Effect of Varying Polyacrylamide Molecular Weight on Tertiary Oil Recovery From Porous Media of Varying Permeability

Abstract

Abstract Three different molecular weight emulsion polyacrylamides (PAA) have been tested for their ability to recover oil from a multi-permeability Berea core system. Injection of a 6.5 × 106, 17 × 106, and 36 × 106 molecular weight PAA resulted in oil recovery from the multiple permeability core systems of 53.4, 63.7, and 57.2 percent of the waterflood residual oil, respectively. In tests with 15 g/kg Na2CO3, injection of the polymer systems resulted in recoveries of 65.6, 75.3, and 68.0 percent of the waterflood residual oil, respectively. In the presence of alkali, the injection of high molecular weight PAA resulted in the reduction of fluid flow into the medium and low permeability cores with 70 to 75 percent of the tertiary oil being recovered from the high permeability core. Decreasing the PAA molecular weight to 17 × 106 and subsequently to 6.5 × 106 resulted in an increasing amount of fluid flowing through the low and medium permeability cores. While decreasing the molecular weight of the PAA resulted in increased fluid diversion, the 6.5 × 106 molecular weight PAA recovery efficiency was less than either of the other two polymers in the high permeability core. The data indicates selection of too large a polymer can result in less than maximum oil recovery from a heterogeneous permeability reservoir as a result of lower sweep efficiency. Selection of too small a polymer can result in less than maximum oil recovery from a heterogeneous permeability reservoir as a result of decreased oil recovery from the high permeability zones. Proper selection of polyacrylamide for a heterogeneous permeability reservoir either in the presence or in the absence of alkali will maximize oil recovery.