Recent applications of isotope geothermometers

Isotopic geothermometers relative to H-O-C stable isotopes in the gas mixture of geothermal fluids were applied in the Larderello area during 1978 within a programme sponsored by the IAEA. The reservoir temperatures obtained differed depending on the isotopic exchange reaction used. Figure 1 shows the differences between calculated (Ti) and measured (Tm) temperatures for selected geothermal wells spread over the field. Depending on the different kinetics of the isotopic exchange reactions considered, Δ(CO2-H2O) gives very similar values to those measured at the well head and Δ(CH4-CO2) shows the strongest deviations (up to 170 deg. C). The CH4-H2 and H2O- H2 pairs are in an intermediate position, with values that are 100 and 40 deg. C higher respectively than the measured ones. The last two groups of temperatures were considered useful for evaluating thermal conditions in the deepest part of the geothermal reservoir. Incongruent results were obtained in 1986 from the application of the same geothermometers to the gas mixtures delivered from the seven productive wells in the Larderello area. A contrasting pattern with respect to the results presented is shown. The geothermometers indicate temperature values lower than those observed at the surface, with the exception of Δ(CH4-CO2), which nevertheless gives values that are generally lower than the 1978 data. This discrepancy may be due either to isotopic disequilibrium conditions at depth or to different thermal conditions in certain parts of the reservoir. Changes in the physico-chemical conditions of the geothermal reservoir may be ascribed to: (1) the effect of continual exploitation of the steam, which may determine pressure differentials between the reservoir and surrounding groundwater; (2) the effect of reinjection of waste waters into the field, a process occurring in the Larderello area since the end of the 1970s. Although the seven wells studied were selected so as to avoid any direct influence of the reinjected waters, some modifications can still be detected in the isotopic composition of the steam produced. The δD/δ18O values of seven samples representative of the seven wells studied with the isotopic composition of steam from wells in the Larderello area before reinjection are compared. Two of the 1986 samples clearly deviate from the 'normal' trend, and a possible extrapolation-of these data seems to converge on an area which corresponds to the isotopic composition of reinjected waters. Studies of the isotopic composition of gas produced by other wells nearer the reinjection zones and of the chemical composition of gas mixtures and gas/steam ratios, measured during the pre-reinjection period, could verify this hypothesis