Comprehensive stability analysis of an inclined wellbore embedded in Colorado shale formation for thermal recovery

Thermal processes have been widely used to recover heavy oil from oil-sands reservoirs, where the overburden shale undergoes cyclic heating and cooling processes. During thermal operations, thermally induced pore pressure can be developed in the shale formation around a thermal well and thermal pore pressure may lead to plastic yielding in the shale. In this study, integrated characterization results on anisotropic thermal-hydro-mechanical (THM) properties of Colorado shale with different clay fractions are presented. Low strength shale intervals along a borehole are identified and the characterized results are used to conduct a stability analysis on an inclined wellbore during thermal operations. The effect of thermally induced pore pressure in Colorado shale on the integrity of a thermal well is investigated using 3D THM-coupled finite element analysis with the consideration of shale's transversely isotropic (TI) elastic characteristics. The results indicate that the wellbore can lose the confinement in high clay fraction shale intervals due to the thermal operation. The mechanisms can be caused by the thermal pore pressure-induced shear plastic yielding behavior in the Colorado shale formation. Tensile fracturing behavior is less likely to happen. The consideration of anisotropy in elastic properties is important in characterizing the shale's stress path in the analysis. Compared with the result based on the TI elastic model, the result based on the isotropic elastic model gives less plastic yielding zone which underestimates the risk.