Effect of interfacial forces on two‐phase capillary pressure—saturation relationships

Abstract

Quantitative descriptions of two‐phase flow in the subsurface require knowledge of the capillary pressure‐saturation relationships. The effect of interfacial forces on the drainage capillary pressure‐saturation relationship for organic liquid‐water systems is usually expressed by the ratio of the liquid‐liquid interfacial tensions as given by Leverett's (1941) function. To assess the appropriateness of this approach for primary drainage of organic liquid‐water systems typical of hazardous waste sites and to evaluate its extendability to spontaneous imbibition, measurements were made of these relationships for various immiscible liquid systems in unconsolidated sand. The results showed increasing deviations with decreasing interfacial forces between the measured values and those predicted by a ratio of interfacial tensions. To improve the predictive capability of Leverett's function, forms including the intrinsic contact angle and roughness were examined. Scaling of the capillary pressure relationships was best achieved by including a correction for both interface curvature and roughness. These corrections became significant for drainage for contact angles larger than 35°–55°, and for imbibition for contact angles larger than 15°–25°. None of the forms of Leverett's function examined predicted the increased residual saturation with decreasing interfacial forces observed in this study. Consequently, their ability to scale the measured data was predicated on posing the saturation of the wetting phase in terms of the variable effective saturation.