Optimal mud pressure design using nonlinear failure criteria for wellbores in shaley sedimentary reservoir

Abstract Wellbore drilling disturbs the equilibrium stress state in the rock mass, resulting in stress redistribution around the opening. Wellbore stability in the altered stress state is vital for engineering applications, as the wellbore instability results in cost overrun. Accurate estimation of rock mechanical properties, in‐situ stresses, and required mud pressure is crucial for safe drilling. If the mud pressure is lower than required, the shear failure of the rock takes place, and conversely, if the mud pressure is higher than the upper limit, tensile failure occurs. A strength criterion that can accurately predict the mud pressure may help significantly reduce non‐productive time and cost in well drilling. The commonly used Mogi‐Coulomb (MGC) failure criterion for estimating critical mud pressure neglects the few fundamental aspects of rock failure characteristics observed in the laboratory, such as nonlinear strength response in major‐minor principal stress space and prediction of multiple failure stress values near the triaxial axial extension boundary. The present study uses the Modified Mohr‐Coulomb true‐triaxial failure criterion (MMC_TT), which predicts the strength of rock better than the MGC in laboratory true‐triaxial tests to overcome the limitations. Moreover, based on the data from previously published five vertical wells in the Krishna‐Godavari basin (K‐G basin), an empirical relationship is proposed to obtain the strength parameters for the MMC_TT criterion for shaley sedimentary rocks as the existing correlations do not cater for the parameters required for MMC_TT criterion. The comparative study of the MMC_TT with MGC, Modified Mohr‐Coulomb triaxial, and Mohr‐Coulomb failure criteria, showed that for most of the K‐G basin wells, the MMC_TT criterion predicted close to the MGC. However, for well‐13, the MMC_TT criterion results are closer to the mud pressure used in actual drilling than the MGC.