Carbonyl sulphide (COS) in geothermal fluids: an example from the Larderello Field (Italy)

The carbonyl sulphide (COS) content in the fluids of 12 wells in the Larderello geothermal field ranges from 0.005 to 0.1 ?mol/mol. Measured data are comparable with the theoretical concentrations, considering a homogeneous gas phase at the temperature and pressure conditions of the reservoir. However, the low temperature dependence of equilibrium constants of reactions involving COS prevents us from using them as geothermometers. On the contrary, PC02 estimates in the gas equilibration zone can be inferred from the H2S/COS ratio. The calculated CO2 partial pressures are comparable with those estimated by means of the H2/CO ratio.