PR-318-06701-R01 Predicting and Mitigating Salt Precipitation

Brine solutions are often produced during gas storage operations, and when these solutions encounter changing temperature or pressure, salt can precipitate. This salt (NaCl) can impair productivity and may even result in abandonment of wells. Dilution with fresh water is the preferred method of mitigating the salt buildup. Existing salt deposits are dissolved with fresh water. Additionally, fresh water is used as a produced water diluent to reduce supersaturation with respect to NaCl. However, this can be expensive depending on the method of application, and as fresh water becomes scarcer, the method will become more expensive. A number of chemicals are reported to reduce or prevent salt deposition. Among them are ferrocyanide and some organic molecules such as nitrilotriacetic acid and nitrilotriacetamide (NTAm). These inhibitors are thought to prevent salt precipitation by crystal modification or by interfering with crystal growth. Their effectiveness, however, varies with their concentration and the chemistry of the brines. For example, ferrocyanide is a very effective salt inhibitor; however, at low pH or in the presence of large amounts of iron it decomposes rendering it ineffective. As shown in Figs. 1 and 2 where supersaturated solutions of NaCl are cooled to room temperature, the performance of both chemicals is reduced as the reservoir water increases in calcium and/or magnesium, eventually becoming ineffective. But even when precipitate is formed, both inhibitors affect the properties of the precipitate so that there is no caking with no tendency to form large crystals associated with sodium chloride scale. The questions concerning the environmental issues associated with ferrocyanide that arose during Phase I are addressed in this report.