Metal sulphide scaling in production wells in Ahuachapan geothermal field, El Salvador

Abstract

Ahuachapán geothermal field has been exploited since 1975. For some of the production wells, deposits have been observed in separators during maintenance breaks of the power plant. X-ray diffraction analysis showed that these deposits were mainly composed of sulphide minerals. In this study, geothermal water collected on surface was used to determinate the saturation state of sulphide minerals such as bornite, chalcopyrite, pyrite, chalcocite, covellite, galena, pyrrhotite, and sphalerite. The selection of the minerals included in the study is based on geological reports. The reconstruction of fluid at reservoir conditions was done by using the WATCH program, as were steam fractions resulting from adiabatic boiling of the reservoir fluid. The saturation state of sulphide minerals was modelled by using the SOLMINEQ.88 program. The results of the simulation show that pyrite and chalcopyrite are expected to be saturated under reservoir conditions in the wells that were considered in this study. Both minerals become super-saturated during the boiling process until the separator temperature is reached. Covellite, bornite, chalcocite, and galena were undersaturated at reservoir conditions but during the boiling process they become saturated. Pyrrhotite and sphalerite were under-saturated in all cases and therefore have no possibility of scaling neither under reservoir conditions nor in the boiling process.