Porochemothermoelastic solutions considering fully coupled thermo-hydro-mechanical-chemical processes to analyze the stability of inclined boreholes in chemically active porous media

Abstract

The present paper presents a fully coupled thermo-hydro-mechanical-chemical (THMC) model for chemically active deformable porous media (such as clay or clay-like shale) under local thermal and chemical equilibrium (LTE and LCE) conditions. The proposed model is developed by combining the phenomenological method and the new entropy production derived based on the non-equilibrium thermodynamics. Our proposed THMC model introduces the following new coupled terms: (1) ultrafiltration effects in the solute flux equation, (2) thermal filtration and Dufour effects into the heat flux expression, and (3) strain, pressure, and solute mass-fraction in the thermal diffusion equation. This model also introduces the influence of bound water that occupies the interlayer space of clay platelets and governs the chemical and mechanical responses as the solid part of porous media. We derive semi-analytical solutions for the specific engineering application of stability of an inclined wellbore in a chemically active clay-like shale formation during petroleum drilling, considering the fully coupled porochemothermoelastic effects under the plane strain assumption. The wellbore loading is decomposed into axisymmetric and deviatoric cases. The time-dependent field variables including displacements, pore pressures, stresses, solute fraction, and temperatures are obtained by performing the inversion of the Laplace transforms. Based on the newly developed semi-analytical porochemothermoelastic solutions, several analyses are conducted to evaluate the influence of the newly introduced terms in the THMC model on pore pressures and stresses around a horizontal wellbore in a shale formation; and on time-dependent failure behavior, including shear failure and tensile fracturing. Results show that the significance of the newly introduced ultrafiltration, thermal filtration, and Dufour effects on induced pore pressure not only depends on the permeability, thermo-osmotic, and thermal diffusion coefficients, but also on the in-situ pressure, temperature, and chemical gradients. We suggest that the drilling engineers consider the time-dependent and location-dependent characteristics of borehole failure that depend on the thermo-hydro-mechanical-chemical process and appropriately design the mud weight required to ensure wellbore stability.