Wellbore Survey and Drilling Margin Optimization for Stress Change and Rotation Around Depleted Reservoirs

Abstract

Uniaxial models are commonly used to describe the change in magnitude of in-situ principal stresses around depleted reservoirs. The accuracy of these models is assessed in this paper through a reservoir-scale analytical solution for the stresses induced by depleting a disk-shaped reservoir embedded within an infinite elastic model of the subsurface. The Hankel-Fourier transform expressions of axisymmetric Green’s functions for a ring-shaped eigenstrain in an infinite elastic medium are used to develop the solution. This solution is validated against numerical simulation results of COMSOL®. The solution is next used to obtain the safest deviation survey, as well as the corresponding drilling margins of infill wellbores that are drilled in the general vicinity of depleted reservoirs. Inside and outside of a reservoir subjected to different faulting regimes are considered for the wellbore-scale stress analysis. Results indicate that despite their common use, the uniaxial models may return significantly erroneous predictions of the in-situ stress state in depleted reservoirs. In particular, the in-situ stress changes are found to strongly depend on the geological azimuth and radial distance from the reservoir center point. Consequently, the uniaxial models may fall short in accurate prediction of the shear or tensile failure gradients of infill wells. These errors would be more substantial for wells that are closer to the reservoir flanks.