Mechanism Of Hydraulic Fracture Propagation In Fractured Shale Reservoir In Three Dimensional Space

Due to the low porosity and low permeability in the shale gas reservoir,a large scale of reservoir stimulation is required to develop this unconventional resource.Hydraulic fracturing produces network fractures and effectively contacts the natural fracture(labeled as ‘NF' in the following text)and pore in the reservoir,which is the purpose of hydraulic fracturing stimulation in shale gas reservoir.This dissertation discusses the impact mechanism of the NF and bedding plane on hydraulic fracture(labeled as ‘HF' in the following text)propagation in shale gas reservoir,and then develops a mechanical model to simulate the multiple fracture propagation in the horizontal well.The research work in this dissertation will provide some theoretical references for stimulating reservoir volume in the unconventional shale gas reservoir.(1)Aiming at the impact mechanism of NF in the shale gas reservoir on the HF propagation,a model is developed to identify type-I HF crossing the NF,based on the fracture mechanics analysis of stress field acting at the natural and HF.Another model is built to identify type I-II HF crossing the NF based on the stresses on the non-planar HF.It is found(1)Propagation path of HF contacting the NF is highly correlated to the occurrence of NF it contacts.It is easy for HF crossing the NF with a high strike angle.No obvious relationship is observed between the crossing behavior of the HF and the dip angle of NF;(2)Propagation behavior of HF contacting the NF is also highly correlated to the tri-axial in-situ stresses.The higher the stress contrast coefficients of tri-axial in-situ stresses and horizontal stresses,the easier the HF crosses the NF.(2)Aiming at the stress interaction of multi-stage fracturing in the horizontal well,stress field between the multiple fractures and fluid flow inside the fracture are researched based on the boundary element method,linearly elastic fracture mechanics and fluid mechanics.A coupled fluid-solid model to simulate multi-fracture propagation in the horizontal well is proposed and coded by MATLAB.This model can respectively simulate the multi-fracture sequential propagation and simultaneous propagation in the multi-stage fracturing of horizontal well.Ideal rigid and elastic proppant respectively affect the closure of HF in the model.This model can simulate the hydro-fracking in the naturally fractured reservoir if combined with the above-mentioned crossing model between the HF and NF.It is found(1)as the HF spacing decreases in the multi-fracture sequential fracturing,the stress interaction from the previous HF will highly act on the subsequent HF,and then the width of subsequent HF will be smaller.In the multi-fracture simultaneous fracturing,the middle HF will be squeezed by the outside HFs and the outside HFs will be repelled shape;as the HF spacing decreases,the stress interaction between the HFs will increase and then the HF path will be more curved.Hence,this coupled fluid-solid model has solve the stress interaction between the multiple HFs in the horizontal well.(2)In the multi-fracture sequential fracturing,the first HF is a planar fracture,but the subsequent HF is a non-planar fracture which is repelled by the first HF.The curved degree of subsequent HF increases leading to a smaller fracture width as the increasing of the stiffness in previously propped HF.Under the same stimulation strategy,the subsequent HF will increase with a higher speed and require an larger injection pressure.In the multi-fracture simultaneous fracturing,the fluid distributes unequally because of HF competition.The outside HF will be much wider and longer than the inside ones.Hence,the coupled fluid-solid model can simulate the configuration of multiple HFs in the horizontal well.(3)The combination of multi-fracture simultaneous and sequential propagation can simulate the multi-stage sequential fracturing in the horizontal well.All the HFs in the subsequent stage will suffer the stress interaction from the previous stage.(4)The closure stress will be balanced by the propped stress from the proppant after the injection pump is shutoff.Compared to the closure of stiff propped HF,the closure of elastically propped HF is greater under a weaker stress interaction.(3)Aiming at the impact mechanism of bedding plane in layered shale on the HF propagation,HF propagation and stress field in the layered medium are researched using boundary element method.The influences of the elasticity contrast in the layered shale on the HF propagation angle,stress intensity factor and HF width are evaluated qualitatively.A model to judge the HF crossing the bedding plane is presented.It is found HF tends to repel the rock with high elastic modulus and attract the rock with low modulus.Compared to the Poisson's ratio,elastic modulus has an obvious influence on the HF propagation and HF width.Due to the influence of elasticity contrast,the HF in the layered shale will propagate as a non-planar plane,not always perpendicular to the minimum principle stress.