Improved Enhanced Oil Recovery Polymer Selection Using Field-Flow Fractionation

Abstract

Summary Various polyacrylamide polymers have been successfully applied in chemical enhanced oil recovery (EOR) projects. These polymers are characterized by high molecular weights (MWs) to achieve high viscosifying power. The selection of polymers for chemical EOR is a crucial step in the field testing and implementation of such EOR projects. The reason is that per-pattern operating expenditures (OPEX) are one of the sensitive cost drivers for such projects. The important parameters for the selection of polymers are the filtration ratio, viscosifying power, polymer retention, and stability of the polymers at reservoir conditions. The MW distribution (MWD) of the polymers has a major impact on polymer properties and performance. Measuring the MWD is challenging using conventional methods. Field-flow fractionation (FFF) enables the determination of the distribution to select and quality check various polymers. Multiangle light scattering (MALS) was used as the main detector. Polymers with high MWs (>1 MDa) are used for EOR to obtain highly viscous aqueous solutions. The MWD of the polymers is crucial for the solution characteristics. Conventional analysis of polymers is performed using either viscometry, which is able to determine the average MW but does not give information on MWD, or size-exclusion chromatography (SEC), which is restricted to MWs of <20 MDa. FFF is based on the analytes flowing at different speeds in a channel-dependent on their size and mass. This effect leads to separation, which is then used to determine the MWD. FFF allows determining the MW and MWD of various ultrahigh MW polyacrylamides (HPAAMs). The FFF measurements showed that, despite similar MWs being claimed, substantial differences in MWD are observed. This technology allowed the quantification of the MWD of HPAAMs up to an MW of 5 GDa. Furthermore, gyration radii of the HPAAM molecules were determined. Selecting polymers on viscosifying power only is not addressing issues related to different MWs and MWDs such as selective polymer retention and degradation of the high molar mass part of the distribution. The results were used to improve the polymer selection for chemical EOR projects. In addition to viscosifying power and price, also the MWD and changes of the MWD in the porous medium are considered in the selection of the polymer. Overall, this work presents a new technique for the analysis of ultrahigh MW EOR polymers, which enables the possibility to determine the full range of polymer MWDs. This available information enhances the EOR polymer selection process addressing selective polymer retention and mechanical degradation in addition to the viscosifying power of polymers.