The viscoelasticity of polymers is known to contribute significantly toward improved displacement efficiency in polymer flood operations. But the contribution of elasticity of viscoelastic polymers in enhanced oil recovery (EOR) still remains largely unexplored. The majority of literature available on polymer-aided EOR, in general, talks about the role played by viscoelasticity of polymers on improved oil recovery with little or no mention of the individual contribution of the elasticity of polymers on EOR. In this work, partially hydrolyzed polyacrylamide (HPAM) solutions, having identical shear viscosity but different elasticity, were flooded to investigate the individual effect of elasticity on improved oil recovery. A transparent, sand packed visual cell, initially saturated with mineral oil, was used for flooding with four different HPAM solutions. Because these polymer solutions differed only in terms of elasticity, a comparative study of the effect of elasticity on sweep efficiency was done. Images taken at regular intervals during the course of flooding were analyzed to study the frontal displacement patterns changing with the elasticity of different HPAM solutions. The mechanism of viscous fingering in immiscible two-phase flow in porous media at different polymer elasticity values was studied. Results from flooding experiments indicate that polymer solutions with higher elasticity not only yield higher oil recovery, but also require less polymer to produce a given amount of oil. These results were further bolstered by the visual analysis. HPAM solutions with lower elasticity showed a high degree of fingering, whereas solutions with higher elasticity produced more stable displacement fronts. Improved microscopic sweep efficiency, due to the greater flow resistance offered by polymer solution with higher elasticity, was visually confirmed.

Polymer flooding has been the most widely used enhanced oil recovery technique in both sandstone and carbonate reservoirs. Ample polymer flooding projects have been conducted with different level of success ever since the technique was introduced 50 years ago. It is a usual practice to select a polymer based on viscosity range, concentration and molecular weight without getting into rheological characterization and its effect on oil recovery. However, in recent times the application of polymer flood has gained critical mechanistic insight with advancement in understanding the role of elasticity on sweep efficiency. Therefore, selecting the type of polymer and understanding how its fluid rheology affects oil recovery are probably among the most critical factors involved in designing a successful polymer flood job.To deal with this, a systematic approach for screening a polymer based on rheological characterization was adapted. Three different polymers, partially hydrolyzed polyacrylamide (HPAM) and Polyoxyethylene (PEO) were first used for fluid rheology study using a cone and plate rheometer and then for oil recovery through a special core holder designed to simulate radial flow through a sand pack -saturated with mineral oil.Effects of various rheological parameters such as; a) Newtonian vs. non-Newtonian rheology (constant shear viscosity vs. shear thinning), b) shear viscosity vs. elasticity, and c) average molecular weight vs. molecular weight distribution (polydispersivity) on oil recovery were investigated. Finally, a parametric characterization study was performed to develop a screening criteria and to correlate oil recovery prediction. The approach could lead to a successful screening process of polymer based on various characteristic parameters such as average molecular weight, polydispersity and Trouton ratio. IntroductionEOR polymers undergo chemical and/or mechanical degradation during the process of polymer flooding depending on the type of reservoir rock, reservoir fluids, reservoir temperature and pressure, injection rate etc. Most commonly used EOR polymers are HPAM and are particularly sensitive to mechanical degradation (W. Littmann, 1988). Khan et al. (2007) explained the elastic effects of extensional deformation of fluids in porous media. Detailed explanation of mechanism of chemical/mechanical and biological degradation of different polymers is given elsewhere (Sorbie, 1991) (Heemskerk et al., 1984.It is of prime importance to know the degradation behavior of polymers used in any polymer flood process. Laboratory investigations are essential for every particular flooding project in order to study the polymer behavior under different reservoir conditions.The study of rheological properties of polymers plays a vital role in the screening of polymers for any polymer flood operation. Rheological measurements and laboratory core floods carried out by Mojdeh et al. (2008) with HPAM polymers with very high molecular weights explains the role of their viscoelastic behavior in improving 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 ∼6% higher recovery with ∼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.

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