Choride and boron behavior in fluids of Los Humeros geothermal field (Mexico): A model based on the existence of deep acid brine

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, delta(11)B +/- 2 sigma = -0.8 +/- 1.6 parts per thousand), 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 similar to 350 degrees 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 degrees C with 0.1 M of NaCl, 0.05 M of H(3)BO(3) and variable amount of HCl (0.001-0.1 M), then a partial condensation of the produced vapor at 250 degrees C and then separation of the steamwater mixture at 150 degrees C. Results of simulation are in a qualitative agreement with the observed data. (C) 2011 Elsevier Ltd.