FRACTIONAL WETTABILITY EFFECTS ON 2-FLUID AND 3-FLUID CAPILLARY PRESSURE-SATURATION RELATIONS

Studies of the relation between capillary pressure (P-c) and fluid sat uration (S) for porous media containing oil-water or air-oil-water, of ten assume that the medium is strongly water-wet. Natural porous media , however, are composed of a variety of mineral constituents; such med ia 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 frac tions of water- and oil-wet sands. The oil-water capillary pressure, d efined as the oil minus the water pressure, was measured during draina ge (primary and main curves) as well as imbibition (main curve only) o f water. A decrease in oil-water pressure was observed as the oil-wet fraction increased in two-fluid media. The pressure became negative du ring 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 dr ainage curves were partly attributed to the effect of initial saturati on. In three-fluid systems with fractional wettability, the observed d ependency of capillary pressures on fluid saturations suggested that t here was no continuous intermediate phase -even for a relatively low o il-wet fraction (25%). The oil-water and air-water capillary pressures decreased, at a particular water saturation, as the fraction of oil-w et sand increased. The water pressure is greater when water acts as th e intermediate fluid than when it is the wetting fluid. The oil pressu re, and hence the air-oil capillary pressure, was relatively insensiti ve to whether oil acted as wetting or intermediate fluid. There is a n eed to model three-fluid P-c-S curves that account for different wetti ng and intermediate fluids.