Case study of wellbore stability evaluation for the Mishrif Formation, Iraq

During drilling operations for the E oilfield in the Mishrif formation in southern Iraq, stuck pipe has been identified as a significant geomechanical problem for several wells. In this study, a 1-D mechanical earth model (MEM) of the Mishrif formation is compiled based on its state of stress and rock strength parameters, and is utilized to assess the contribution of borehole collapse leading to the stuck pipe problems. The results of this study show that wells characterized by stuck pipe are drilled along azimuths which promote wellbore collapse. Three different failure criteria, the Mohr-Coulomb, Mogi-Coulomb, and Modified Lade rock failure criteria, are investigated in order to determine feasible drilling trajectories (i.e. azimuths and inclinations) and mud pressure conditions for many different wells in the Mishrif Formation. If a specific azimuth for a well cannot be altered, an optimum inclination is recommended to reduce the severity of the borehole collapse. However, as the intermediate principal in-situ stress increases the optimum drilling inclination progressively changes. The presented study shows that 1-D MEMs are an important tool to both assess and address existing wellbore stability problems and to provide guidance for future well plans for better drilling efficiency by reducing non-productive time.