Flow-coupled Cohesive Interface Simulations For Complex Hydraulic Fracture Morphologies

Due to the extremely low natural productivity,the effective production of tight oil and gas reservoirs is highly dependent on the complexity of hydraulic fracture networks.Influenced by various factors,such as the properties of fracturing fluid,heterogeneity in formation and fracturing technology,the hydraulic fractures in tight reservoirs have complex morphology with many branches,which brings many difficulties to the design and the evaluation of fracturing operation.Therefore,developing effective numerical simulation methods and studying the propagation of complex hydraulic fractures are very important to the development of tight reservoirs.So far,the finite element method is widely used in simulating hydraulic fracturing,but it still has many limitations,such as simplified cohesive constitutive relationship,calculation divergence problem and inefficiency in simulating the propagation of complex hydraulic fractures with multi-branches.To solve these problems,the Xu-Needleman cohesive model is adopted to characterize the fracturing process in rock.A technique for avoiding convergence problems in finite element simulations of cohesive cracks is introduced by adding a fictitious viscosity to regularize the snap-back instability.Considering the fluid flow in multi-branch fractures,a complex hydraulic fracture propagation model is established.The coupling of the flow and fracturing processes is made possible via the crack surface pressure field,and iterative implicit-explicit algorithms and staggered time incrementation is adopted to ensure numerical performance.Combined with the ABAQUS user-defined element UEL function,a user-friendly and generic finite element framework for simulating arbitrarily topological hydraulic fracture is developed.With this simulating method,the influences of fracturing fluid type,insitu stress differences,cemented natural fractures and beddings,anisotropy and heterogeneity in rocks on the hydraulic fracture morphologies are studied.The results show that:（1）For supercritical CO₂ fracturing,a large number of shear failure fractures are generated in the rock,which leads to the formation of complex fractures with multiple branches.The complexity of the fractures increases with the decrease of the insitu stress difference.The simulation results are in good agreement with the experimental results,which proves that the new method is reasonable and the code calculation results are accurate and reliable.（2）When hydraulic fracture interact with the cemented fractures,the fracturing kinking are more likely to happen in small approaching angles and weak cemented strength case.When the cemented strength is strong,the hydraulic fractures is stopped and propagate along the outside of cemented fractures.（3）In the anisotropic rock,when the perforation direction is not parallel to the rock anisotropy direction,shear failure fractures will occur in the anisotropy direction.When the hydraulic fractures propagate in a heterogeneous rock,shear failure occurs when they encounter weak natural fractures,and creates many fracture branches.The hydraulic fractures are trapped and form a ring-shaped fracture network around the injection point.The strong natural fracturs will block the hydraulic fracture propagation and deflect the hydraulic fracture propagating direction.（4）As a result of stress shadow between main fractures,a complex fracture network can be created under the multistage fracturing.The fracture network in simultaneous fracturing is the densest,but the fracture network is limited between the main fractures.Sequential and alternative multistage fracturing will lead to a sparse and wildely spreading fracture network.The flow-coupled cohesive interface framework for simulating complex hydraulic fractures developed in the work can provide a new solution on simulating complex hydraulic fractures,and is of great significance for realizing the efficient development of tight oil and gas reservoirs.