Adapting the type-curve approach for estimating reservoir rock permeability

摘要

This study introduces a type-curve technique forestimating rock permeability from capillary pressuremeasurements. The Brooks and Corey model in conjunction withthe Wyllie and Gardner model were basis for the development ofthis type-curve approach. The Wyllie and Garden permeabilitymodel was used as the starting point for obtaining an analyticalexpression for permeability as a function of displacement pressure(Pd), pore geometrical factor (λ), interfacial tension (γ), fractionof pore volume available for flow (φ*), and porous mediumconfiguration parameters (βand n). The last parameters (βand n)are assumed to be functions of porosity and irreducible watersaturations, respectively. Parameters λ and Pd are determined bymatching the actual rock capillary pressure versus wetting-phasesaturation (Pc-Sw) profile with that obtained from the Brooks and Corey model.Capillary pressure data for 62 rock samples representing avariety of sandstone and carbonate facies were used to test thinapproach. Permeability (k) of these samples ranged from less than7 md to over 2500 md, and porosity (φ) ranged from 12 to 32%.In general, laboratory-measured permeability values matchedreasonably well estimated permeability values using the proposedtype-curve approach. An average absolute relative error of 7% wasobtained with a standard deviation of 6%. For carbonate rockscharacterized by a bimodal pore- size distribution, the capillarypressure data match in some cases two distinct type-curves ratherthan one. The use of an average pore geometrical factor for thesecases is recommended, since it gives an accurate estimate ofpermeability. The use of this type-curve approach is restricted toair-brine drainage capillary data. This approach improvesconsiderably the permeability evaluation from capillary pressuremeasurements, since it is based on the whole capillary pressureprofile data rather than a single point and since it does not requirean advance knowledge of fudge factors related to rock poregeometry. The approach is direct and is not plagued by any trial-and-error procedures.