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

• Main Authors: N. Segovia, F. Sandoval, E. Portugal, V. M. Arellano Gómez, R. M. Barragán
• Format: Article
• Language: English
• Published: Istituto Nazionale di Geofisica e Vulcanologia (INGV) 2005-06-01
• Series: Annals of Geophysics
• Subjects: Los Azufres geothermal field; gas geochemistry; geothermal gas equilibria; reservoir exploitation
• Online Access: http://www.annalsofgeophysics.eu/index.php/annals/article/view/3189

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.