Wellbore Stability of Deviated Wells in Depleted Reservoir

Abstract

Abstract Reservoir depletion alters formation pressure, far-field in-situ stresses, and stresses around the wellbore. All of these factors influence collapse and fracture gradients, and therefore the drilling mud weight window. For deviated wells in depleted reservoirs, this situation is even more complicated due to the effect of well inclination and azimuth on the mud weight window. Analytical model results are presented in this paper for mud weight window calculations, considering formation pressure depletion, initial in-situ stress ratio (SHmax/Shmin), well inclination and azimuth. In-situ stresses change due to reservoir depletion are first determined from poroelastic theory, assuming the reservoir experiences vertical deformation, but no deformation in the lateral directions. Next, far-field in-situ stresses after depletion are transformed to a deviated wellbore with arbitrary well inclination and azimuth. Then, utilizing the Mohr-Coulomb shear failure criterion and tensile strength failure criterion, collapse and fracture gradients are calculated for various ratios of initial horizontal stress, degree of depletion, wellbore inclination, and azimuth. Finally, the sensitivities of collapse and fracture gradients relative to these factors are compared and discussed. The most problematic situation is to drill highly deviated or horizontal wells in reservoirs with large depletion along the maximum horizontal stress direction. In this case, fracture gradient can reach extremely low levels, resulting in very narrow drilling window and very small margin for mud weight adjustment. Drilling along or near the direction of minimum horizontal stress is preferred. An interesting finding in this study is that fracture gradient does not always decrease for deviated wells, which is contrary to common understanding that fracture gradient always decreases as the pore pressure gradient declines.