The decrease in the residual oil saturation by polymer flooding with a high visco-elasticity solution has been widely addressed in the past few years. The effect of the polymer solution visco-elasticity on the microscopic sweep efficiency was studied experimentally in a range of length scales, from micromodels through core floods to full field application. In most of the micromodel and core flooding experiments, a comparison was made between the effect of glycerol and polyacrylamide solutions on the oil recovery, because glycerol does not exhibit elastic behavior (only viscous), while polyacrylamide solutions do.
In this paper it is discussed that the use of glycerol in combination with brine and/or polymer solutions could yield erroneous results due to a very strong dependence of the glycerol viscosity on temperature and on mixing with water. Instead, in the current study a series of core floods was done in which several polymer solutions were injected with a wide spread in viscoelasticity but the same apparent viscosity to study the effect of visco-elasticity on oil recovery.
The main conclusion of the study is that for the crude oil of high viscosity (~300 cP), hardly any effect was observed of increasing visco-elasticity on the oil recovery, even when both viscosity and flow rate were increased up to 300 cP and 3 ft/day, respectively. However, for the low viscosity crude (~9 cP), extra oil was recovered upon increasing the viscosity and/or flow rate of the polymer of high elasticity. No extra oil was recovered when using polymers of low elasticity even when using similar viscosity and flow rate, which indicates that it is an effect of elasticity and not of viscous stripping of residual oil saturation.
Thus, this paper confirms that polymers of high elasticity can reduce the residual oil to water. However, since this effect was only observed by either increasing the polymer viscosity or injection rate, the main challenge for field application is injectivity of the polymer at such high viscosity or rate, therefore before embarking on field application an injectivity test is required.