Selection of failure criteria for estimation of safe mud weights in a tight gas sand reservoir

Abstract

In an undisturbed state, a stress field present in the earth consists of virgin stress in the rock. A change in this stress occurs due to drilling the borehole and disturbing the original stress by either introducing drilling fluids or by getting reservoir flow in the borehole. This change in stress leads to a classical problem of well bore instability. To resolve wellbore instability, evaluation of rock mechanical properties and in-situ stresses is essential. The output of this study is evaluation of safe mud weights which is critical to safe drilling. If the mud weight used for drilling is higher than those predicted by the failure criteria, the mud percolates into the formation, causing tensile failure (fracture stress) which induces fluid losses. Conversely, the lower mud weight results in shear failure (collapse stress) of rock leading to borehole breakout and collapse of wellbore. Three types of failure criteria i.e. Mohr-Coulomb, Mogi-Coulomb and Modified Lade are considered in this study. The stress around the wellbore is obtained using Kirsch equation. The input parameters required for estimating the safe mud weights from these failure criteria are rock mechanical properties, friction angle and cohesion which are estimated from well log data. The rock mechanical properties like Poisson's ratio and Uniaxial Compressive Strength are computed using sonic derived compressional and shear velocities. Internal friction angle is obtained using gamma ray log and is validated with core data before it is used in further analysis. The failure criteria have been applied to two wells located in a field in Gulf of Oman. The analyses show that the Mohr-Coulomb overestimates the mud weight while the Modified Lade criterion underestimates it. The result for a tight gas sand reservoir suggests that the Mogi-Coulomb predicts better mud weight values that are in agreement to those measured values chosen for drilling.