Strength And Fracture Of Anisotropic Poroelastic Media

Oriented drilling and hydraulic fracturing are two key engineering steps in the shale gas mining project,both of them are happening in the shale rock,which is a highly anisotropic material.At the macroscopic level,shale rocks show the anisotropy of the constitutive model,strength property,and fracture behavior;at the microscopic level,shale rocks consist of the solid skeleton and pores with fluid.This thesis is devoted to the research on strength and fracture problems of the anisotropic poroelastic material,e.g.,shale rocks.In the poroelastic fluid-infiltrated constitutive model,the disconnected pores?i.e.,isolated fluid islands?are considered as parts of the solid skeleton,and the composite consisting of the solid skeleton and the connected pores with fluid,is modeled as a homogeneous medium.The coupling of the deformation and the diffusion of the medium can be analyzed,without regard to the details of the microstructure of the medium.In this thesis,a unified framework of the anisotropic poroelastic fluid-infiltrated constitutive model is constructed,and the number of independent material constants is clarified.The influence of disconnected pores with fluid inside,i.e.,the fluid islands,which results in the inhomogeneity on solid skeleton,is also discussed.After comparing different basic assumptions introduced in the previous poroelastic fluid-infiltrated constitutive models,four levels of assumptions are formulated.It is proved that all the differences between the four levels of assumptions consist only in a particular independent material constant C_CH.Several measurable and unmeasurable material constants are lumped together in different ways in the expression of C_CH.Based on the anisotropic poroelastic fluid-infiltrated constitutive model,the borehole drilling problem is also discussed in this thesis.The time-varying solution for all the domain?a?r<??is given and three particular sets of?instant t,location r?are chosen as critical for strength check,namely:the instantaneous solution?t=0⁺,r=a?,the short-time solution?t?0⁺,r?a?,and the long-time solution?t??,r=a?.Two types of tensile failure and six types of shear failure are checked,and corresponding critical borehole pressures,failure time and locations are obtained.All the safety conditions for the applied borehole pressures are given in linear algebraic expressions,which are convenient for field engineers.After comparing with the results based on the classical Hooke's constitutive model,it is found that poroelastic fluid-infiltrated constitutive model in the borehole safety checking is necessary,and the classical Hooke's elastic model,if used in the drilling engineering,may lead to incorrect results?possibly unsafe?.Besides the anisotropic constitutive model,the fracture toughness of shale rock can also be anisotropic.Theoretical and numerical analysis on crack path prediction in such materials with anisotropic fracture toughness is shown in this thesis.A weak plane model is adopted to characterize the fracture toughness property of the shale rock with its fracture toughness of a bedding plane lower than the other directions.By using the maximum energy release rate?MERR?criterion,the crack deflecting direction is predicted theoretically in this thesis,and the forbidden area of crack path phenomenon is discussed.The weak plane model is also embedded in the extended finite element method?XFEM?for numerical modeling the crack path.To model the crack path precisely,a mesh independent piecewise linear crack model is developed and embedded in the XFEM.Some numerical examples are given by XFEM,which show the extendability of the algorithms introduced in the thesis.The periodically oscillatory crack paths are also studied numerically.