Gas geochemistry for the Los Azufres (Michoacán) geothermal reservoir, México

R. M. Barragán, V. M. Arellano Gómez, E. Portugal, F. Sandoval, N. Segovia

Abstract


Gas data of the Los Azufres geothermal field were analyzed using a method based on equilibrium of the Fischer-
Tropsch (FT) reaction: CH4 + 2H2O = 4H2 +CO2 and on the combined pyrite-hematite-magnetite (HSH2) reactions:
5/4 H2 +3/2 FeS2 +3/4 Fe2O3 + 7/4 H2O = 3 H2S +Fe3O4 in order to estimate reservoir temperature and
excess steam. The solution of equilibrium equations produces a grid (FT-HSH2). This method is suitable for
reservoirs with relatively high H2S but low H2 and NH3 concentrations in the fluid as is the case of the Los
Azufres well discharges. Reservoir temperature and reservoir excess steam values were estimated for initial and
present conditions in representative wells of the field to study the evolution of fluids, because of exploitation and
waste fluids reinjection. This method was very useful in estimating reservoir temperatures in vapor wells, while
in two-phase wells it was found that as the well produces a smaller fraction of water, the reservoir temperature
estimation agrees qualitatively with results from cationic or silica geothermometers. For liquid-dominated wells
the reservoir temperature estimations agree with temperatures obtained from the well simulator WELFLO. This indicates
that FT-HSH2 results provide the temperature of the fluid entering the well where the last equilibrium
occurs. Results show a decrease in reservoir temperatures in the southern zone of the field where intensive reinjection
takes place. With exploitation, it was also noted that the deep liquid phase in the reservoir is changing to
two-phase increasing the reservoir steam fraction and the non-condensable gases in well discharges.

Keywords


Los Azufres geothermal field;gas geochemistry;geothermal gas equilibria;reservoir exploitation

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References


DOI: https://doi.org/10.4401/ag-3189
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X