Quantitative Risk Assessment of Wellbore Collapse of Inclined Wells in Formations with Anisotropic Rock Strengths

Shale is a typical sedimentary rock that exhibits anisotropic strength characteristics owing to the presence of planes of weakness, such as bedding planes and joints. However, conventional quantitative risk assessment (QRA) models for wellbore stability often assume that shale is isotropic, leading to inaccurate predictions of the risk of wellbore collapse. This study proposes a novel QRA model of the wellbore collapse risk in inclined wells that considers the combined effects of anisotropic rock strength and parameter uncertainty. First, the semi-analytical model of wellbore collapse was revisited using various coordinate system transformations, the classical Bradley's solution, and a single plane of the weakness criterion. Second, based on the reliability theory, a QRA method of wellbore collapse was established for inclined wells in formations with anisotropic rock strengths. Third, the K-S test and normal information diffusion (NID) method were used to determine the uncertainty of the logging interpretation results. The key parameters with significant uncertainty were obtained through sensitivity analysis. Finally, Monte Carlo simulation was conducted to quantitatively evaluate the risk of wellbore collapse. The results show that when the anisotropic rock strength is considered, the assessment of the equivalent mud weight (EMW) of the collapse pressure is more accurate. The failure of vertical wells primarily occurred within the rock matrix, whereas that of inclined and horizontal wells predominantly occurred along the planes of weakness. The pore pressure, Biot coefficient, and in-situ stress are the parameters with the greatest influence on wellbore stability. When considering the uncertainty of the weak plane occurrence, the collapse pressure EMW for inclined and horizontal wells significantly increased. The NID method yielded a more precise quantification of the parameter uncertainty compared with the normal distribution. The novel model proposed in this study incorporates the anisotropic rock strength and parameter uncertainty, resulting in more consistent predictions under actual conditions. This model and analysis provide a crucial theoretical foundation for drilling trajectory and drilling mud weight optimization to maintain wellbore stability of horizontal wells in shale formations. A quantitative risk assessment method for wellbore collapse was proposed for inclined wells in formations with anisotropic rock strengths.The combined effects of anisotropic rock strength and parameter uncertainty were considered in this study.It is imperative to consider the impact of the uncertain properties of the plane of weakness when analyzing the stability of inclined wells.The parameters with the greatest influence on the stability of inclined wells are pore pressure, Biot coefficient, and in-situ stress.The normal information diffusion method yielded a more precise estimation of the parameter uncertainty in comparison to the normal distribution.