Abstract
An exhaustive review of the available data related to the reservoir of the volcano-hosted Copahue geothermal system is presented. Reservoir interpretation to date suggests a vapor-dominated nature. However, the review provided reveals the limitations of this model, regarding the transition from shallow argillic to deep propylitic alteration zones, gas geothermometers calibrated to near-neutral pH environment, production tests, and low recharge of the feed zone, among the other features. We propose that a layered model with a shallow vapor zone above a fluid pressurized reservoir better accounts for the known features of the geothermal reservoir. Temperature profiles define the 800-m-thick upper thermal boundary that includes the impermeable clay cap. An argillic alteration layer (smectite + chlorite + illite + kaolinite) with low electric resistivity response, consistent with the presence of smectite, composes the clay cap. A shallow steam cap develops immediately below the impermeable layer. This vapor zone has 200–215 °C, low seismic activity, and high electrical resistivity, and is highlighted by the isothermal segment of the well logs. The bottom part of the reservoir is subjected to controversy because of the lack of direct measurements. The deep liquid-dominated reservoir has a temperature of ~ 280 °C, develops below 1500 m depth under near-neutral pH condition, and probably has a pervasive propylitic alteration, and wairakite + laumontite + chlorite + epidote + prehnite + actinolite is the likely mineral assemblage. Pressure collapse might trigger the development of the shallow steam cap during deflation–inflation volcanic cycles or Pleistocene-to-recent tectonic deformation. This work points out the main weak points of the previous conceptual model, providing an alternative one based on the first comprehensive compilation of data of the Copahue reservoir. Further research, including a deep exploration drilling stage, is required to establish the physicochemical reservoir state precisely and to validate the proposed layered model.
Highlights
The definition of a conceptual model is essential during the feasibility stage of a geothermal project, given that it drives both surficial and underground exploration, constraining the inferences about the reservoir nature
This study aims to analyze the available data of the Copahue geothermal reservoir, testing the current conceptual model nature
We propose that the Copahue geothermal system is driven by the dynamics of a stratified reservoir composed of a steam-heated vapor zone above a liquid-dominated reservoir (Fig. 10)
Summary
The definition of a conceptual model is essential during the feasibility stage of a geothermal project, given that it drives both surficial and underground exploration, constraining the inferences about the reservoir nature. Barcelona et al Geotherm Energy (2019) 7:7 tectonic settings and shows different surficial manifestations, deep geological structures, and permeability conditions. These features, in turn, define the geometry of the system (e.g., Jolie et al 2015; Moeck 2014, Stelling et al 2016). Vapor-dominated systems display greater geothermal potential and have significant engineering benefits over liquiddominated ones They require about a third of the mass flow rate to achieve the same power output of a power plant, accessing a two-phase reservoir at the same conditions of temperature and pressure (Zarrouk and Moon 2014; Dipippo 2016). The reservoir conceptual model has to be carefully analyzed to avoid incorrect planning of the deep-drilling stage, inaccurate assessment of the resource, and to perform better risk and financial models to prevent a delay or cancelation of the project
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