On the Effect of Polymer Elasticity on Secondary and Tertiary Oil Recovery

Typically, a polymer for enhanced oil recovery (EOR) is selected on the basis of the viscosity range or average molecular weight, concentration, and brine composition, besides other reservoir properties. There is not much emphasis given on how the elasticity of polymers could enhance the oil recovery. In this study, in an effort to find a systematic approach for selecting the best polymer for water flooding, the effect of molecular weight distribution (MWD), a direct measure of a polymer's elasticity, was studied on oil recovery performance. The individual effect of the elasticity of polymers on oil recovery, breakthrough and overall recovery, and residual resistance factor (RRF) was determined by keeping the viscosity constant and varying the elasticity during secondary and tertiary recovery experiments. Within two different groups of polymers each with similar average molecular weight studied here, nearly 10% higher recovery for the highest elastic polymer was observed during secondary recovery, whereas for tertiary flood similar to 6% higher recovery with similar to 5 times higher RRF value was observed for the highest elastic polymer solution studied here. Results have shown that average molecular weight by itself might not be the best criterion to select the optimum polymer fluid composition for polymer flooding operations. Polymer elasticity should be weighted more than the average molecular weight, as it could correspond to higher sweep efficiency due to the stretching of polymer along the pores. Considering the polymer elasticity or MWD together with average molecular weight seems to be a better approach for achieving higher oil recovery performance at lower polymer concentrations.