Acid Fluids in Geothermal Systems: Case Study of Cerro Prieto IV sector, México

Abstract

Fluids of low pH are common in active geothermal fields under exploitation. More often they occur in geothermal systems associated with recent volcanism and may also occur in superheated steam containing HCl. In Mexico two of the main geothermal fields have produced corrosive fluids. In the Los Humeros geothermal field during the early exploitation stage some wells produced highly aggressive fluids. In the Cerro Prieto geothermal field, particularly in the Cerro Prieto IV sector, some wells started to produce low pH fluids; the main evidence is observed as accelerated corrosion of conduction pipe lines and changes in the chemistry of fluids.

In both fields the formation of acid fluids may be related to a deep zone where the liquid fraction is low and the highest temperatures have been recorded. In a recent study (Arellano et al., 2009) found that in the early exploitation stages the Cerro Prieto IV sector seems to be divided in two blocks by the Fault H. One with high enthalpy (> 2000 kJ/kg) and low salinity compared to the block with enthalpy between 1400 and 1800 kJ/kg and high salinity. Evidence of acid fluids in Cerro Prieto IV are present in wells from the high enthalpy block.