Capillary-Gravity Equilibria for Hydrocarbon Fluids in Porous Media

Abstract

ABSTRACT A theory and a general formulation have been developed to describe the influences of gravitational and capillary forces on reservoir fluid PVT properties and phase behavior. The theory assumes thermodynamic equilibrium for the fluid system and makes use of the Leverett J-function concept to approximate the in-situ pore curvature as a function of wetting saturation at a given height in the reservoir. The general formulation then permits interpretation and prediction of fluid phase and compositional variations in the capillary transition zone and beyond. The effects of capillarity (interfacial curvature) and gravity on reservoir fluid PVT and VLE were investigated, for a gas-condensate system and assuming different curvature functions. It is shown that the interfacial curvature has no practical effect on fluid PVT and VLE for typical sand packings used in the laboratory. However, for in-situ reservoir conditions, it is possible to have quite high interfacial curvature with pore radius in the order of about 1 micron. Under such condition, the capillarity effect was found to become appreciable. A lower reservoir temperature would also make the capillarity effect more pronounced. A second example was presented to show the variations of composition and saturation of the gas-condensate system above the gas-oil contact. It is shown that the capillary behavior of the reservoir rock controls the oil saturation variation above the gas-oil contact.