Thermal-hydro-mechanical coupled dual-medium model of inclined wellbore in fractured anisotropic formations

Abstract

The deep shale formation exhibits anisotropic and fractured properties. Previous models of shale wellbore stability have primarily focused on fractured or mechanical anisotropies of shale. Furthermore, thermal effects are inevitably considered when drilling deep shale formations. Nevertheless, the instability mechanism of a wellbore under the combined effects of anisotropy, fractures, and thermal-hydro-mechanical coupling is unclear. Thus, based on the assumption of generalized plane strain, anisotropic porothermoelastic theory, and dual-porosity medium theory, this study established a thermal-hydro-mechanical coupled dual-porosity medium model for inclined wellbore considering complete material anisotropy. The finite element formulation was employed to solve this model. Parametric analysis was performed to investigate the effect of dual-porosity medium properties and material anisotropy parameters on effective stress, fracture pore pressure(p^II), and matrix pore pressure(p^I). Through model comparison, the effective stress, pore pressure, and failure zone were observed to be completely different from those of the traditional elastic isotropic dual-porosity medium model and elastic anisotropic single-porosity medium model when subjected to the combined action of influence dual-porosity medium and anisotropy. With the elastic anisotropy index increases, the elastic anisotropic p^I is smaller than the elastic isotropic p^I. The effective stiffness of the rock increases with the elastic anisotropy index, which leads to the generation of ‘negative’ thermal stress, reduces the effective radial stress and hoop stress. When the well inclination exceeds 60°, the evolution of the induced p^I in elastic anisotropy is significantly different from that in elastic isotropy in the X direction, but p^II in is not sensitive to the change of well inclination. When a horizontal well is drilled parallel to the bedding direction, the risk of wellbore shear failure will be reduced for a higher ratio of anisotropy in elasticity, solid thermal expansion, and permeability.