Qualifying an “Emulsion” Polymer for Field Use - Lab-scale Assessments on Adsorption and Injectivity

Polymers prepared as concentrated inverse emulsions (“emulsion” polymers) appear valuable for offshore EOR as they involve simple dilution procedures. However, recent studies have shown that these polymers could entail permeability reduction effects which question their injectivity.

Initial screening tests, for a specific high temperature and high salinity field case, showed no sign of plugging. Adsorption was, however, reported to be extremely high and a more elaborate work package was therefore initiated.

An “emulsion” polymer and its “dry” equivalent (from which oil and most surfactant were removed) were put through coreflood tests focused on injectivity (including plugs with intermediate pressure taps) and adsorption (evaluated from measurements of the irreversible retention). Injection in porous media, at reservoir type rates, revealed increases of the resistance factors, RF, in two successive fronts: a “quick” front with RF values consistent with the polymer viscosity and a “slow” front with much higher RF. The quick front corresponds to the propagation of the viscous polymer, as is it associated to the polymer breakthrough. The slow front is attributed to the deposition of oil droplets coming from the synthesis process of the polymer. Similar behavior has been observed from tests carried out in monophasic conditions, in presence of residual oil and at different injection velocities, but not with solutions of the “dry” polymer sample. As oil droplets impact the polymer inaccessible pore volume and hence the volumes at breakthrough, a specific procedure had to be developed to determine polymer adsorption. With this procedure the “emulsion” and the “dry” polymer samples yielded comparable results. Following this work package adsorption was consequently no longer a concern, while injectivity again proved questionable. Finally a new version of the “emulsion” polymer, with an improved surfactant formulation package, was injected through porous media with no sign of plugging/pressure build-up.

In conclusion we do not foresee problems associated with the chosen “emulsion” product. In the laboratory we have, however, not mapped all rate regimes nor with actual mineralogy. Hence sufficient emulsion propagation cannot be guaranteed (in particular as the permeability damage could propagate in-depth). Injectivity therefore still remains as one major factor to be investigated in the coming field trial.