Development and Validation of a New Wellbore Stability Prediction Model for Complex Reservoirs: Application to the Keshen Gas Field in the Tarim Basin

The objective of this paper is to propose a new method for predicting the safe mud density window of drilling fluid in the Keshen gas field, located in the Tarim Basin. The study focuses on applying this method to analyze wellbore stability in the complex and high-pressure environment of the field. The new method incorporates the Mogi-Coulomb criterion, which considers intermediate stress as the shear failure criterion of the wellbore. Additionally, the method accounts for the shear breakdown of rock when calculating the fracture pressure. It predicts the shear slip of the reservoir's weak surface using the Jaeger criterion. Furthermore, it takes into account the weak surface of drilling fluid invasion when calculating the tensile failure of the wellbore. The application of wellbore stability analysis in Well A reveals that the dislocation of natural fractures and the tensile fracture after the invasion of drilling fluid are significant factors contributing to wellbore instability, such as chipping and lost circulation, particularly in formations with developed natural fractures. In the case of a strike-slip stress field, shear sliding is more likely to occur when the fluid invades the crack or weak surface, as opposed to tensile failure. Based on the prediction results of the new method, the mechanism of wellbore instability is interpreted, and engineering countermeasures are proposed to mitigate the risk of wellbore instability. 1. INTRODUCTION The Keshen gas field in Tarim Basin poses significant challenges due to its complex structure, high temperature, high pressure, complicated stress state, and developed natural fractures. These factors contribute to issues such as wellbore instability, a high accident rate, and prolonged drilling periods (Zhang et al., 2015; Cai et al., 2019; Zhu et al., 2014). The primary cause of wellbore instability lies in the disruption of the original mechanical equilibrium near the wellbore, leading to the redistribution and concentration of stress that exceeds the rock's strength once the drill bit penetrates the formation (Nelson et al., 2005; Abbas et al., 2019). The most common manifestations of wellbore instability are collapse and leakage. However, proactive determination of critical mud weights can often mitigate these drilling issues. To achieve this, a constitutive model is employed to estimate the stresses surrounding the wellbore and is complemented by failure criterions. Thus, establishing an accurate and reliable safe mud weights window is of utmost importance.