Chemical enhanced oil recovery (CEOR) is emerging as a vital method to increase recoverable oil from aging reservoirs, but economics and the achievable incremental recoverable oil will dictate its widespread adoption. Water chemistry of the injectant plays an important role in the effectiveness and cost of CEOR. For instance, in polymer floods using partially hydrolyzed polyacrylamides (PHPAM), it is well established that the realizable viscosity for a given polymer concentration is a function of the injection water salinity, with divalent cations having a more detrimental impact than monovalent ions. Studies have shown that five to ten times less PHPAM is required using desalinated or lower salinity water when compared to seawater, resulting in an equivalent cost savings related to polymer consumption. In surfactant flooding, the relationship between the injectant water chemistry and the effectiveness of the flood is a more complex equation. Considerable research is ongoing to predict optimal salinities for surfactant flooding and the added benefit of using salinity gradients through the life of the surfactant flood. And lastly, alkalinity addition to either a surfactant and/or polymer flood dictates the injectant water be softened to prevent calcium and magnesium carbonate precipitation if sodium carbonate is the source of cost-effective alkalinity. Further complicating matters, the injection water salinity must be kept at a level sufficient to prevent clay swelling in the reservoir, and sulfate levels must be low enough to prevent precipitation of the sparingly soluble salts of barium or strontium, if present.

This paper analyses the economics associated with an alkalinity-surfactant-polymer (ASP) flood using general characteristics of an ASP flood project and demonstrates how optimizing the water quality of the injection water can change the project economics. Creating customized water chemistries of optimal blends throughout the CEOR flood is possible by carefully choosing membrane and polishing technologies which allow for a range of possible chemistries. Reducing the cost of CEOR through optimized water chemistries may further enhance oil recovery on a global scale by expanding the number of reservoirs in which CEOR is economically attractive.

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