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Documenting Porosity Stress Sensitivity in Conventional and Fractured Reservoir Rocks

M. T. Holland, T. M. Little, S. R. Bereskin

Most measurements obtained from routine and special core analyses are dependent upon the physical configuration of the reservoir pore system. Factors such as porosity type, tortuosity, presence of pore-filling minerals, size of pores and pore throats, particle size, grain roughness, and angularity are but a few of the pore system characteristics that control non-wetting phase flow. Porosity, permeability, mercury porosimetry, and derivations obtained therefrom are commonly used for reservoir evaluation and engineering calculations. However, without a description of the physical parameters controlling these measurements (i.e., the reservoir pore system), gross errors in interpreting test results can occur.

Most geologists and engineers are now aware of the large differences in core analysis measurements performed at bench and overburden conditions. The magnitude of change observed in these measurements is highly variable not only for different rock types, but also for similar lithologies within a single cored interval. Several rock fabric physical properties contribute to the variable stress sensitivity of reservoir rocks; however, we believe that the effects of coring, core recovery operations, natural dilation and/or contraction of the rock fabric, and preservation procedures are primarily responsible for increased porosity and permeability at bench conditions.

Measurements of pore system parameters in fractured and conventional reservoirs can be achieved using new petrographic methods and special core testing techniques. Fluorescent epoxy impregnations of core samples performed under bench and simulated in-situ conditions of effective stress offer the ability to observe fracture and intergranular porosity at surface and reservoir conditions. Fluorescent microscopy, computer image analysis, and special core permeability testing are combined to document the variation of stress sensitivity within several different reservoir rock types.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.