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.
Highlights
Gas geochemistry is useful in geothermal exploration and exploitation, especially in vapordominated fields where very little is known about the deep reservoir liquid
The method was suitable for gas data from the Los Humeros geothermal system in order to understand the dynamics of the reservoir at initial and present conditions (Arellano et al, 1998; Barragán et al, 1999, 2000a,b)
The dark mark corresponds to the reference point found by Nieva et al (1987), calculated through equilibrium of the FT reaction for the measured reservoir temperature
Summary
Gas geochemistry is useful in geothermal exploration and exploitation, especially in vapordominated fields where very little is known about the deep reservoir liquid. In this method more local oxidant conditions are assumed, implying the occurrence of high concentrations of H2S and relatively low concentrations of both H2 and NH3 in the fluid. The method was suitable for gas data from the Los Humeros geothermal system in order to understand the dynamics of the reservoir at initial and present conditions (Arellano et al, 1998; Barragán et al, 1999, 2000a,b). The objective of this work is to investigate the effects of exploitation in Los Azufres reservoir with time through the changes in both the reservoir temperature and the excess reservoir steam of wells
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