TEUFEL, LAWRENCE W., Sandia National Laboratories, Albuquerque, NM

ABSTRACT: Production-Induced Changes in Reservoir Stress State: Applications to Reservoir Management

Knowledge of in situ stress and how stress changes with reservoir depletion and pore pressure drawdown is important in a multi-disciplinary approach to reservoir characterization and management. Stress affects nearly all petrophysical properties and hence the measurement and interpretation of laboratory, well test, and geophysical data. Hydrostatic (isotropic) loading is the conventional test procedure followed by the petroleum industry to determine the stress dependence of reservoir properties. However, hydrostatic tests do not truly reflect the stress anisotropy and deviatoric stress state that exists in most reservoirs and do not adequately simulate the evolution of stresses in a reservoir during production. In situ stress measurements made during pore pressure drawdown show that m ny reservoirs follow a stress path (defined as the change in effective horizontal stress/change in effective overburden stress from initial reservoir conditions) that is significantly less than either a constant total-stress boundary condition (hydrostatic loading) or a uniaxial-strain boundary condition (i.e., no lateral displacement of the reservoir boundaries). Triaxial-compression laboratory tests on a variety of reservoir rocks show that compressibility, permeability, and velocity vary markedly with stress path. Hence, changes in reservoir properties measured under hydrostatic loading conditions to predict reservoir response during production and pore pressure drawdown can be inaccurate and very misleading if applied to a reservoir that follows a non-hydrostatic stress path. Represe tative measurements of reservoir properties should be measured in the laboratory under loading paths that duplicate the stress path followed by the reservoir during production. Case studies from carbonate and clastic reservoirs illustrate the importance of reservoir stress path and the appropriate laboratory testing necessary to develop effective reservoir management strategies. The results of this study suggest that optimum reservoir management may require that the reservior stress path be determined by measuring in situ stresses early in the production history of a reservoir and periodically thereafter as the reservoir pore pressure is reduced.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.