Thermo-poro-elastic analysis: The effects of anisotropic thermal and hydraulic conductivity on borehole stability in bedding formations

Abstract

Laminated or layered structures exist widely in shales, which resulted in costly and laborious wellbore unstable problems. Due to the existence of bedding structures, not only the strength of rocks (like shale) is proved to be anisotropic, but also the hydraulic and thermal conductivity are tested to be anisotropic by many experiments in the literature. However, these effects are seldom illustrated by former studies when building a wellbore stability model. Aiming at the anisotropic conductivity effects (thermal and hydraulic), we derived a new model of wellbore stability: the effective thermal and hydraulic conductivity coefficients around the wellbore are first obtained based on coordinates transformation; the temperature and stress distributions in the surrounding rocks are then calculated according to thermoporoelastic theory; the failure area, the collapse and fracture pressures under different conductivity conditions are calculated and compared considering both the failure of bedding planes and rock matrix. The results show that the distributions of temperature and stresses around the wellbore, the collapse area, the collapse and fracture pressures calculated are quite different both in radial and circumferential directions under different conductivity (thermal and hydraulic) conditions. The effects of anisotropic conductivity condition (ACC) can evolve with time and at certain times these effects can be significant, which makes the consideration of ACC quite necessary. The thermoporoelastic solutions, incorporated with the effects of anisotropic conductivity, may lead to more accurate results of inclined wellbores when encountering bedding shale in drilling practice.