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

Studies of the relation between capillary pressure (Pc) and fluid saturation (S) for porous media containing oi l -water or a i r -o i l -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 waterand oil-wet fractions. This study reports on twoand three-fluid Pc-S data for media of different fractions of waterand oil-wet sands. The oi l -water capillary pressure, defined as the oil minus the water pressure, was measured during drainage (primary and main curves) as well as imbibition (main curve only) of water. A decrease in o i l -water pressure was observed as the oil-wet fraction increased in two-fluid media, q-he pressure became negative during imbibition of water for relatively oil-wet media. The Pc-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 e v e n for a relatively low oil-wet fraction (25%). The oi l -water and a i r -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-oi l capillary pressure, was relatively insensitive to whether oil acted as wetting or intermediate fluid. There is a need to model three-fluid Pc-S curves that account for different wetting and intermediate fluids. * Corresponding author. Elsevier Science B.V. SSDI 0 1 6 6 3 5 4 2 ( 9 5 ) 0 0 0 2 7 5 90 S. Bradford, F. Leij /Journal of Contaminant Hydrology 20 (1995) 89-109