Parametric sensitivity study of wellbore stability in transversely isotropic medium based on polyaxial strength criteria

Abstract

Borehole instability in laminar formations often occurs during drilling operations without a clear explanation. Two dominant factors, rock lithology and stress concentration effects on borehole instability, are investigated in this study. The microstructure and mineral composition are evaluated to determine the possible reactions of the drilling fluid and shale interaction. A borehole stability model in a transversely isotropic medium is established using three different types of polyaxial strength criteria to study the effect of the degree of elastic anisotropy and intermediate principal stress on the lower boundary of the safe drilling window. Research has indicated that Poisson's ratio anisotropy does not markedly change the collapse pressure at low anisotropy. However, the effect of the elastic modulus anisotropy is notable, and the collapse pressure predicted by polyaxial criteria is more sensitive to elastic anisotropy. The collapse pressure calculated by the Mohr–Coulomb criterion is the upper limit; the results predicted by the Modified Lade and Modified Wiebols–Cook criterion are similar, corresponding to the wellbore orientations. This study explores the failure mechanisms of wells drilled in layered shales, which can help determine critical mud pressure and well orientation design. The outcomes of this investigation indicate the implications of fully accounting for the degree of elastic anisotropy and the influence of intermediate stress in conducting wellbore stability analysis.