Are Field Polymer EOR Projects Reaping the Benefits of Sor Reduction Due to Polymer Viscoelasticity?
Authors: Azad Madhar Sahib , Seright Randall S 
Published: October 2024

For EOR polymer solutions, viscoelasticity is a rheological phenomenon that has a strong time (therefore flux) dependency and has been tied to significant reductions in residual oil saturation (Sor) during lab core floods at high flux conditions. However, the question of whether the polymer's viscoelastic effects reduce Sor over a significant portion of a polymer-flooded reservoir remains unanswered.

Two methodologies are used in this paper to examine whether polymer-flood projects conducted using vertical and horizontal wells across nine countries (Argentina, Austria, Canada, China, India, Oman, Russia, Suriname, USA) are experiencing Sor reduction due to shear thickening induced-viscoelastic effects. 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 effect of variables such as radial distance from the wellbore, well-spacing, horizontal well length, and thickness on Darcy velocity are examined. In Method 2, the Sor reduction potential of the polymer systems used in various field projects is evaluated by analyzing relevant core flood experiments conducted in various labs. The observations from the lab 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 are too low (in the range of ~0.01 to 0.2 ft/day) and unlikely to reach the onset velocity for viscoelastic behavior (i.e., &gt;1 ft/day for most field conditions). For most vertical polymer injectors in permeable reservoirs, only 2-3% of the reservoir will experience fluid velocities high enough for viscoelasticity to potentially be important. Less permeable reservoirs (&lt;200 md) could experience the onset of viscoelasticity at low rates (e.g., ~0.17 ft/d), but even so, only a very small fraction of the reservoir is expected to achieve this onset flux. For an extreme case of a low thickness (10 ft), short horizontal well length (1210 ft), and a shorter well spacing of 656 ft, an average velocity of 1.16 ft/day and a pressure gradient of 7.65 psi/ft was estimated for the Matzen field polymer flood. At a very short well-spacing of 100 ft in the Pelican Lake polymer flood, the average velocity is 2.05 ft/day. Although both these average velocities are 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. It also discusses the potential role of other effects for Sor reduction in existing polymer floods, including wettability alteration by the polymer and secondary-versus-tertiary polymer flooding effects. EOR researchers are advised to use the realistic field-relevant fluxes during laboratory assessments while studying Sor reduction.