Crustal Fault Zones (CFZ) as Geothermal Power Systems: A Preliminary 3D THM Model Constrained by a Multidisciplinary Approach

Abstract

The Pontgibaud crustal fault zone (CFZ) in the French Massif Central provides an opportunity to evaluate the high-temperature geothermal potential of these naturally permeable zones. Previous 2D modeling of heat and mass transfer in a fault zone highlighted that a subvertical CFZ concentrates the highest temperature anomalies at shallow depths. By comparing the results of these large-scale 2D numerical models with field data, the depth of the 150°C isotherm was estimated to be at a depth of 2.5 km. However, these results did not consider 3D effects and interactions between fluids, deformation, and temperature. Here, field measurements are used to control the 3D geometry of the geological structures. New 2D (thin-section) and 3D (X-ray microtomography) observations point to a well-defined spatial propagation of fractures and voids, exhibiting the same fracture architecture at different scales (2.5 μm to 2 mm). Moreover, new measurements on porosity and permeability confirm that the highly fractured and altered samples are characterized by large permeability values, one of them reaching 10-12 m2. Based on a thermoporoelastic hypothesis, a preliminary 3D THM numerical model is presented. A first parametric study highlights the role of permeability, stress direction, and intensity on fluid flow. In particular, three different convective patterns have been identified (finger-like, blob-like, and double-like convective patterns). The results suggest that vertical deformation zones oriented at 30 and 70° with respect to the maximum horizontal stress direction would correspond to the potential target for high-temperature anomalies. Finally, a large-scale 3D numerical model of the Pontgibaud CFZ, based on THM coupling and the comparison with field data (temperature, heat flux, and electrical resistivity), allows us to explore the spatial geometry of the 150°C isotherm. Although simplified hypotheses have been used, 3D field data have been reproduced.

Tectonic Regime as a Control Factor for Crustal Fault Zone (CFZ) Geothermal Reservoir in an Amagmatic System: A 3D Dynamic Numerical Modeling Approach

“…Fluids in a reservoir are affected by stresses, whether on their pressure (undrained conditions in low-permeable media with, e.g., increase in pressure under compressive stress state), or on their circulation (drained conditions in permeable media with, e.g., convection from more to less compressed regions). Our previous study, which included poro-elasticity, suggested that vertical deformation zones oriented at 30°and 70°to a maximum horizontal stress could correspond to potential targets for high-temperature geothermal energy (Duwiquet et al, 2021a;. Thus, the stress orientation has an effect on fluid flow, as already suggested by Jiang et al (2019).…”

“…In addition, weathering and fracturing processes present in fault zones tend to increase the value of the Poisson's ratio (Heap et al, 2020). The physical properties of the fluids were identical to the study of Duwiquet et al, (2021a), as well as the incorporation of the stress evolution with depth as boundary conditions. However, as detailed in the following paragraph, here we used Andersonian assumption to consider different tectonic regimes in the 3D dynamic numerical models.…”

“…This heat flux represents the sum of the mantle heat flow and the heat emitted by the decay of radioactive elements in the underlying crust. The fluid was assumed to be pure water, and the fluid density depended on pressure and temperature conditions (as detailed in Duwiquet et al, 2021a). The details on Darcy law, heat equation and Hooke law can be found in Duwiquet et al, (2021a).…”

“…The fluid was assumed to be pure water, and the fluid density depended on pressure and temperature conditions (as detailed in Duwiquet et al, 2021a). The details on Darcy law, heat equation and Hooke law can be found in Duwiquet et al, (2021a). All thermal, hydraulic and mechanical parameters are detailed in Table 1.…”

Assessment of deep geothermal research and development in the Upper Rhine Graben

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The Upper Rhine Graben (URG) is a major target for deep geothermal exploration in Central Europe due to the highly elevated geothermal gradient of locally more than 100 K/km (e.g., Soultz-sous-Forêts) (Schellschmidt and Clauser 1996;Pribnow and Schellschmidt 2000;Agemar et al 2012;Baillieux et al 2013) and the abundance of large fault zone providing potential fluid pathways (Bächler et al 2003;Guillou-Frottier et al 2013;Duwiquet et al 2021). According to Kock and Kaltschmitt (2012), the technical potential for deep geothermal utilization in the URG sums up to 186 TWh/a, which is approximately 5% of the annual primary energy demand of Germany (AG Energiebilanzen 2021).

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