Effect of polymer concentration on the polymer adsorption-induced permeability reduction in low permeability reservoirs

Abstract

This study presented the effect of polymer adsorption-induced permeability reduction on oil recovery, particularly in low permeability system. The polymer injecting experiments were conducted in a quarter of five-spot pattern with concentrations generally applied. Especially, by using the field oriented two-dimensional sandstone, we attempted to analyze the combined effect of polymer retention for concentration and high velocity near injection on oil recovery.Particularly in low permeable sandstone, oil recovery was not proportionally enhanced with increase in polymer concentration, rather decreased at concentrations above 1,000 ppm in this experiment. This could be understood by polymer adsorption phenomena corresponding to concentrations. The polymer adsorption layer formed at high concentration clogged effective pore radius by up to 59.8%, compared to clean sandstone and the permeability greatly decreased from 56 to 1.5 mD, resulting in rather decreased oil recovery. For this reason, there should be an optimum concentration for maximizing the oil recovery. It was also confirmed by shear rate analysis. At high concentration, the shear rate near injection where has the highest velocity was greater than critical rate. This means that polymer molecules adsorbed to mono-layer of adsorption by hydraulic force, and the adsorption became additionally thicker and hindered the oil flow.