L Thermodynamic Classification of Geothermal Systems

A thermodynamic approach is presented to classify geothermal reservoir as vapor or liquid dominated. According to it, the vapor dominated reservoir has specific volume of fluid (i.e. combined vapor and liquid) greater than the critical volume of water whereas the liquid dominated reservoir has lesser specific volume. Enthalpy is not a conservative entity in a geothermal system. Apart from it, the measured enthalpy depends on the pressure and temperature conditions of well head and separator. It means that the enthalpy measured at well separator is not reservoir enthalpy. It should be called as production or discharge enthalpy instead of reservoir enthalpy at the temperature and pressure conditions of well head and separator. Using these concepts and a simplified two phase flow, a method is developed to calculate the specific volume of reservoir fluid. The approach is applied in case of well M-19A of Cerro Prieto geothermal system. The Cerro Prieto reservoir under this well is vapor dominated.