Fractional wettability effects on two-and three-fluid capillary pressure-saturation relations

Abstract

Studies of the relation between capillary pressure ( P c) and fluid saturation ( S) for porous media containing oil-water or air-oil-water, often assume that the medium is strongly water-wet. Natural porous media, however, are composed of a variety of mineral constituents; such media are typically composed of water- and oil-wet fractions. This study reports on two- and three-fluid P c- S data for media of different fractions of water- and oil-west sands. The oil-water capillary pressure, defined as the oil minus the water pressure, was measured during drainage (primary and main curves) as well as inhibition (main curve only) of water. A decrease in oil-water pressure was observed as the oil-wet fraction increased in two-fluid media. The pressure became negative during imbibition of water for relatively oil-wet media. The P c- S data could be adequately described by modifying the van Genuchten model for water retention. The observed differences between primary and main drainage curves were partly attributed to the effect of initial saturation. In three-fluid systems with fractional wettability, the observed dependency of capillary pressures on fluid saturations suggested that there was no continuous intermediate phase — even for a relatively low oil-wet fraction (25%). The oil-water and air-water capillary pressures decreased, at a particular water saturation, as the fraction of oil-wet sand increased. The water pressure is greater when water acts as the intermediate fluid than when it is the wetting fluid. The oil pressure, and hence the air-oil capillary pressure, was relatively insensitive to whether oil acted as wetting or intermediate fluid. There is a need to model three-fluid P c- S curves that account for different wetting and intermediate fluids.