The Optimum Gas Saturation for Maximum Oil Recovery from Displacement by Water
Authors: Land Carlon S. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.

This paper was prepared for the 43rd Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Houston, Tex., Sept. 29-Oct. 2, 1968. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

The influence of a free gas saturation on recovery by water drive is investigated. A digital computer is used to calculate the combined oil recovery from solution-gas drive to various states of depletion and then oil recovery by water displacement at these states of depletion. Results of these calculations are compared, and optimum conditions for the maximum oil recovery are found for various fluid-rock systems. For most of the fluid-rock systems, the calculations show an optimum gas saturation other than zero.

Using fluid and rock properties as parameters, graphs are plotted showing the optimum parameters, graphs are plotted showing the optimum gas saturation and the increase in oil recovery brought about by the gas saturation. The effects of fluid properties and relative permeability on the optimum gas saturation are permeability on the optimum gas saturation are seen in these graphs.

The graphs can be used as guides in connection with other considerations in determining the optimum time to start a waterflood. They should also be helpful in determining the MER of a water-drive reservoir.

Introduction

Petroleum engineers are not in agreement on the state of depletion that a solution-gas-drive reservoir should have reached before a waterflood is initiated, such that the maximum oil recovery is obtained. Probably the most widely held belief is that the maximum recovery is obtained by waterflooding when the reservoir pressure is near the bubble point. This pressure is near the bubble point. This viewpoint is in agreement with the influence of hydrocarbon fluid properties on waterflood production, since the oil formation volume production, since the oil formation volume factor and oil viscosity are most favorable for oil recovery by waterflood at the bubble point.

On the other hand, evidence has been accumulating over the years that indicates that a gas saturation at the time of waterflood reduces the residual oil saturation, thereby increasing oil recovery. In laboratory experiments, the residual gas saturation varies directly with the initial gas saturation. As the residual gas occupies pore space that would be occupied by oil if the gas were not present, oil recovery is improved by the presence of a residual gas saturation. Under reservoir conditions, the change in fluid properties and the decreasing oil permeability with increasing gas saturation tends to offset the beneficial effect of the free gas saturation.