Thermodynamic behaviour of the Bagnore geothermal field

In 1959 production began from the Bagnore reservoir near the Mt. Amiata Volcano. The reservoir gas was initially at a pressure of approximately 23 ata. Its noncondensable gas content was more than 80% by weight, most of which was CO₂. During the first few years of production the noncondensable gas content and the reservoir pressure dropped simultaneously to about 10% by weight and 7 ata respectively. They have been changing very slowly since that time.Detailed studies of hydrogeological data from the Bagnore field were made as a part of this work. The initial reservoir temperature was estimated to be between 170 and 180°C. The history of watering-out of individual wells on the periphery of the field was examined. The depth of fractures in these wells can be correlated with the gas-water interface in the reservoir which is assumed to rise in direct proportion to the drop in reservoir pressure.A mathematical model which accounts for thermodynamic and chemical equilibria between the vapor, liquid and solid carbonate phases in the reservoir was developed and applied to a study of the initial conditions in the reservoir. A lumped-parameter model of a 2-phase, 2-component system was then developed. This CO₂-H₂O, liquid-vapor model was used to calculate history of pressure and composition for the reservoir. These calculated histories compared favourably with those observed in the field.This research confirms the hypothesis that there was initially a large accumulation of noncondensable gas in the reservoir, and that it was drawn off during the first years of exploitation. Model calculations for the initial state of the reservoir indicate the CO₂ initially present could not be derived solely from local carbonate rocks. Calculations with the producing-state lumped-parameter model, furthermore, indicate that the long-term producing concentration of CO₂ cannot be accounted for by assuming reasonable amounts of CO₂-saturated liquid-water influx into the reservoir. These results point out that further investigation into the nature of CO₂ and water influx into the reservoir are required.