Abstract
The shale reservoir serves as the primary driver of global oil and gas production growth. However, during the exploitation of these reservoirs, frequent instability in shale formations often leads to complex situations and significant nonproductive time. The underlying mechanisms of shale formation instability are not yet fully understood. Shale’s distinctive transverse isotropy, resulting from its depositional processes, differentiates it from other lithologies. To explore the mechanisms of instability, we perform triaxial compression experiments on shale samples under varying bedding dip angles and confining pressures. The experiments reveal shale’s strength anisotropy properties, and an anisotropic strength criterion is established using the least squares method. Based on this, a wellbore collapse pressure prediction model incorporating elastic and strength anisotropy is developed. The model is used to study the effects of elastic modulus and Poisson’s ratio anisotropy on the stress distribution around the wellbore. In addition, differences in collapse pressure predictions using three strength criteria are analyzed. The results indicate that the single plane of weakness (SPW) and Pariseau criteria exhibit significant prediction errors for layered rock strength, whereas the plane of patchy weakness (PPW) criterion provides more accurate predictions of anisotropic rock strength. Poisson’s ratio anisotropy has a minimal effect on tangential stress around the wellbore compared with elastic modulus anisotropy. The stress distribution is highly sensitive to variations in elastic modulus anisotropy. This, combined with Poisson’s ratio anisotropy, amplifies stress concentrations around the wellbore. Among the evaluated strength criteria, the SPW criterion tends to overestimate collapse pressure, whereas the PPW criterion underestimates it. In contrast, the Pariseau criterion provides predictions that fall between the two extremes, the PPW criterion is considered the most reasonable overall. These findings enhance the accuracy of wellbore safety predictions and the determination of optimal drilling fluid density in shale formations, aiding in the mitigation of complex subsurface challenges and improving operational efficiency.
Paper Information:
Gao, X., Wang, M., Shi, X., Dai, P., Zhang, M., & Ramezanzadeh, A. (2025). Effect of anisotropy on wellbore stability based on transversely isotropic strength criteria in shale formation. Geophysics, 90(5), D101-D110.
