Hydrogeologic Model of the Geothermal Reservoirs from Los Humeros, Puebla, Mexico

Abstract

Petrological and geochemical studies of water and gases and reservoir engineering from geothermal wells, show two geothermal reservoirs. This is confirmed by petrographic and structural correlations and the conditions of the casing in the productions intervals in the geothermal wells. The upper reservoir in andesites with augite has neutral pH and is non-corrosive. The lower reservoir in basalts and andesites with hornblende has very high temperatures and an acid pH. They are separated by a layer of vitric tuffs (the Toba Humeros). Differences in elevations between potentiometrics levels in the wells do not allow the direction of the flow of the deep geothermal fluids to be inferred.

Lithological, hydrogeochemical and piezometric results from the gradient wells drilled inside the Caldera of Los Humeros, also show two superficial zones of ground water one relatively cold and the other hotter. Due to the distance between these wells and their geologic environment, neither zone (cold on warm) can be used to infer the direction of the fluid flow.

Regional studies of hydrology, geophysics, hydrogeochemistry and structural geology, confirm that the shallow ground waters (cold and warm) do not have hydraulic connection with the deep wells, water wells or springs around Los Humeros. Therefore, the recharge of the shallow waters (cold and warm), occurs inside the closed basin of the caldera of Los Humeros, delimited by topography. Those ground waters recharge the geothermal fluids through faults and fractures inside the limits of collapse of Los Humeros.

The regional geological-structural sections supported by evidence of the lithology from geothermal wells, show that the outcrops of granitic rocks and clay limestones appear at the level of geothermal reservoirs, obstructing the regional lateral recharge. The annular faults of the collapse Los Humeros and Los Potreros form impermeable barriers to the lateral fluids. The supply to the geothermal reservoirs is produced only inside the limits of the Los Humeros, Caldera.