Does Prior Gas Injection Increase Waterflood Recovery? Reservoir Simulator Results Vs Old Theory
Authors: Negron-Perez E.A. , Ali S.M. Farouq 
Published: December 2007

This paper was prepared for the Eastern Regional Meeting of the Society of Petroleum Engineers of AIME, to be held in Pittsburgh, Penn., Nov. 7–9, 1973. 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 granted upon request to the Editor of the appropriate journal 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 SPE magazines.

Abstract
For many years, gas injection prior to waterflooding has been practiced in Pennsylvania - New York oilfields, with an aim to enhance oil recovery. A number of investigators have reported laboratory results supporting the contention that "prior gas injection" helps to increase oil recovery. The Buckley-Leverett theory has provided the main theoretical basis for such studies. The present investigation aims to re-examine the role of prior gas injection on oil recovery by a waterflood, using a three-phase, two-dimensional simulator. Such an approach permits the inclusion of capillary, gravity, and three-phase flow effects which the Buckley-Leverett theory does not consider.
Three-phase flow equations were formulated, and solved by the alternating direction implicit procedure. The simulator was tested for procedure. The simulator was tested for stability, and material balance errors, with excellent results.
A number of situations encountered in gas injection prior to waterflooding were simulated. The main finding was that oil recovery is slightly increased for those cases where a small free gas saturation is present. The results showed that the increase due to prior gas injection is greater for higher viscosity oils than for the less viscous oils. Values obtained for the increase in oil recovery ranged from 0.50 percent for a 0.81 - centipoise oil to a high of percent for a 0.81 - centipoise oil to a high of 3.52 percent for a 40.5 - centipoise oil.
It is felt that the previous work utilizing laboratory models was not completely representative of the conditions existing in a reservoir. It is concluded that a careful assessment should be made to determine the probable benefit of a prior gas injection probable benefit of a prior gas injection project before it is undertaken, since any project before it is undertaken, since any increase in oil recovery may be outweighed by economic considerations.

Introduction
The research directed toward increasing the oil recovery efficiency of a waterflood operation has followed three basic approaches:improving the mobility ratio, in order to increase displacement efficiency and volumetric sweep efficiency,elimination of interfacial forces by use of solvents and surface-active agents, andsubstitution of an injected fluid, such as carbon dioxide or natural gas for a portion of the residual oil.
It is toward this third category that we direct our attention in this study.