Fluid production from oil/gas reservoirs perturbs the local stress state, both through variations in pore pressure and also changes in the local total stress. In stress sensitive reservoirs, these changes in the stress state cause deformation in the solid part of the rock which, in turn, affects the fluid-flow properties and, consequently, the reservoir productivity. Different reservoir conditions and production scenarios yield different degrees of reservoir productivity reduction. Therefore, a model that can be applied to study the effect of fluid flow/ deformation processes on the productivity of stress-sensitive reservoirs is essential for optimum reservoir management.

This paper presents a 3D finite difference, fully implicit model to simulate the physical phenomena occurring during the production from reservoirs with stress-sensitive mechanical and fluid-flow properties. The model considers two different physical domains: (i) an inner porous domain representing the reservoir, where fluid-flow and rock deformation occurs, and (ii) a surrounding domain representing the extended stress-disturbed region caused by the reservoir depletion. The inclusion of the surrounding domain leads to a more realistic modeling of the actual reservoir geomechanical boundary conditions. The reservoir is treated as a poro-elastic system consisting of a deforming solid skeleton and a moving compressible fluid. Nonlinear elastic deformation is assumed for both domains.

The model has been applied to illustrate the effect that rock deformation has on reservoir productivity. Results show that for stress-sensitive reservoirs, calculated flow rates obtained from conventional fluid-flow models can be significantly different from flow rates calculated from coupled fluid-flow/geomechanics models. Also, production rates calculated from fully coupled, single-domain models may differ from flow rates calculated from fully coupled, two-domain models.

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