Research Article Crustal Fault Zones (CFZ) As Geothermal Power Systems: a Preliminary 3D THM Model Constrained by a Multidisciplinary Approach
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Hindawi Geofluids Volume 2021, Article ID 8855632, 24 pages https://doi.org/10.1155/2021/8855632 Research Article Crustal Fault Zones (CFZ) as Geothermal Power Systems: A Preliminary 3D THM Model Constrained by a Multidisciplinary Approach Hugo Duwiquet ,1,2,3 Laurent Guillou-Frottier,1,2 Laurent Arbaret,2 Mathieu Bellanger,3 Théophile Guillon,1 and Michael J. Heap4 1BRGM, Georesources Division, Av. C. Guillemin, BP 36009, F-45060 Orléans Cedex 2, France 2ISTO, UMR 7327, University of Orléans, CNRS, BRGM, 1A Rue de la Férollerie, 45071, Orléans, France 3TLS-Geothermics, 91 Chemin de Gabardie, 31200 Toulouse, France 4University of Strasbourg, CNRS, ENGEES, Institut Terre et Environnement de Strasbourg (ITES), UMR 7063, 5 Rue Descartes, Strasbourg F-67084, France Correspondence should be addressed to Hugo Duwiquet; [email protected] Received 18 September 2020; Revised 7 December 2020; Accepted 16 December 2020; Published 4 February 2021 Academic Editor: Hang Lin Copyright © 2021 Hugo Duwiquet et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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. 1. Introduction Russia [9]). The cost-effectiveness of a reservoir depends on permeability, heat, and fluid flow. These conditions are often As potential geothermal reservoirs, crustal fault zones (CFZ) related to a specific geodynamic context and the physical con- are still largely unexplored and unexploited (e.g., South Erzge- ditions of heat transport. birge deep fault zone, Germany [1]; Gravberg, Sweden [2, 3]; A CFZ is generally not a simple plane but rather has a Badenweiler-Lenzkirch Sutur, Germany [4]; Dewey’sFault complex geometry defined by a succession of fault and Zone, Canada [5]; Longmen Shan Fault Zone, China [6, 7]; fracture intersections reflecting intense deformation [10–12]. Pasmajärvi Fault Zone, Finland [8]; and Luchlompal fault, CFZ are manifestations of strain localization and modify the 2 Geofluids mechanical properties of the crust above the Brittle-Ductile twofold: first, a simple synthetic model is dedicated to the Transition (BDT). These heterogeneities, characterized by demonstration of the effect of stress intensity and direction the presence of a succession of undeformed protolith, damage on fluid flow and thermal regime within the fault zone. Sec- zones, and fault cores, will cause a significant variation in the ond, we implement mechanical effects within a 3D TH model permeability of the crust [13]. Permeability variation in the of the Pontgibaud CFZ, with the unique objective of crustal domain has been widely discussed since the 1990s questioning consistency of the obtained results with field [14, 15]. Overall, the permeability of a CFZ is controlled by data. To our knowledge, THM models are generally applied (i) 3D distribution of structural features such as brittle planes to purely pressure-driven flow, where fluid density is con- or lithological interfaces, (ii) the physical properties (e.g., stant and thus with no buoyancy-driven components (see, ’ Young s modulus and porosity) of lithologies along and adja- however, [54, 55]). However, Vallier et al. [24] used a similar cent to the fault zone, (iii) the state of regional stress, and (iv) approach as ours but with another objective being the study more generally by the coupling between thermal (T), hydraulic of mechanical effects on gravity anomalies, so that no infor- – (H), mechanical (M), and chemical (C) processes [10, 16 24]. mation on the coupling between fluid flow and mechanical At depths below the BDT, the deformation of granite leads to effects can be retrieved. reductions in matrix porosity and, presumably, matrix perme- Thus, in this paper, we (i) provide additional qualitative ability (e.g., [25]); however, these authors also note that micro- ° and quantitative data necessary for the understanding of a cracks can remain open in granite at high temperature (900 C) hydrothermal system such as the Pontgibaud CFZ; (ii) insert and high pressure (100 MPa), resulting in matrix permeabil- these new data in a 3D geological model; (iii) explore by ities higher than that of the intact material. numerical modeling the effect of stress intensity, stress Convective heat transfer is often invoked to explain tem- fl fl – direction on uid ow, and temperature anomalies; and perature anomalies within faults [26 33]. Many parameters (iv) evaluate in 3D the geothermal potential of the Pontgi- seem to condition the convective regime: the permeability baud CFZ. ratio between the fault and its host, the CFZ permeability distribution, the fluid properties (viscosity and density varia- tions), the CFZ thickness, and the depth and the dip of the 1.1. Geological Setting CFZ [32–36]. The stress field is well known to influence CFZ permeability [20, 37–39] especially by influencing the 1.1.1. Geological Formation. Bounded to the east and west by the Aigueperse Saint Sauves fault and the Sillon Houiller void properties (fractures, pores, channels, and cracks). In “ ” ff fact, these voids under stress are expected to evolve over time fault, the La Sioule licence (Figure 1) is largely a ected by late Variscan tectonics. Metamorphic formations and the and, according to several authors, favour shapes that will fi facilitate the circulation of fluids and stress propagation at Gelles granite, which are intrusive, characterize the eld. different scales [40–45]. Magmatic and hydrothermal veins are widely represented. Covering a large part of the mapped region, metamorphic In order to estimate the geothermal potential of CFZ, it is ff necessary to consider a multidisciplinary approach [13, 15, formations (green in Figure 1) can be separated into di erent 46–50]. Using such an approach (structural field measure- lithologies. Biotite and sillimanite paragneiss extends south- ments, laboratory porosity and permeability analysis, and wards near the Miouze river and northeastwards to the Ville- numerical modeling) Duwiquet et al. [51] studied the geother- longue region. Orthogneiss associated with biotite and mal potential of the Pontgibaud CFZ in the French Massif sillimanite paragneiss are present sporadically. Cordierite ffi Central (FMC) and highlighted (i) the heterogeneous 2D gneisses are more widely represented, but it is di cult to distribution of permeability within the fault zone, (ii) CFZ with assign a cartographic boundary. Rare amphibolitic levels a near vertical dip as potential targets for high-temperature are interspersed in the cordierite gneisses. As an intrusive ° geothermal energy, and (iii) the position of the 150 Cisotherm in metamorphic series, the porphyroid granite of Gelles at a depth of 2.5 km. Although all these results were in agree- (red in Figure 1) extends southwest of Pontgibaud, between ment with previously acquired field data (geology, geophysics, the Sioule Valley to the East and the village of Tortebesse to and temperature measurements) [52, 53], the understanding the West. To the North, the last granitic points appear imme- of the 2D distribution of permeability has been made on a diately to the west of La Goutelle. limited number of samples, and the modeling estimates have The presence of thermomineral springs (such as Pranal been made in 2D and based on TH coupling only. In addition, and Ceyssat) is an indicator of present-day fluid circulation the structural framework poorly considered neighbouring (Figure 1). Temperature measurements have been carried faults to the Pontgibaud fault system. Using more field observa- out