Wellbore stability analysis of layered shale based on the modified Mogi–Coulomb criterion

Abstract

Borehole instability was frequently encountered during shale gas drilling. Most conventional models are not applicable to layered formation's wellbore stability analysis on account of anisotropic strength characteristic. In this study, an empirical equation for predicting anisotropic strength was implemented in the Mogi–Coulomb criterion to describe variations of cohesive strength and friction angle of shale formations. A collapse pressure model and its appropriate solution method for layered shale formations were proposed. The impact of different strength criteria and rock anisotropy type on rock strength and collapse pressure was investigated. The analysis indicated that the predicted strength of our modified criterion was usually higher than the weak plane failure criteria. The collapse pressure calculated by the modified Mogi–Coulomb criterion was lower than the weak-plane failure criteria. Furthermore, it was more consistent with real mud weight. Additionally, the anisotropy type of rock notably influences wellbore stability. More significant anisotropy coefficients correspond to higher strengths, which results in smaller collapse pressure values. Improper anisotropy coefficients can over- or under-predict the collapse pressure. Reasonable estimates of collapse pressure of anisotropic rocks can be made through the modified Mogi–Coulomb criterion using limited experimental data and the anisotropy rock type.

Keywords

• Shale
• Wellbore stability
• Collapse pressure
• Anisotropy