Numerical Simulation Of Hydraulic Fractureing In Shale By Using Cohesive Finite Element Method

Cohesive finite element method(CFEM)is an efficient fracture simulation method.It discretizes the interesting body with the bulk triangular elements.The interface is setup at the co-edge of two neighboring elements.The bulk element is characterized by the linear elastic constitutive relation while the interface by the cohesive law.The cohesive law contains both the strength and the fracture criterion of material,which makes the CFEM free of the separate fracture criterion.The fracture is only allowed to grow along the interface network.This makes the CFEM free of the mesh modification.Shale contains distributed directional bedding planes,which make the shale transverse isotropic.To model shale with consideration of bedding plane,the interfaces that are aligned with the bedding plane are assigned the bedding plane cohesive law while the rest interfaces are assigned the matrix cohesive law.By this method,the anisotropy of shale can be well represented.The simulation results demonstrate that the weaker the interface is,the more significantly the interface impacts on the fracture propagation.In the hydraulic fracturing process,the hydraulic field and the mechanical field are coupled together in the fracture.On one hand,the fluid in the fracture exerts pressure on the fracture faces.The fluid pressure can widen the fracture,which significantly enhances the fracture permeability.On the other hand,the increased permeability in return affects the hydraulic field.To describe this coupling process,the fully coupled hydraulic-mechanical coupled equation of the interface element is derived,which is verified by the KGD model.This method is used to simulate a hydraulic fracture experiment of shale with strong bedding planes.The simulation results show that the propagation of hydraulic fracture is strongly influenced by the bedding planes.Moreover,with the in-situ stress difference decreasing,the hydraulic fracturing process can produce more fracture branches due to the activation of the bedding planes.The traditional CFEM(or global CFEM)set up the interface elements in the whole interesting domain,which usually leads to the low computational efficiency and the stiffness reduction problem.To address these problems,a local-CFEM method is proposed in that the interfaces are only generated in the possible fracturing zone instead of the whole domain.At the handshaking area between the local-CFEM and the normal FEM zone,there exist special interface elements.Such special interface element has three nodes,which is termed as the triangular interface element.The mechanical and the hydraulic matrices of the triangular interface element are derived.Compared with global CFEM,the efficiency of the local CFEM has been significantly improved due to that the interface number is considerably reduced.Meanwhile,the stiffness reduction problem of the global CFEM is also relieved to great extent.The local-CFEM is more likely to be extended to the large scale computation due to its efficiency.This thesis extends the CFEM to the hydraulic fracture simulation of shale with consideration of the bedding plane effect and the hydraulic-mechanical coupling effect.The proposed local-CFEM can significantly improve the computation efficiency and relieve the stiffness reduction problem of CFEM.It provides an effective and efficient approach to the hydraulic fracture simulation in shale.