The Importance of Interfacial Tension on Fluid Distribution During Depressurization

Abstract

Summary Many North Sea reservoirs produce water at very high cuts under current waterflooding conditions. As oil production drops below economical limits, procedures for recovering residual and bypassed oil are being investigated. One technique identified as potentially suitable for at least ten candidate fields in the United Kingdom Continental Shelf (UKCS) is to depressurize these fields. During pressure depletion, improved hydrocarbon recovery has been shown to result from the release of solution gas and subsequent expansion of the freed gas. The precise nature of gas nucleation and its subsequent growth phenomena, however, have not been well understood. For example, there exists experimental evidence supporting two apparently conflicting models describing gas nucleation. High pressure micromodel studies have shed light on the physical processes occurring at the pore scale during pressure depletion in waterflooded systems. Specifically, tests performed with fluids characterized by different gas/oil interfacial tensions (IFT's) have shown the important role gas/oil and gas/water IFT's play during bubble formation and growth in porous media. Analysis of visual information from water-wet micromodel tests have shown that the magnitude of the liquid-gas IFT is one of the factors that determines in which phase the bubbles nucleate, and how many bubbles form per unit volume of pore space. A low gas/oil IFT results in a relatively large number of bubbles forming in the oil phase only, with little or no supersaturation. Higher values of gas/oil IFT result in increasing levels of supersaturation with relatively fewer bubbles nucleating at cavity sites on the water-solid interface. Not only is the gas/oil IFT at the bubblepoint important in determining the nucleation loci and densities, but the increasing gas/oil IFT values with decreasing pressure also affect the distribution of the three phases during the process. Increasing gas/oil IFT's, combined with the effect of disjoining pressure in thin water films separating oil ganglia, result in delayed oil production and expulsion of water from the system below a certain threshold pressure. This suggests that there is an identifiable minimum pressure below which hydrocarbon production will not be viable.