The behavior of a waterflood is affected, among other factors, by the initial gas saturation at the start of injection. To illustrate this effect on oil rate and on the water-oil ratio (WOR), calculations were made with the Dykstra-Parsons layered system model. Two cases were chosen, one for a high gravity oil with a low mobility ratio, and one for a low gravity oil with a relatively high mobility ratio.
The factors that effect the recovery of oil are described. The ideal situation for calculating a waterflood performance, would be that the values of all of the parameters to be used in the model correctly define the reservoir and fluid properties and the displacement process. The more closely the parameters define the system, the greater will be the confidence in the predicted results. If production history is avaIlable, a history match can be made. Certain parameters can then be adjusted to obtain a better match with actual performance before predictions are made of future performance. Examples are given for two waterfloods
The fluid properties and saturation data used in the analysis were based on pressures obtained from depletion calculations to establish initial conditions prior to the start of the waterflood For Case 1, a 40 deg API oil having an initial bubble point pressure of 3500 psia at 220 deg F was depleted down to a gas saturation of 30 percent. For Case 2, a 20 deg API oil having an initial bubble point pressure of 1500 psia at 130 deg F was depleted down to a gas saturation of 12 percent.
The results of the study show a substantial decrease in maximum oil rate and a substantial rise in the level of the WOR-recovery curve with increase in initial gas saturation. These types of results, showing the effect of an initial gas saturation, have not heretofore been published.
A considerable number of papers have been published on methods of calculating waterflood oil recovery. references to these methods are given in a Monograph by Craig.