Polymer flooding optimization, minimizing fouling in heat exchangers

Polymer flooding is a well-known enhanced oil recovery (EOR) technique, commonly deployed after water flooding as tertiary recovery. Management of the water produced is an important aspect in production operations, particularly in terms of flow assurance. There has been a great deal of attention to impact of water cycle in production facilities, in terms of inorganic mineral scale deposition, hydrate formation and corrosion. However, the interaction of the water produced and the injected EOR chemicals has not been as thoroughly studied, despite the fact of the significant impact on surface production facilities. Fouling in heat-exchangers is not commonly considered in polymer flooding EOR strategies at the front end engineering and design (FEED) stage. The structure of EOR polymers makes them susceptible to multiple factors within a reservoir environment, such as thermal hydrolysis and the presence of divalent ions the produced water. Polymers in the presence of divalent cations precipitate, known as cloud point, where compatibility reduces with temperature. Therefore, fouling in heat exchangers is expected, when polymer breaks through, and as a consequence the rate of heat transfer decreases. Although, the exact mechanism is extremely complex, due to the numerous chemical and physical phenomena, it depends mainly on the nature of the crude oil and the composition of the produced brine, particularly the concentration of divalent ions. In this study, the impact of fouling in the production facilities is described by the Fouling Index (FI), which is the product of divalent ions concentration in the produced water and the produced polymer concentration. The purpose of this manuscript is to identify optimum polymer flooding strategies in a five-spot pattern heterogeneous synthetic reservoir model, minimizing the level of fouling in the heat exchangers, by minimizing FI. Fouling in heat exchangers prevent the efficient production of hydrocarbons; with the corresponding halt in production and loss of revenue. The optimization results identified the optimum injection polymer concentration and optimum injection water salinity. The results highlighted that reducing the salinity around 50% of the original value, the project net present value (NPV) was optimized, minimizing FI and therefore achieving an optimum oil production. The reduction of salinity can be achieved economically using nano-filtration at the lowest level of rejection, 60%. In conclusion, the results identified optimum polymer flooding strategies, where oil recovery and NPV is maximized, fouling is minimized, aiming for the most efficient continuous oil production.