Numerical Simulation Of Hydraulic Fracturing In Heterogeneous Shale

The efficient exploitation of shale gas brings opportunities and challenges to countries worldwide.Shale gas is exceptionally abundant globally and has gradually become an essential supplement to conventional oil and gas resources,which are at the latter stage of development,playing an increasingly important role in the world’s energy composition.However,on the one hand,the compactness of shale rocks determines that the efficient extraction of shale gas depends greatly on hydraulic fracturing technology.On the other hand,the heterogeneity of shale rocks determines the complexity of fracture evolution in the shale formation reconstruction by hydraulic fracturing technology.Therefore,correctly describing the evolutionary behavior of hydraulic fractures in heterogeneous shales during hydraulic fracturing is an important issue in shale gas exploration and development and a vital fracture mechanics problem under the effect of fluid-solid coupling.This thesis focuses on the critical scientific problem of the evolutionary behavior of hydraulic fractures in heterogeneous shales,integrates the application of the extended finite element method(XFEM)and the phase-field method(PFM),establishes the discontinuous and continuous fracture mechanics model of hydraulic fractures with fluid-solid coupling,reveals the interaction mechanism of hydraulic fractures with reservoir interfaces and natural fractures in heterogeneous shales from the perspective of numerical simulation,and develops the corresponding extended finite element hydraulic fracture procedure.The details are as follows.(1)A weak interface model for shale reservoirs with material differences is established,and an energy-based propagation criterion for the hydraulic fracture intersecting with such a weak interface is proposed.The effects of the cement strength of the weak interface,the intersection angle between the weak interface and the hydraulic fracture,and the differences in rock materials on both sides of the weak interface on the hydraulic fracture propagation behavior are analyzed.The results show that the smaller the fracture toughness ratio between the interface and the rock matrix,the more the hydraulic fracture tends to deflect along with the interface after intersecting with the interface;the smaller the intersection angle between the weak interface and the hydraulic fracture,the more the hydraulic fracture tends to deflect along with the interface after intersecting with the interface;when the hydraulic fracture expands from the soft layer to the hard layer,the more significant the difference between the softness and hardness of the two layers,the easier the hydraulic fracture is to pass through the weak interface and enter into the other layer of the rock matrix.In addition,the numerical model captures the kink phenomenon of the hydraulic fracture,i.e.,the hydraulic fracture is captured by the interface and then will escape from the interface again and expand toward another layer of the rock matrix.(2)A hybrid fracture model combining discontinuous fracture model and continuous fracture model for hydraulic fracturing is developed,and based on which a globallocal numerical simulation method for hydraulic fracturing based on the extended finite element method(XFEM)and phase-field method(PFM)is proposed to investigate the intersection behavior of hydraulic fractures with different types of natural fractures in shale.The computational results show that the proposed hybrid numerical computational method can effectively predict the propagation path of hydraulic fractures,accurately describe and characterize the geometry of fractures,and has good computational efficiency.In addition,the proposed method successfully realizes the simulation of the interaction behavior of a single hydraulic fracture with cemented and frictional natural fractures.It demonstrates its adaptability in simulating complex fracture networks by applying it to multiple fracture interactions.(3)The two-dimensional(2D)hydraulic fracturing model established by XFEM is extended to the three-dimensional(3D)case.The 3D numerical model integrates the fracture propagation criteria that can describe the mixed fracturing mode of planar fractures,including the determination of the propagation direction(deflection and torsion direction)and the selection of the advancing length.In addition,an adaptive propagation algorithm for the crack tip vertex of a planar crack is given to automatically select the appropriate crack advancing length based on the level of the stress intensity factor of the crack tip,including the approaching,intersection,and coalescence processes of two planar cracks.The preliminary numerical test results show that the 3D numerical model can effectively simulate the propagation process of hydraulic fractures.It is expected to establish a numerical simulation framework for the intersection of 3D hydraulic and natural fractures to reveal the complex 3D crack morphology.The two-dimensional and three-dimensional numerical models of hydraulic fracturing,the algorithm for solving the nonlinear system of equations,and the corresponding fracture propagation criterion established in this thesis not only provide a numerical analysis scheme for the mechanical behavior and mechanism of fracture in heterogeneous shale under the action of fluid-solid coupling but also provide critical theoretical support for hydraulic fracturing technology in petroleum engineering.