The Importance of Interfacial Tension on Fluid Distribution During Depressurisation
Authors: Mackay E.J. , Henderson G.D. , Teharani D.H. , Danesh A. 
Published: December 2007

Abstract
Many North Sea reservoirs are producing water at very high cuts under current waterflooding conditions. As oil production drops below economical limits, procedures for recovering residual oil are being investigated. One technique identified as being potentially suitable for at least ten candidate fields in the UKCS involves depressurising 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 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 characterised by different gas-oil interfacial tensions (IFTs) have shown the important role gas-oil and gas-water IFTs 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 plays an important role in determining 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, only in the oil phase, 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 bubble point 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 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.

Introduction
Over two billion US dollars is being spent on the UK's largest oil and gas producer, the Brent field, refitting its four offshore platforms for a new phase in the field's life. An additional 5.4×106 m3 (34 million barrels) of oil and 3.6×1012 m3 (1,500 billion cubic feet) of gas is expected to be produced by depressurising the waterflooded field over the next decade. This represents a considerable investment and anticipated return, and yet the physical processes occurring at the pore scale during pressure depletion in waterflooded reservoirs are not, as yet, fully understood. Most of the studies conducted on depressurisation have investigated the nature of bubble or how to determine the critical gas saturation. Core tests have been the most common investigative tool. Since the acquired information is only external to the core, for example production of fluids and pressure changes, the physical basis of the processes that take place, within the core, had to be inferred. Micromodel experiments, however, permit the direct observation of the process of gas nucleation and bubble growth. In the tests described in this paper, the flow of gas, oil and water within the porous media is observed through a powerful magnifying camera and recorded on video tapes for further study and analysis.
A geometrically designed network of pores or a picture of an actual thin section of a real reservoir rock is used to construct the glass micromodel by etching it with hydrofluoric acid. The micromodel is placed in a vessel that exerts an overburden pressure outside the model and allows the operator to conduct high pressure experiments on the live fluids within the model. P. 779^