Analytical model for studying the effect of weak bedding plane on wellbore stability

Abstract

Optimum mud weight estimation in wellbore is one of the most important steps to prevent instability. In wellbore stability studies, media (rock) is usually assumed to be isotropic but errors occur when weak bedding planes cause the rocks to be anisotropic. In this study the effect of weak bedding plane in stability of wellbore was studied. Also, the effect of bedding plane parameters on stability of vertical and horizontal wellbore was investigated. Through the use of the geometric relations of bedding plane and wellbore, new equations were presented to calculate the attack angle. Sensitivity analysis on the dip and dip direction of weak bedding plane in the vertical and horizontal wellbore were also performed. On the basis of the porous elasticity theory and Jaeger theories, an analytical model was proposed to analyze the wellbore stability with regard to the dip and dip direction of the weak bedding plane. A code in MATLAB was written based on an analytical model and the effect of the dip and dip direction of the weak bedding plane can be reviewed. By using real data from a wellbore, a comparative analysis was carried out between the new analytical model and the intact rock failure model. Minimum drilling mud weight was calculated in two phases, without weak bedding planes, and with weak bedding planes. Results show that the existence of weak bedding planes causes more instability in the wellbore in some azimuths and deviations. The dip and dip direction of weak bedding planes have a significant impact on the wellbore stability and in the horizontal wellbore according to dip and dip direction, the optimum wellbore trajectory is different. By applying the code, geomechanical engineers can calculate the amount of mud weight based on the dip and dip direction of the weak bedding plane.

Keywords

• wellbore stability
• weak bedding plane
• in-situ stress
• analytical model and determination of mud weight