Abstract
In oil/gas reservoirs, the state of stress changes as fluid production/injection from/into the reservoir takes place. Also, petrophysical and geomechanical properties may change due to the variation of the effective stress. However, this consideration is seldom taken into account in well test analysis.
This paper presents a numerical, fully coupled, fluid- flow/geomechanical model to perform well test analysis in stress-sensitive reservoirs. The governing equations are developed in cylindrical coordinates honoring the geometry of the flow lines characterizing the drainage area in most well tests. The model is a 3D, point distributed, finite-difference simulator which applies a fully implicit discretization scheme to ensure maximum stability.
The model assumes isothermal, single-phase fluid-flow (slightly compressible fluid). Infinite and finite acting behavior is allowed during the well test. Reservoir properties are allowed to change from one layer to another. Both cases isotropic and anisotropic rock property behaviors are considered in the model. The rock behaves as elastic system whose deformation is described by nonlinear theory (the mechanical properties are function of the mean effective stress).
The results show that in stress-sensitive reservoirs the permeability decreases with production time reaching its minimum value near the wellbore and moving more and more into the reservoir as production time increases. After a certain production time, the permeability distribution reaches a constant value. An important finding from this study is that the damage caused by rock deformation is irreversible; therefore, its early detection and treatment is essential for optimum reservoir management.
