Are Field Polymer Enhanced Oil Recovery Projects Reaping the Benefits of Residual Oil Saturation Reduction Due to Polymer Viscoelasticity?

Summary For polymer solutions used in enhanced oil recovery (EOR), viscoelasticity is a rheological phenomenon that has a strong flux dependency and has been tied to significant reductions in residual oil saturation (Sor) during laboratory corefloods at high flux conditions. However, an unanswered question is whether the polymer’s viscoelastic effects reduce Sor over a significant portion of a polymer-flooded reservoir. In this paper, two methodologies are used to answer this question for polymer-flood projects across nine countries (Argentina, Austria, Canada, China, India, Oman, Russia, Suriname, and USA). In Method 1, the average Darcy velocity in each field is compared with the corresponding predicted velocity for the onset of shear thickening. Then, the effects of variables on Darcy velocity are examined, such as radial distance from the wellbore, well-spacing, horizontal well length, and thickness. In Method 2, the Sor reduction potential of the field polymer solutions used is evaluated by analyzing relevant coreflood experiments conducted in various laboratories. The observations from the laboratory results are considered in view of the fluid velocity, oil viscosity, permeability, mode of flooding, and pressure gradient of the various field projects. For most polymer floods with horizontal injectors, the highest possible Darcy velocity for various combinations of thickness, injection rate, horizontal well length, and well spacing is too low (in the range of ~0.01–0.2 ft/D) and unlikely to reach the onset velocity for viscoelastic behavior (i.e., >1 ft/D for most field conditions). For most vertical polymer injectors, less than 1% of the reservoir will experience fluid velocities high enough for viscoelasticity to potentially be important. Less-permeable reservoirs (<200 md) could experience the onset of shear thickening viscoelasticity at low rates (e.g., ~0.17 ft/D), but even so, a very small fraction of the reservoir is expected to achieve this onset flux. At a very short well spacing of 100 ft in the Pelican Lake polymer flood, the average velocity is ~1.7 ft/D. For an extreme case of a low thickness (10 ft), short horizontal well length (1,210 ft), and a small well spacing of 656 ft, an average velocity of ~1.2 ft/D and a pressure gradient of ~7.7 psi/ft were estimated for the Matzen field polymer flood. Although the average velocity is higher than the average onset flux rate, Sor reduction appears unlikely based on the macroscopic pressure gradients. This paper conveys the improbability of shear-thickening induced-viscoelasticity causing Sor reduction in field applications. It also discusses the potential role of other effects for Sor reduction in existing polymer floods, including wettability alteration by the polymer and secondary-vs.-tertiary polymer-flooding effects. EOR researchers are advised to use realistic field-relevant fluxes during laboratory assessments while studying Sor reduction.