Residual oil saturation (S_or) and irreducible water saturation (S_wi) have been considered primarily to be a function of wettability. If wettability is the dominant factor in relative permeability, then a fundamental understanding of the process must await a major breakthrough in measuring and understanding the wettability effect. There is no disputing the effect of wettability on oil production, but is it the only factor? In the context of a single field, it appears that commonly wettability does not change much. Coskun and Wardlaw have demonstrated that strong statistical relationships exist between characteristics obtained by image analysis of petrographic thin sections and relative permeability end points.

In the results described here Fourier Transforms of large high-resolution images of thin section porosity are used to delineate and describe the mechanism that permits end points to be predicted from thin section. Characteristics of the radial power spectrum and of filtered images show that S_wi mostly occurs in domains of close packed grains with a relatively small contribution from water films, pendular rings etc. Oil and water coexist in the intervening circuits of packing flaws. Mobile oil exists in a subset of such circuits characterized by low aspect rations between pores and throats. Residual oil occurs in circuits with slightly higher aspect ratios. This characteristic microstructure is mainly the product of depositional process and early dewatering. Understanding of the fabric-controlled aspects of multi-phase flow should permit relatively straightforward advances in wireline log development and in scaling relative permeability from core scale to reservoir scale.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah