Effects of Polymer Concentrations, Slug Size And Permeability Stratification in Viscous Waterfloods

Abstract

Abstract A streamline type mathematical model has been developed which may be used to study oil recovery by viscous waterfloods for any desired well spacing. The model was used to study the combined effects of permeability stratification, polymer concentration and viscous water slug size for a five spot injection pattern. Five different permeability profiles ranging from a homogeneous to a highly stratified reservoir were studied. It was found that viscous water floods show greater percentage increase (over conventional water percentage increase (over conventional water floods) in oil recovery for stratified reservoirs than for homogeneous reservoirs. A highly stratified reservoir containing viscous oil showed a 96% increase in recovery for a particular viscous water flood. it was also found that the adverse effect o connate water on oil recovery becomes insignificant as the degree of permeability stratification approaches a large number. Guide-lines for the determination of an optimum slug size and viscosity were derived. Generally speaking, it is advisable to use a high viscosity, small size slug in flooding a homogeneous formation, while stratified reservoirs do better with the use of a large diluted slug. It was found that there exists an optimum slug size for a given amount of thickening chemical depending upon the oil viscosity and degree of permeability stratification. The increase in oil recovery by selecting an optimum slug size may be sizeable in some instances. Introduction The role of mobility ratio in water flooding has been well established. Thickening of the injected water or otherwise increasing its resistance to flow brings about this improvement through a decrease in its fluid mobility. Early attempts to decrease the mobility of the injected water were uneconomic, but, during the past few years, the use of a high molecular weight water-soluable polymer has shown promise as an effective polymer has shown promise as an effective material in reducing the water mobility. Results from both laboratory and field pilot tests indicate that the concentrations of polymer required in the injection water to polymer required in the injection water to effect substantial increase in flooding efficiency are usually low enough to satisfy normal profitability criteria. Much has been written postulating the mechanisms by which the mobility of water is reduced by the addition of a polymer.