Chloride and Boron behavior in fluids of Los Humeros geothermal field (Mexico): A model based on the existence of deep acid brine
2011
Bernard, Ruben | Taran, Yuri | Pennisi, Maddalena | Tello, Enrique | Ramírez, Alejandro
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 (500–600mg/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 0.1M of NaCl, 0.05M of H₃BO₃ and variable amount of HCl (0.001–0.1M), 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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