The presence of HCl in fluids in the Los Humeros geothermal reservoir has resulted in accelerated corrosion of wells in the area known as Colapso Central and along the east direction of the field side. The main objective of this work is to recognize the origin of the fluid acidity, on the basis of the evaluation of available chemical and mineralogical information.

Considering the expected alteration due to HCl in other geothermal systems, no relation to the available information from Los Humeros was found. It is possible that acidity in the geothermal fluids is recent, and generated when the deep low liquid saturation reservoir was reached by drilling. On the other hand, the occurrence of H2SO4 is evident due to the advanced argillic alteration of surface and subsurface rocks in some areas of the field. From the paper of DíAmore et al., 1990 we made the hypothesis that in the natural state in Los Humeros a high salinity brine could exist bellow the two known reservoirs. This high salinity brine favored the formation of the HCl gas; which moved to the low liquid saturation zone when exploitation began. HCl gas was carried by the vapor phase toward the upper reservoir (high liquid saturation) forming aqueous HCl.

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