A Mud Weight Window Investigation Based on the Mogi-Coulomb Failure Criterion and Elasto-Plastic Constitutive Model

Abstract

A successful stable borehole scheme results in saving time and money. To reach this aim many parameters such as bottom hole pressure, and stress distribution around the borehole must be considered in the design of a well. During drilling there are two main wellbore stability problems, namely, borehole collapse and fracture. These stability problems can lead to the need for fishing, stuck pipes, sidetracking, and lost circulation. These drilling problems can be mitigated or often eliminated by proper determination of the critical mud pressure.

The most common approach for these studies is a linear elastic and isotropic constitutive model in conjunction with linear failure criteria like Mohr-Coulomb. Numerous researchers have found situations in which the Mohr-Coulomb criterion was deficient because mid-principle geotechnical stress is not incorporated. In addition, the conventional mechanical model is based on the linear elasticity condition, while in reality, it is believed that the fluid barrier and a part of the bore hole wall behave plastically, which provides higher fracturing pressure than Kirsch's equation.

In this article, the mud window is estimated using Mogi-Coulomb failure criterion and the elasto-plastic model. This is based on a hypothesis that wellbore behavior indeed is not strictly elastic and intermediate principle stress plays an important role in rock strength. Therefore, an elasto-plastic model and the Mogi-Coulomb failure criterion should be incorporated. This hypothesis is verified and used in this article for the South Pars gas field (phases 6, 7, and 8) in the Persian Gulf. The results indicate that the newly developed model is more accurate than the conventional approach.