Gas-mineral equilibrium in the Berlín geothermal field, El Salvador

Abstract

The aquifer fluid composition at the depth of the volcanic geothermal system of Berlín, El Salvador, was reconstructed based on samples of steam and liquid from discharge fluids of 14 two-phase producer wells sampled during the period from July to October 2013 using the chemical speciation program WATCH 2.4. Berlín geothermal field is a liquid-dominated field with discharge enthalpies close to that of the reservoir liquid, except for well TR18B which has higher enthalpy because of phase segregation between the liquid and fluid phase in the depressuration zone around the well. The objectives of this study were to interpret deep fluid gas composition with the purpose of understanding gas behaviour in the geothermal system. The state of equilibrium between CO2, H2S and H2 in the aquifer water and selected mineral assemblages that could potentially fix the concentrations of these reactive gases, in the temperature range from 200 to 300 °C, were studied. Field scale distribution of these gases, along with N2, indicates that the H2S and H2 are temperature controlled; CO2 follows a similar pattern in field scale distribution as N2 which is a conservative gas in geothermal systems. Aquifer liquid H2S concentration at the Berlin geothermal field is controlled by a close approach to equilibrium with the mineral assemblages. Hydrogen concentration for most of the wells lies close to equilibrium with the epidote-wollastonite-grossular-magnetitequartz hydrothermal mineral assemblage. The aquifer concentration of CO2 species was observed to fall below equilibrium with respect to the mineral buffers, suggesting that the concentration of the gas is source controlled.