Modeling amorphous silica precipitation: a strategy to reduce silica precipitation near coso injection wells

To reduce long-term silica precipitation near injection wells, we evaluated three strategies to reduce silica precipitation: (1) “acid injection”, (2) “alternating acid and alkali injection”, and (3) injection through “multiple small diameter injection wells”. For each case, we modeled mineral precipitation from the injection point toward the production well. We found that “alternating acid and alkali injection” was the most effective strategy to reduce long-term silica precipitation near the injection point. (1) Acid injection reduces the kinetic rate of silica precipitation by reducing the pH. This strategy significantly reduces silica precipitation but the feedback effect of acid injection causes a more long-term precipitation problem. (2) Alternating acid and alkali injection reduces silica precipitation by alternating the fluid pH. The kinetic rate of silica precipitation is reduced when acid is injected. When alkali is injected, silica becomes undersaturated and dissolves by the formation of NaHSiO3(aq). Since the pH of mixed water is not modified by this treatment, no significant feedback effect is expected. (3) The use of small diameter injection wells and slow injection rate allows fast temperature increase down the well. The increased temperature results in higher solubility of silica and prevents silica precipitation near the injection point. This strategy, however, is limited by the slow injection rate and silica and calcite precipitation in the pipeline and wellbore.