Abstract
We review the methods based on the measurement of CO2 emissions for the computation of geothermal heat flow, both at a local (hydrothermal sites, a few km2) and regional scale (hundreds km2). At the local scale, we present and discuss the cases of the Latera caldera and Torre Alfina (Italy) geothermal systems. At Torre Alfina and Latera, the convection process sustains a CO2 emission of ~1 kg s−1 and ~4 kg s−1, and heat flows of 46 MW and 130 MW, respectively. At the regional scale, we discuss the case of the central Apennine (Italy), where CO2 mass and enthalpy balances of regional aquifers highlights a wide and strong thermal anomaly in an area of low conductive heat flow. Notably, the CO2/heat ratios computed for the central Apennines are very similar to those of the nearby geothermal systems of Latium and Tuscany, suggesting a common source of CO2-rich fluids ascribed to the Tyrrhenian mantle.
Highlights
Since the late seventies numerous studies have pointed to high carbon dioxide emissions in tectonically active regions frequently hosting geothermal systems [1,2,3]
The heat flux of a region is a central parameter in geothermal prospecting and it is normally measured through expensive deep drillings
This classical method allows the estimation of the conductive heat flux but not of the convective-advective heat transported by the fluids
Summary
Since the late seventies numerous studies have pointed to high carbon dioxide emissions in tectonically active regions frequently hosting geothermal systems [1,2,3]. In detail we will show how the measurement of soil CO2 degassing can be used to compute both the thermal emission from the hot soils of hydrothermal sites and the total heat associated with the convection of geothermal liquids. The thermal energy involved in the upflow of the geothermal liquid of the Torre Alfina system was computed with Equations (1) and (2), considering temperature and mCO2 data of the wells reported in [34] Among these data we selected the maximum temperature (150 ◦C) at which corresponds a mCO2 equal to 0.33 mol kg−1. Considering the CO2 diffuse emission of 1.06 kg s−1, we estimated a geothermal fluid upflow (QL) equal to 73.2 kg s−1 and an associated thermal energy (QH) equal to 46 MW
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