The Research On The Mechanism Of Wellbore Stability In Shale Formation With Dual-Porosity And Dual-permeability

The stress field,which the rock around a borehole subjects to,will redistribute during the drilling process,since it generates the stress difference(pore pressure difference)before and after the excavation.Especially for low permeability formation,pore fluid can not immediately escape and then produce excess pore pressure due to the resistance to the deformation of solid matrix,under the instantaneous change of external load.Owing to the effect of excess pore pressure,the effective stress field will change and naturally affect the changes of petrophysical parameters,which determine the seepage characteristics.Therefore,it is necessary to introduce the solid-fluid coupling effect into the wellbore stability analysis for the low permeability formation.The fractured shale formation has two sets of coupled seepage systems of matrix and fractures,and hence the solid-fluid coupling effect in dual-porosity shale is more complex than of the single-porosity case.Furthermore,correctly predicting the perturbed stress field after bored is key to objectively evaluate the key mechanism of wellbore stability.To study on the mechanism of wellbore stability in fractured shale,the fractured shale were conceptualized as two separate and overlapping continua: matrix and fractures,and each has its own mechanical characteristics and physical parameters.Namely,it is a medium with dual-porosity and dual-permeability.Under the framework of the poroelastic theory accounting for the fluid-solid coupling effect:(1)Based on the phenomenological method,the mathematical expressions of the poromechanics parameters appearing in the poroelastic constitutive equation,which describes the elastic deformation of the dual-porosity medium,are determined from the combined experimental and theoretical analysis at the micro-level.(2)On account of the system macroscopic approach,utilize the conservation equations including mass,momentum and energy and entropy production theory to establish different forms(including local thermal equilibrium and local thermal non-equilibrium theory)of Clausius-Duhem inequality for Dual-porosity and Single-porosity models.The Clausius-Duhem inequality is used to derive the three-dimensional anisotropic analytical solutions,which only explain the case that the direction of borehole axis coincides with the normal direction of homogeneous surface of transversely isotropic medium,about the field variables around arbitrary deviated borehole under non-hydrostatic stress field in the fractured shale formation.The solutions are applicable to the coupled thermal-hydraulic-mechanical model and coupled chemical-hydraulic-mechanical case.Moreover,the anisotropic solutions can be reduced to the isotropic solutions by a simplified model,and are divided into different forms based on the permeable boundary and non-permeable boundary conditions.The derivation process holds under the assumption of plane strain condition.In addition,this paper also derives the analytical solutions of single-porosity model corresponding the abovementioned coupling models to facilitate comparative analyses of different models.The results from this paper mainly show that(1)the coupled thermal-hydraulicmechanical model for Dual-porosity(or Single-porosity)medium: The thermos-osmosis effect will induce the free water of the drilling mud into the pore space of shale under permeable boundary condition,and cause the pore pressure build-up on condition that the thermal osmotic coefficient is significantly large and wellbore is cooled.While the wellbore is heated and the thermal osmotic coefficient remains significantly large,it will produce backflow to pull the pore fluid out of formation and hence shield against wellbore failure,since the effective stresses are increased resulting from the reduced pore pressure.(2)the coupled chemical-hydraulic-mechanical model for Dual-porosity(or Singleporosity)medium:the transverse fracturing caused by the effective tensile axial stress will appear nearby the wellbore,when it overemphasizes the backflow effect arising from the higher concentration value of drilling mud than that of the pore fluid.When predicting the shear failure of fractured shale formation,the Single-porochemoelastic model is more conservative than Dual-porochemoelastic case.