| Title | Chemistry and Control of Silica at Los Azufres Geothermal Field, Mexico |
|---|---|
| Authors | Oleh WERES |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Los Azufres, silica, reinjection, modeling, pH control, carbon dioxide |
| Abstract | Injection wells at the Los Azufres geothermal field in Michoacan, Mexico are damaged by silica deposits. Silica polymerization in Los Azufres fluids were modeled to improve understanding of silica chemistry and propose solutions. A mixture of flashed brine and excess condensate is reinjected. The wells at Los Azufres produce both steam and brine, whereby the ratio of steam to separated brine is greater than would be the case where steam is generated by flashing brine. Therefore, the proportion of excess condensate in the reinjected mixture is unusually large, typically about 25% by volume. Modeling the kinetics of silica polymerization using computer code SILNUC showed that dilution of the brine by condensate strongly retards silica polymerization, and indicated that polymerization of silica and formation of silica deposits could be retarded by an additional order of magnitude by decreasing pH of the brine. The most practical way to decrease pH is to contact excess condensate with condenser vent gas (which comprises mostly carbon dioxide) before combining it with the brine. Decreasing pH and introducing oxygen will make the brine mildly corrosive, but corrosion can easily be controlled using a suitable inhibitor. It was further determined that silica polymerization involves heterogeneous nucleation induced by particles of aluminosilicate minerals which begin to form immediately after steam is separated from the brine. Therefore, silica polymerization can be further retarded by adding a silicate inhibitor to the brine downhole in the production wells. A test of silica control utilizing carbon dioxide is underway at Los Azufres with results expected in time to be reported at the Congress. |