Gas geochemistry in modeling geothermal reservoirs

Abstract

Geochemical models for Los Azufres and Los Humeros geothermal reservoirs are discussed. Los Azufres is defined as vapour-liquid (VAP-LIQ) and Los Humeros is mainly a vapour-dominated field. The geochemical conceptual model for Los Azufres reservoir was based on the spatial distribution of isotopic and chemical species in the reservoir fluid. Based on equilibrium of the Fischer-Tropsch (FT) reaction (CO2+4H2=CH4+2H2O), the “reservoir excess steam” parameter was estimated. This parameter was used to correct the total discharge composition of fluids to obtain the composition of fluids at reservoir. Results indicated that concentration of volatile species was higher at shallower levels while non-volatile species concentration was higher at depth. This pattern suggested the occurrence of a convective process with reser- voir vapour separation and condensed vapour down-flows. For the Los Humeros geothermal field, where the liquid phase is scarce, two reservoirs were found, a hotter vapour-domi- nated reservoir located at depth and a liquid-dominated shallower one. Gas equilibria based on the Fischer-Tropsch (FT) reaction and on the pyrite-hematite and pyrite-magnetite (HSH2) combined reactions provided both the reservoir temperature and the “excess steam” parameters. With exploitation it seems that fluids from the hotter reservoir rise through fractures to the shallower one.