An Investigation of the Role of Capillary Forces in Laboratory Water Floods

Abstract

Capillary forces play a controlling role in waterdrivedisplacement processes both in laboratory experiments and in actualreservoirs, but their quantitative importance may be quite differentin the two cases. Because of the importance of conducting laboratoryexperiments which are representative of field conditions, it is necessaryto understand exactly the role of capillary forces in the displacementprocess. Though a number of experimental investigations related tothis subject are contained in the literature, there appears to bea lack of information pertaining to unsteady-state experiments inwater-wet media. This experimental study was conducted to obtainadditional laboratory data to clarify further the role of capillaryforces in both the macroscopic and microscopic flow of oil and waterin porous materials. Theoretical Considerations The capillary pressureis defined as the difference in pressure between a continuous oilphase and a continuous water phase in a porous material. The magnitudeof this pressure difference depends on the interfacial curvatureand the interfacial tension. The interfacial curvature is determinedby the geometry of the pore spaces, the wettability of the rock surfaces,and the quantity of each phase present. Capillary forces are involvedin a water-drive displacement process in that they exert a controllinginfluence on the microscopic fluid distribution which in turn isreflected in the saturation or macroscopic flow behavior. MicroscopicFluid Distribution Because of the microscopic nature of the displacementof oil by water, it is necessary to consider the flow and the fluiddistribution in individual pores. On this microscopic scale the capillaryforces, which act over a distance of one or two sand grain diameters,control the distribution of oil and water under static equilibriumconditions. When an external force is applied to the fluids, suchas in a water-injection experiment, the applied forces tend to distortthe oil-water interfaces. However, in most fine-grained, water-wetsands, the applied pressure difference across one or two grain diametersis usually several orders of magnitude less than the capillary pressuredifference.