| Abstract |
The Los Azufres, Mexico geothermal field has been under continuous exploitation since 1982. The vapor phase at this reservoir has an heterogeneous composition, showing a wide range of non-condensible gas concentrations between 1% and 9% of total gas weight. NCG's measured in this reservoir typically contain CO2, H2S, NH3, CH4, O2, H2, He, N2 and Ar. Carbon dioxide is the major constituent found in the entire field, representing between 70% and 99% in total NCG weight. The highest CO2 content is found in shallow wells with high steam quality, located near important conductive faults. H2S is the second most important gas in total weight, varying between .2 and 13%. The chemistry of fluids in this reservoir originated from volcanic processes and is controlled by temperatures at depth, mineral solubility, pH values and mineral equilibrium. Simultaneously to fluid extraction, significant amounts of cold liquid and air are injected into this reservoir, modifying its geochemistry and changing, in a complex way, the NCG spatial distribution. Total gases in some wells have increased because of the extra amount of atmospheric N2 and Ar injected. The molar ratio N2/Ar has been falling with time reaching in some cases the same value as in the atmosphere. Both gases follow flow paths related to high permeability, pushing the reservoir gases in the same directions and changing their natural spatial distribution. This geochemical information can be used together with a multi-component numerical simulator to deduce global permeability values between both zones and to analyze non-isothermal pressure tests with real geothermal fluid. The thermodynamic effect of reinjection has been beneficial for the energy production and longevity of this geothermal field. In this paper the observed evolution and spatial distribution of NCG's at Los Azufres geothermal field are reported for the period 1981-1999. |