| Abstract |
Vapor phase at volcanic reservoirs has a heterogeneous composition, showing very often a wide range of non condensible gases (NCG) concentration. For example, the chemistry of fluids in the Los Azufres, Mexico geothermal field originated from volcanic processes and is controlled by temperatures at depth, mineral solubility, pH values and mineral equilibrium. The NCG concentration at this field ranges between 1% and 9% of total gas weight in the steam phase, and it typically contains CO2 , H2S, NH3 , CH4 , O2 , H2 , N2, He and Ar. It may also contain Ne, Kr and Xe, but these gases are not recorded. Carbon dioxide is the major constituent found in the field, representing between 70% and 99% of total NCG weight. H2S is the second most important gas, varying between 0.2% and 13% in the total weight. Simultaneously to fluid extraction, significant amounts of cold liquid and air are injected into the reservoir. Total gas concentration found in some wells are increased because of the extra amount of atmospheric N2 and Ar injected. Rare gases such as He and Ar measured in this field show a low, but general trend to increase when the corresponding wells are subjected to continuous production, with few unusual exceptions. In other cases, the dependence of both gases on production is not clear. Nevertheless, important anomalies in He concentration at Los Azufres were observed before the occurrence of some earthquakes in Mexico. |