The oxygen isotope exchange between carbon dioxide and water in the Larderello geothermal field (Italy) during fluid reinjection

The oxygen isotope compositions of CO₂ and water vapor samples collected from Larderello geothermal wells after the start of the fluid reinjection program suggest that if the oxygen isotope exchange in the vapor phase does, in fact, exist, it is a very slow process when compared with the residence time of the fluids in the geothermal reservoir. This is because carbon dioxide and water vapor phases could not have equilibrated significantly in the vapor-dominated reservoir. This conclusion implies that the oxygen isotope composition of carbon dioxide may possibly be used as a tool in geothermal exploration for revealing the presence of liquid water in deep geothermal systems. Based on the interpretation of the oxygen isotope data of the CO₂, I propose that the origin of the low oxygen isotope ratios of carbon dioxide at Larderello is the high-temperature exchange with liquid water in the lower reservoir. In Larderello, the liquid water–rock interaction in the lower reservoir may have increased the ¹⁸O/¹⁶O ratio of the recharge meteoric component. By contrast, lack of high-temperature liquid water in the upper reservoir suggests that the large “δ¹⁸O shift” described for the upper-reservoir steam during the last decades reflects varying degrees of dilution of the lower-reservoir fluid by the low-¹⁸O vaporized liquid water of meteoric origin that recharges the field at shallow depth, with local contribution from still deeper high-¹⁸O water vapor of magmatic origin. The low oxygen isotope composition of the Mesozoic carbonaceous rocks that form the upper reservoir, consequently, likely represents a “fossil” record of the past hot-water geothermal stage.     

The Cerro Prieto IV (Mexico) geothermal reservoir: Pre-exploitation thermodynamic conditions and main processes related to exploitation (2000–2005)

The Cerro Prieto IV (CP IV) reservoir, located in the northeastern part of the Cerro Prieto (Mexico) geothermal field, was studied in order to define its pre-exploitation conditions and initial (2000–2005) response to exploitation. Bottomhole thermodynamic conditions were estimated by modeling heat and fluid flows using the WELLSIM program and well production data. Produced fluid chemical and isotopic data were also analyzed to investigate characteristic patterns of behavior over time, which were then compared against simulation results to obtain a conceptual model of the CP IV reservoir. According to the proposed model, two zones in the reservoir – separated by Fault H and producing fluids of different characteristics – were identified under pre-exploitation conditions. Wells in the area to the east-southeast (south block) produce very high-enthalpy fluids (≥2000 kJ/kg), with very low chloride (≤7000 mg/kg) and high CO₂ (>6‰ molar) and δD (<−94‰). In contrast, wells toward the west-northwest (north block) show moderate-enthalpy fluids (1400–1800 kJ/kg), with high chloride (∼12,000 mg/kg) and relatively low CO₂ (<6‰ molar) and δD (<−94‰). Dilution caused by cooler water entry, boiling due to steam gain, both occurring in the north block, and steam condensation in the south block were identified as the three main reservoir processes associated with exploitation. Also, it was found that the dynamics of the CP IV reservoir is controlled by the Fault H system.