Transport of EOR polymer solutions in low permeability porous media: Impact of clay type and injection water composition

Abstract The application of polymer flooding for enhanced oil recovery (EOR) to low-permeability porous media (below 100mD) can be very challenging as high polymer retention and poor injectivity are frequently observed. The challenges are mostly related to polymer solutions properties (ionic strength and hardness) and porous media mineralogy (clay content). This paper reports on an experimental study that aims at drawing a better picture of the mechanisms governing the transport of polymer solutions in low permeability clayey porous media. Results confirm the major role played by the injection water composition (salinity and hardness) on polymer conformation and on polymer-minerals interactions. Strong interactions between polymer and clay are also evidenced with significant differences according to the clay type: good propagation and high polymer retention in an uncharged and non-swelling clay (kaolinite) and poor propagation with lower than expected retention in charged or swelling clays (illite, smectite). For kaolinite, the results are interpreted in terms of fast formation of a polymer adsorbed layer on the solids surface whereas, for illite and smectite, they can be explained by the slow diffusion of polymer into clay aggregates, whose presence was evidenced by microscopic analysis. These outcomes stand as new elements for understanding and modeling the transport of polymer solutions in low permeability sandstone reservoirs. They also allow classifying the clays in view of their practical impact on the feasibility of polymer flooding operations.