Wellbore stability analysis for an extended-reach well on Ekofisk

Abstract

The drilling of extended-reach wells is an increasingly common practice to reduce costs in exploration and development of mature fields. Extended-reach wells are technically challenge, as high inclination trajectories may predispose to borehole instability. This thesis studies an extended-reach well drilled in the Ekofisk field in the North Sea where borehole stability issues were observed and eventually resulted in the loss of the well. A wellbore stability assessment is performed with well-specific stress and formation strength data that explores the possible failures the well may have suffered from. Uncertainty is propagated both in rock strength and anisotropy to generate a mud window that acknowledges the variability present in geomechanical data using a Monte Carlo approach. It is observed that three parameters from the plane of weakness present a high degree of uncertainty: cohesion, friction factor and orientation. The impact this parameters have in the failure along the planes of weakness pressure is studied. A safe mud window generated with a stochastic approach is presented. Acknowledging for uncertainty and the failure along the weakness planes in extended-reach wells to be drilled in Ekofisk may generate safer mud windows that in turn reduce the occurrence of wellbore instability in the field.