Wellbore stability analysis of inclined wells in transversely isotropic formations accounting for hydraulic-mechanical coupling

Abstract

As the main hydrocarbon reservoirs, sedimentary formations are usually anisotropic in elasticity, strength, and permeability. Traditional wellbore stability analysis assumes that the surrounding rocks are isotropic and elastic, whereas some analytical and numerical methods assume that the surrounding rocks are anisotropic in both elasticity and strength. Since the anisotropy of permeability is usually ignored, the wellbore instability mechanism in anisotropic formations may not be identified. Therefore, based on the poroelasticity theory and the assumption of generalized plane strain, this study proposed a pseudo-3D coupled hydraulic-mechanical finite element model by considering the anisotropies of elasticity, strength, and permeability. Taking the L203-x well as an example, the elastic and permeability anisotropies and the wellbore trajectory and stability in any bedding plane directions were analyzed parametrically. Results showed that the model simultaneously considering the strength and elastic anisotropies was in line with the actual hole enlargement rate of the L203-x well. With the increased anisotropy index of the elastic parameters, the anisotropic induced pore pressure is opposite to that of isotropic situation on the 2D cross-sectional plane of the wellbore, and the effective stress increased and was deflected at a certain angle. The effective hoop stress was biased toward the direction of higher stiffness, and the shear failure area that grew smaller gradually decreased. With the increased anisotropy index of permeability, the pore pressure rapidly increased along the direction parallel to the bedding plane, the effective radial stress of shape became rectangular along the bedding direction, the high-stress concentration zone of effective hoop stress changed from four to two, and the shear failure area grew smaller. The relative importance order of the anisotropy influences on the wellbore stability was thus: strength anisotropy > elastic parameter anisotropy > permeability anisotropy. A comparison of the well inclination and bedding plane orientation showed that, when the relative angle between the well axis and bedding normal was small, the influence of the bedding plane declined. In practical drilling engineering, avoiding drilling parallel to bedding planes is conducive to borehole stability.