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The production of oil by water flooding can be substantially increased bythe maintenance of free gas saturation in the reservoir during the floodingoperation. This effect is accomplished by the alteration of oil relativepermeability characteristics and the occupation by gas of pore space that wouldotherwise be filled with residual oil. The amount of reduction in residual oilcan be calculated from appropriate water-oil relative permeabilitycharacteristics. This paper presents experimental data in support of the foregoingconclusions and an example of the calculations. The microscopic pore saturationconcepts of the mechanism are discussed. A method of practical application tofield floods is presented together with discussion of certainlimitations. Introduction The presence of free gas has been reported by a number of investigators tosignificantly affect the oil recovery which can be obtained from sandstone flowsystems by water flooding. The effect of gas, noted in every instance, has beento cause lower residual oil saturations than could be obtained by waterflooding the same systems in the absence of free gas. The degree of improvementin recovery has been observed to vary widely, depending on the systems used andthe conditions of the tests. The increased oil recovery obtained because of thepresence of gas during a water flood has been variously attributed to changesin physical characteristics of the oil, selective plugging action of the gas, inclusion of oil mist in the free gas phase, and the additional sweeping ordriving action of the free gas. All but the first of these suggestions imply changes in the displacementmechanism. The change in viscosity and interfacial tension of the oil phase, within the pressure range used for all the experimental work, is certainly notsufficient to account for the differences in residual oil saturation notedunless there is a drastic change in the displacement process. One other effect which logically seems capable of causing differences inresidual oil saturation of the magnitude noted in the experimental work is thatof simple replacement. In a water-wet system containing oil, water, and gas, itis to be expected that the gas will exist inside the oil. T.P. 3055

Effect of Free Gas Saturation on Oil Recovery by Water Flooding