Screen Factor Polymer Characterization: Improved Screen Factor Technique, Apparatus, and Analysis

Abstract

Abstract Improvement of mobility control in conventional oil reservoirs is commonly achieved through polymer flooding. This enhanced oil recovery (EOR) technology involves the addition of hydrolyzed polyacrylamide (HPAM) to the injection fluid to increase the viscosity of the displacing phase in the reservoir. The mobility improvement of a polymer flood is defined as the resistance factor (RF), which is experimentally measured by comparing the flow characteristics (e.g., pressure drop, flow rate) of the polymer solution against its solvent (or other simulate fluid for the waterflood). Equations for these comparisons are built upon Darcy's law for fluid flow, which applies in low shear rate conditions (like deep reservoir flow) where the pressure drop in a given section of the porous media is linearly related to the flow rate for a given fluid viscosity. However, the viscosity of HPAM solutions follow non-Newtonian behavior that changes with shear rate, typically following a shear-thinning trend. Flow through complex porous media that is representative of the reservoir can introduce elongational (or extensional) flow, which can cause a "shear-thickening" region where the polymer's apparent (in-situ) viscosity increases according to its viscoelastic characteristics. Since predicting the RF potential of a polymer solution is a primary goal of laboratory screening and formulation work for EOR projects, polymer evaluations often incorporate experimental methods that probe this viscoelastic potential. Screen factor (SF) is a long-established method that is often considered to characterize polymer solutions' viscoelasticity with a relatively simple apparatus and fast measurement. This study introduces a new method for conducting screen factor measurements that improves upon the original design and protocol (as described in API RP 63). Validating the efficacy of the new design required an in-depth examination into the nature of SF measurements. The proposed novel design and methodology was able to replicate benchmark results generated according to API RP 63 while improving ease of use, measurement precision and accuracy, and level of data generation to allow for in-depth measurement analysis. While investigating the principles that govern standard gravity drainage screen factor, it was found that the solvent flows under non-linear conditions, precluding the application of linear flow equations (such as Darcy's law) and explaining why SF is a wholly unique value that cannot be directly related to other measurements (e.g., porous media RF or in-situ viscosity). Through rate controlled experiments with the screen pack from a SF setup (five 100 mesh screens), it was determined that screen factor does not appear to be a purely viscoelastic measurement, but rather exerted a shear rate in the transition regime from viscous to viscoelastic flow under the studied conditions. While useful applications of screen factor are recognized, the discussed analyses bring attention to the limitations of SF. In reference to RF results generated in porous media (Berea core), alternative laboratory experiments (e.g., CaBER evaluation or RF with an in-line filter) are shown to provide more effective characterization of the studied polymers' viscoelastic potential compared to screen factor measurements.