Effects of Polymer Adsorption and Desorption on Polymer Flooding in Waterflooded Reservoir

Abstract

Polymer Flooding is one of the most well-known methods in Enhanced Oil Recovery (EOR) technology which can be implemented after either primary or secondary recovery. Polymer can lower the mobility ratio of the displacement mechanism by means of increasing viscosity of injected water. Therefore, polymer flooding can increase volumetric sweep efficiency. Moreover, polymer adsorption onto rock surface can decrease permeability contrast in reservoir with high heterogeneity. Due to reduction of the absolute permeability, effective permeability to water is also reduced. Once polymer is adsorbed onto rock surface, polymer can also be desorbed when different fluids are injected. This study focuses on polymer adsorption/desorption behavior in the reservoir as well as effects on operating parameters and interest reservoir conditions. Reservoir simulator called STARS® commercialized by Computer Modeling Group is utilized to assess interest effects. The results show that performing polymer flooding after water pre-injection sooner can increase oil recovery factor in the shortest period and similar results are also obtained from higher injection rate. The difference of polymer mass injected into the reservoir, represented by polymer concentration affects the optimum polymer slug size. Higher polymer concentration requires smaller slug size of polymer to attain constant oil recovery factor, vice versa. Polymer desorption causes polymer re-employment from the previously adsorbed onto rock surface, resulting in improving of sweep efficiency in the further period of polymer flooding process. However, the optimum value of polymer desorption degree in difference polymer concentration exists. When using high polymer concentration, total polymer desorption allows chasing water to bypass polymer slug, causing adverse effect on sweep efficiency. In reservoir with variation in permeability, polymer can be injected to retard early water breakthrough. However, degree of polymer desorption should be zero to allow permanent adsorption as reduction of relative permeability to water can be maintained to ensure slow movement of inject fluid. Magnitude of polymer adsorption does not affect much on oil recovery factor when degree of polymer desorption is varied. In high magnitude of polymer adsorption, zero polymer desorption causes less effective polymer to maintain viscosity whereas effects of reduction of relative permeability dominates effects of maintaining viscosity in case of 50 percent polymer desorption. Combination of total desorption with high residual resistant factor shows favorable results. Sweep efficiency is improved also at boarder of flood pattern and moreover, diverting of chasing water to border zones results in very favorable sweep efficiency.