Study Of Mechanical-chemical-thermal Coupling Effect On Wellbore Stability In Anisotropic Shale Formation

Shale oil and gas,which is a kind of nonconventional resources,is receiving growing attention at home and abroad in these years.However,wellbore instability problems have significantly affected project progress and cost during drilling operation,so it is necessary to get wellbore instability mechanism and law based on experiments study and theory analysis.Lab experiment were done for shale from downhole in Chongqing district,these experiments includes structure and component analysis,physical test and rock mechanical experiments.Thin-section analysis and scanning electron microscope(SEM)observation suggest that the bedding architecture is obvious for shale,so the shale can be regarded as transversely isotropic(TI)material.Brittle mineral takes a large part of shale,the porosity and permeability is very low,hydration swelling experiments states that hydration capacity of shale is weak too.In order to get elastic parameters and strength along and perpendicular to the bedding plane respectively,Shale cores are drilled with different angle comparing to bedding plane.The mechanical and chemical effect between shale and drilling fluid are taken into consideration,mechanical-chemical coupling model are developed for anisotropic shale and governing equations are discretized to apply FEM.Mechanical-chemical coupling experimental apparatus was designed to verify the model and permeability and solute diffusivity coefficient along and perpendicular to the bedding plane are got by FEM inversion.The mechanical-chemical coupling model is extended to mechanical-chemicalthermal model by including thermal effect of shale and drilling fluid,discretized governing equations are developed in user-defined element(UEL),coordinate transformation method is applied and one layer elements is enough to simulate wellbore and shale formation according to generalized plane strain theory.Pore pressure and effective stresses around well are in agreement with analytical solution,this method could greatly reduce FEM computation comparing to normal three-dimensional(3D)model.Pore pressure and effective stresses are analyzed by changing temperature and chemical boundary condition,thermal and chemical parameters,elastic and material parameters anisotropic ratios,the results show that fluid migration can be restrained by reducing temperature and rising solute mass fraction(SMF)of drilling fluid.Thermal coupling diffusion coefficient in fluid transport equation and chemical refection coefficient,Young’s modulus and permeability anisotropic ratios play an import role in pore pressure and effective stresses distribution,instead,Poisson’s ratio nearly has no effect on them.Strength of anisotropic shale varies continuously according to rock mechanic experiment and Weak plane model cannot describe the character correctly.Cohesion and internal friction angle change with bedding plane dip angle,so a 3D strength parameters model is developed to make Mohr-Coulomb strength criterion suitable for shear failure of anisotropic shale.The analysis of mud weight,temperature and SMF on wellbore failure is applied and the results state that there are three kinds of failure: shear failure,radial tensile and circumferential tensile failure.Lower mud weight and SMF,higher mud temperature could cause shear and radial tensile failure,which could cause the well collapse;higher mud weight and lower mud temperature lead to circumferential tensile failure and drilling induced fractures(DIF)may happen.Wellbore stability analysis is done by taking a high temperature well as an example,the solution suggests that the cooling effect of circulating mud plays a key part in DIF formulation.The anisotropic mechanical-chemical-thermal coupling model and 3D MohrCoulomb strength criterion will be favorable for guiding drilling operation and improving wellbore instability problem.