Assessment of the Mechanical Stability of High-Angle Wells in Block 22/11, Nelson Field Discovery

Abstract

Summary This paper summarizes the theory of using a practically oriented simulator to assess the mechanical stability of a wellbore in a triaxial, linearly elastic stress field. The model can be used to determine the range of mechanically stable well inclinations in a given formation and to produce mud-weight programs tailored to efficient and safe drilling, even in difficult conditions. In the Nelson discovery in Block 22/11 of the North Sea, the model was used to evaluate mechanical instability experienced during drilling of deviated appraisal wells through overpressured Oligocene shale. Simulation results showed that, within a range of drilling-mud densities, wells of any orientation can be drilled through the shale. However, practical considerations of formation heterogeneity, swab and surge pressures, and possible chemical interaction dictate that inclined wells beyond about 65° should not be attempted until more field data are acquired to enhance confidence in the use of the model in Block 22/11.