Role of CO2 in low to medium enthalpy geothermal systems in the Central Betic Cordillera (Spain)

Abstract

3There is growing interest in geothermal energy, which is considered as an efficient energy 4 solution to mitigate rising atmospheric CO2. Besides known high enthalpy geothermal systems, 5 increasing attention is paid to low temperature geothermal systems, as they are suitable for local 6 use. Although geothermal production seems to be an environmentally advantageous renewable 7 energy, it might result in significant CO2 emissions. In this study, we investigate the relationship 8 between temperature, fugacity of CO2 (fCO2), and mineral buffers in the reservoir conditions, 9 taking the low-to medium-enthalpy thermal waters in the Central Betic Cordillera as case 10 study. Using geochemical modeling, three main groups of waters have been identified 11 depending on temperature, buffering mineral assemblage, and fCO2 in their reservoir. A group 12 of waters with a reservoir temperature ranging from 70 to 90°C and located in the intramountain 13 sedimentary basins shows a fCO2 in depth ranging from ~ 6 x10 -2 and 6 x10 -1 . The reservoir 14 chemistry of this water group seems to be mainly controlled by carbonates and evaporites 15 displaying a fCO2 variation between depth and surface (ΔfCO2) of 10 -1 . Another intermediate 16 group of waters, located in an active extension zone, displays lower temperature (50-60°C) and 17 fCO2 in the reservoir (from 10 -3 to 10 -2 ). Finally, the third group of waters, located on the 18 metamorphic complexes contacts, show the highest estimated temperatures (130 -140°C) and 19 fCO2 in the reservoir (1 to 10 2 ). The two latter groups suggest increasing buffering effect of 20 alumino-silicates, in addition to carbonates and quartz. Therefore, we evidenced a strong 21 relationship between temperature and fCO2 in the reservoir as well as the potential mineral 22 buffers. We discussed the potential of geothermal systems as clean energy source based on the 23 estimation of the CO2 emissions generated by the investigated thermal systems for a practical 24 case of household heating. 25 chemical water-rock interactions, is an important parameter to consider as it "buffers" the fluid 51 chemical composition and the CO2 degassing. Therefore, understanding the relationship 52 between the mineral buffer, the fugacity of CO2 (fCO2) (or its partial pressure of CO2, pCO2), 53 and the temperature in the reservoir, is crucial to estimate the CO2 release in geothermal 54 systems. These links between fCO2 and reservoir temperature have been subject to Chiodini 55 and collaborators' investigations since the early 1990's, although they focused mainly on 56 volcanic geothermal systems (e.g., Chiodini et al.

The effect of lunar declination on CO₂degassing from central Italian Apennines

“…18,34 When tidal potential is high and the Moon is close, aquifer depths are highest and fluids are oversaturated with respect to calcite; whereas when tide potential is low, fluids are close equilibrium with calcite. Although lunar tides exert only weak stress on rock 35 without fracturing it, 31 they do indeed produce weak vertical fluid movements 18,34 causing ex-solution from CO 2 -rich waters. This process, similar to the mechanical bubbling that occurs when you shake a bottle of carbonated soda, modifies the system's overall chemical equilibrium 36 and generates fluid releasing processes.…”