Assessment of the Stability of Inclined Wells

Abstract

Abstract The need for wellbore stability analysis during the planning stage of drilling is rising due to the economic considerations, and because of the increasing use of highly-deviated, extended-reach, and horizontal wells. Wellbore instability can cause lost circulation when tensile failure occurs, and can cause caving and hole closure when collapse failure occurs. This paper presents the method and results of wellbore stability analysis for three common reservoir lithologies consisting of a consolidated sandstone, a shaly sandstone, and a limestone formation. The effect of stress anisotropy on the mechanical stability of wellbores is evaluated while varying the inclination angle from 0 to 90°, for both the Mohr-Coulomb and the Drucker-Prager failure criteria. The selected failure criterion, and the in-situ rock stress regime are found to have significant effects on the safe drilling fluid density required to maintain wellbore integrity. According to some field examples, the Drucker-Prager failure criterion appears to systematically mimic rock conditions more realistically than the Mohr-Coulomb failure criterion. The simulated consolidated sandstone formation is found more stable with lesser drilling fluid density, at any inclination angle, than the simulated shaly sandstone formation. The simulated limestone formation is even more stable than the consolidated sandstone at all inclination angles since it requires lighter fluid density to prevent wellbore collapse. For all these rock types, the higher the deviation angle (from vertical), the higher the drilling fluid density needed for maintaining wellbore integrity. For the depth and rock conditions simulated, both consolidated and shaly sands are unstable in a strike-slip stress regime, but stable in an extensional stress regime. The simulated limestone formation was found stable in both stress regimes. However, in an extensional stress regime, the limestone formation required lighter fluid density to maintain wellbore integrity than in a strike-slip stress regime. This study provides a valuable insight into the influence of inherent factors such as the in-situ stress regime, pore pressure, and formation properties, and conditionally variable factors such as wellbore directions and inclination angle on wellbore stability. Results of this investigation can be used to improve the management of wellbore integrity, thereby increasing drilling efficiency and reducing drilling costs.