Intermittent Recharge Processes at the Los Humeros (Mexico) Geothermal Reservoir Indicated by Analysis of Gas Data

Abstract

Gas data from wells of the Los Humeros geothermal field were analyzed in order to investigate reservoir processes occurring as a result of fluid extraction and reinjection. Equilibrium of the gas phase with mineral buffers controlling H2S concentration in fluids was assumed to occur according to the Fischer-Tropsch (FT) and the combined pyrite-hematitemagnetite (HSH2) reactions. At Los Humeros two reservoirs have been identified: a shallower liquid-dominated one with temperatures between 300 and 330ºC and a deeper low-liquid saturation one with temperature between 300 and 400ºC. In this work the behavior of gas data of two representative wells: H-1 that produces from the upper reservoir and H-7 from the deeper reservoir, is shown. The 1987-1995 and 2000-2005 trends observed in the FT-HSH2 diagram in most wells indicated that important recharge processes of hotter deeper fluids have occurred for such periods of time with maximum temperatures occurring in 1995 and 2005. Temperatures estimations for 1995 data were 310°C in well H-1 and 338°C in well H-7 while higher temperatures were estimated for 2005 data, 333°C (H-1) and 350°C (H-7). These results could be related to some increases in reservoir permeability due to tectonics. The analysis of data also evidenced the presence of steam phase reinjection returns in well discharges from 1995 to 2000.