The physical basis of the displacement of oil from porous media by other fluids: a capillary bundle model

Abstract

The displacement of oil from a bundle of short capillary tubes of various sizes, by another fluid, demonstrates many of the characteristic features of oil displacement from porous media. With a properly chosen capillary diameter-number distribution, experimental observations on the oil: water ratio during immiscible and miscible displacement from sand-packs and cores can be quantitatively reproduced. Moreover, the same distribution can quantitatively account for the observed effects of viscosity ratio on the recovery. The introduction of a term to take account of interfacial drag at the rear end of the oil demonstrates qualitatively the known effects of interfacial tension and wetting angle on the shape of the recovery curve, on the residual oil and on the effect of displacement flow rate. The model can also be used to demonstrate the effect of temperature on recovery. Recovery during a two-stage process such as water-flooding followed by polymer flooding, can also be predicted with good reliability by proper choice of the capillary diameter-number distribution. The success of this model in replicating the experimental behaviour of cores or sand-packs suggests that during displacement from cores or sand-packs, flow takes place preferentially through paths of least flow-resistance, displacing only oil until one path is completely depleted of oil. After water breakthrough along such a path, oil must be produced from three-dimensional islands of oil, which are erroded by penetration of water into successively smaller islands and filaments of oil in the most highly resistant paths, until the oil is broken down into small oil globules. Implications pertinent to other aspects of oil recovery are also discussed.