Chloride and Boron behavior in fluids of Los Humeros geothermal field (Mexico): A model based on the existence of deep acid brine |
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Authors: | Ruben Bernard Yuri Taran Maddalena Pennisi Enrique Tello Alejandro Ramirez |
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Affiliation: | aInstitute of Geophysics, Universidad Nacional Autónoma de México, 04510 Mexico D.F., Mexico;bIstituto di Geocronologia e Geochimica Isotopica, Vie Maffi 36, 56125 Pisa, Italy;cComision Federal de Electricidad, Volta 20, Morelia, Mexico;dUniversidad Autonoma de Guerrero, Taxco, Mexico |
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Abstract: | Geothermal field Los Humeros, Mexico, is characterized by a high steam fraction in the well fluids, by extremely high B concentrations in separated water (grams per liter, with a magmatic B signature, δ11B ± 2σ = −0.8 ± 1.6‰), by the absence of correlation between B and Cl concentrations and by positive correlation between B content in separated water and fluid enthalpy. Such behavior is consistent with the existence of moderately acid brine (pH 3–5) at depth with a high B concentration (500–600 mg/kg). In this case a 3-level model can be suggested for Los Humeros: (1) immature, moderately acid brine at depth which is boiling at a temperature ∼350 °C producing the HCl-bearing vapor with a high B content; (2) partial condensation of this vapor at the upper level accompanying water–rock interaction and neutralization; (3) formation of a shallow water-dominated aquifer above a lithologic low-permeability boundary as has been proposed by other authors. A thermochemical computer code has been used to model boiling of an aqueous fluid at 350 °C with 0.1 M of NaCl, 0.05 M of H3BO3 and variable amount of HCl (0.001–0.1 M), then a partial condensation of the produced vapor at 250 °C and then separation of the steam–water mixture at 150 °C. Results of simulation are in a qualitative agreement with the observed data. |
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