Effects of fractional wettability on capillary pressure–saturation–relative permeability relations of two-fluid systems

Abstract

Capillary pressure ( P c)–saturation ( S)–relative permeability ( k r) relationships must be quantified to accurately predict non-aqueous phase liquid (NAPL) distribution in the subsurface. Several experimental techniques are presented here for two-fluid P c– S– k r relationships for various saturation paths to better define the effect of fractional wettability on these relationships. During the primary drainage path of the P c– S curves, the air–water system showed no distinct trend as a function of the fraction of sand treated by organosilane (S) to render it non-water wetting. In a NAPL–water system, however, a consistent decrease of capillary pressure with increase of the fraction of non-water wetting sands was observed. The much lower contact angle for air–water (a–w) system may result in the observed insensitivity of the a–w P c– S curves to fractional wettability, at least for the PD pathway. For the main imbibition path of NAPL–water system, capillary pressure decreased as the fraction of the S component increased, requiring forced imbibition (negative capillary pressures) for a certain range of saturations. Systems with an increasing percentage of the S component also exhibited a higher water k r and lower NAPL or air k r at a given saturation for the primary drainage and main imbibition paths in both air–water and NAPL–water systems. The increase of water k r with increase of the fraction of the S component can be explained by the ability of water to occupy larger and highly conductive pores in such a system. Experimental k r– S data for the primary drainage path of NAPL–water system presented here were used to test the Bradford et al. [Bradford SA, Abriola LM, Leij FJ. Wettability effects on two- and three-fluid relative permeabilities. J Contam Hydrol 1997;28:171–91] model and the modified Mualem model for estimating the k r– S curves from measured P c– S data as a function of fractional wettability. Both models predicted significantly less variation in the k r– S curves than measured indicating that they did not adequately represent the system under investigation.