Geochemical evolution of the Ahuachapán geothermal field, El Salvador, C.A.

Abstract

More than 2800 fluid samples have been collected during the 20 years history of production from the Ahuachapán reservoir. A systematic analysis of this data shows a trend of dilution for the majority of the production wells. Also observed is a decline in enthalpy, both measured and based on geothermometers. This trend of dilution and declining reservoir enthalpy characterizes wells in the northern and northeastern sectors of the present wellfield. In the south and southwest, however, stable conditions are observed and the Na-K-Ca geothermometer indicates unchanged temperature. This is interpreted as two sources of recharge to the wellfield. The colder and less saline fluid recharges the reservoir in the northern sector and is most likely taken from the saturated reservoir zone above. The hot and deep recharge, on the other hand, comes into the wellfield from south. A simple lumped parameter model study indicates that 10-20% of the total mass produced is taken from the colder recharge. The mineral state of the reservoir fluid shows a low scaling potential for most of the present wells.