Discrete fracture network model of the vapor zone leakages at the Copahue geothermal field

Abstract

The volcano-hosted geothermal system of Copahue is one of the most promising resources of renewable energy in Argentina. It is driven by a layered reservoir composed of a steam cap above a deep liquid-dominated reservoir. We study the surficial leakages of the shallow steam zone by performing a discrete fracture network model and a 3D forward strain-stress model of the upper caprock unit. The hydrothermal zones linked to steam cap leakages are subordinated to deep-rooted N60-striking faults. Associated damage zones exhibit higher values of fracture intensity, vertical and anisotropy of permeability than the host rocks. Joints and cross-joints likely control leakage zones, where the formers are subject to high dilation tendency. The Anfiteatro monocline might constitute the northwestern boundary of the shallow steam zone. Variations of H2O–CO2 ratio, helium isotopic signature, and CO2 between hydrothermal zones are explained in terms of petrophysical properties of the upper ignimbrites and changes in their structural control. Our results suggest that the upper caprock ignimbrite avoids the decompression of the reservoir because it provides the low fractured and low permeability layer that seals the geothermal system; conversely, ignimbrites affected by the extensional faults system enhance the shallow vapor zone given its high fracture density and permeability.