Abstract
Optimum preflush size for a 1440-acre polymerflood in the North Burbank Unit, Osage County, Oklahoma was estimated as 120 days at 63,000 BWPD (0.073 pore volumes) with a cross-sectional reservoir simulator that incorporates a realistic treatment of the compositional dependencies of resistance and residual resistance effects. The optimum preflush size differs from tract to tract, varying from 150 days to less than 30 days of fresh water injection. These differences can be attributed to the different degrees of brine crossflow which are associated with the layer structures of the nine different tracts. Brine cross-flow reduces the incremental oil recovery due to the polymer since resistance and residual resistance effects are significantly reduced in the presence of high-salinity brine. The detrimental effects of salinity can result in a loss of up to 20% of the incremental oil.
A plot of predicted tertiary oil versus preflush size for the total project shows that a preflush does not significantly improve the performance of the polymer. The amount of tertiary oil is almost constant for preflushes between 60 and 180 days with a broad maximum at 120 days. A fresh water preflush of 120 days was recommended to maximize incremental oil from these tracts and to fit the construction schedule for the required facilities. The observed dependence of predicted recovery on preflush size resulted from a realistic treatment of the effects of salinity, polymer adsorption and polymer concentration on polymer performance in the computer model.
Injection of fresh water preflush began on September 1, 1980.
