effect of different position and geological conditions on hydraulic fracturing pattern,the obtained results are as follows:the hydraulic fractures in distressed zones gradually deflect from the vertical direction to the horizontal direction with the working face advancing,while the vertical hydraulic fractures in the stress-increasing and stablility zone deflect to both sides.The hydraulic fractures at the front of the working face is less affected and only 90°deflection occurs in the low-position hard roof;the deflection degree of hydraulic fracture increases with the decrease of hard roof height and stress difference.