Evaluation model and application of wellbore stability based on an improved three-dimensional Hoek-Brown strength criterion

In drilling engineering, the analysis of wellbore stability is directly related to the design of wellbore structure, the design of drilling fluid density window and the safety of drilling operations. Traditional strength criteria, due to mostly linear assumptions and the neglect of the intermediate principal stress effect, are difficult to accurately characterize the nonlinear failure characteristics of rocks under deep and complex geological conditions. Therefore, the selection of an appropriate strength criterion is the key to accurately evaluating wellbore stability. This paper uses an improved three-dimensional Hoek-Brown strength criterion and combines theoretical research with model application to establish a wellbore stability evaluation model that is more in line with actual working conditions. The research results show that this criterion not only retains the nonlinear characteristics of the Hoek-Brown strength criterion in the meridian plane, but also considers the influence of the intermediate principal stress on rock strength. The yield surface of the improved three-dimensional criterion in the principal stress space is a conical surface, and the yield curve satisfies the properties of being outwardly convex, smooth and closed. Through the comparative analysis of the true triaxial test data of six different rocks, it is proved that this criterion has good applicability to various rocks. When this criterion is applied to the wellbore stability analysis of the Jialingjiang Formation limestone strata in the Tongluoxia Gas Field in the eastern Sichuan Basin, the calculated collapse pressure equivalent density is 1.16 g/cm³. When drilling along the direction of the maximum horizontal principal stress, the collapse pressure equivalent density is the lowest, which is in line with the actual working conditions and is superior to the other four traditional strength criteria. The improved three-dimensional Hoek-Brown strength criterion provides a more accurate and practical method for wellbore stability analysis.