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

Abstract

Members SPE-AIME Abstract Three different molecular weight emulsion polyacrylamides (PAA) have been tested for their ability 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 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. permeability core. The data indicates selection of too large a polymer can result in less than maximum oil recovery 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 permeability zones. Proper selection of polyacrylamide for a heterogeneous permeability polyacrylamide for a heterogeneous permeability reservoir either in the presence or in the absence of alkali will maximize oil recovery. Introduction Partially hydrolized polyacrylamides are currently the most widely used polymers in enhanced oil recovery applications. These polymers have been used as mobility control agents as well as for permeability profile correction agents, either with permeability profile correction agents, either with or without a crosslinking agent. While polyacrylamide polymers have demonstrated success in both polyacrylamide polymers have demonstrated success in both applications, the selection of the best product for a specific application, from the wide variety available, has always been difficult. The absence of any defined guidelines for selection of polymers to optimize performance in a given reservoir has undoubtedly resulted in many projects suffering, both technically and economically. In heterogeneous permeability reservoirs, both improved mobility control and permeability profile correction are important. Selection of a polymer which will perform well in the high permeability zones yet leaves the lower permeability zones unswept is undesirable. This is especially true for reservoirs in which the lower permeability zones contain the higher oil saturation. For optimum performance in a sustained polymer flood, the performance in a sustained polymer flood, the polyacrylamide must demonstrate good injectivity and polyacrylamide must demonstrate good injectivity and flow properties (improved mobility) in all oil saturated zones, as well as either maintaining or diverting as much flow as possible into those zones with the highest oil saturation. The evaluations described in this paper are designed to compare the effect of varying polyacrylamide molecular weight on oil recovery in a polyacrylamide molecular weight on oil recovery in a reservoir with vertical permeability heterogeneity. The oil recovery is characterized in terms of enhanced performance due to improved mobility in each of three individual permeability zones as well as concurrently determining the enhanced performance due to profile modification. In addition to evaluating the effects of polymer in this reservoir situation, the results of a series of alkali/polymer combination tests will be discussed. P. 201