On morphology and amplitude of 2D and 3D thermal anomalies induced by buoyancy-driven flow within and around fault zones

Abstract

Abstract. In the first kilometres of the subsurface, temperature anomalies due to heat conduction processes rarely exceed 20–30 °C. When fault zones are sufficiently permeable, fluid flow may lead to thermal anomalies much higher, as evidenced by the emergence of thermal springs or by fault-related geothermal reservoirs. Hydrothermal convection triggered by buoyancy effects creates thermal anomalies whose morphology and amplitude are not well known, especially when depth- and time-dependent permeability are considered. Exploitation of shallow thermal anomalies for heat and power production partly depends on the volume and on the temperature of the hydrothermal reservoir. This study presents a non-exhaustive numerical investigation of fluid flow models within and around simplified fault zones, where realistic fluid and rock properties are accounted for, as well as appropriate boundary conditions. 2D simplified models point out relevant physical mechanisms for geological problems, such as thermal inheritance or splitting plumes showing a pulsating behaviour. When permeability is increased, the classic finger-like upwellings evolve towards a bulb-like geometry, resulting in a large volume of hot fluid at shallow depth. In the simplified 3D models, where fault zone dip angle and fault zone thickness are varied, the anomalously hot reservoir exhibits a kilometre-sized hot air balloon morphology, or, when permeability is depth-dependent, a funnel-shape geometry. For thick faults, the number of thermal anomalies increases but not the amplitude. The largest amplitude (up to 80–90 °C) is obtained for vertical fault zones. At the top of a vertical, 100 m wide, fault zone, temperature anomalies greater than 30 °C may extend laterally over more than 1 km from the fault boundary. These preliminary results should motivate further geothermal investigations of more elaborated models where topography and fault intersections would be accounted for.

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

“…This will produce a temperature anomaly within the Pontgibaud hydrothermal system, but lateral thermal diffusion is observed within the fault zone itself, thus defining a larger geothermal reservoir. Indeed, considering a simplified 3D geometry, this lateral temperature diffusion has been highlighted in recent numerical models of fluid flow around faulted zones [33].…”

“…López and Smith [31] showed that temperature anomalies can be directly influenced by the permeability ratio between the fault and the host. 3D modeling based on TH coupling has shown the influence of this permeability ratio on the morphology of the temperature anomalies [33]. Our parametric study (Figures 11 and 12) made it possible to approach three factors influencing fluid flow: stress intensity, stress direction, and the permeability ratio between the fault and the basement.…”

“…Convective heat transfer is often invoked to explain temperature anomalies within faults [26][27][28][29][30][31][32][33]. Many parameters seem to condition the convective regime: the permeability ratio between the fault and its host, the CFZ permeability distribution, the fluid properties (viscosity and density variations), the CFZ thickness, and the depth and the dip of the CFZ [32][33][34][35][36]. The stress field is well known to influence CFZ permeability [20,[37][38][39] especially by influencing the void properties (fractures, pores, channels, and cracks).…”

3D Basin‐Scale Groundwater Flow Modeling as a Tool for Geothermal Exploration: Application to the Geneva Basin, Switzerland‐France

“…These focused on the thermal state of the basins (Bonté et al., 2018; Duddy et al., 1994) or on hydromechanical processes taking place at depth (Hairuo Qing & Mountjoy, 1992; Montegrossi et al., 2018). However, only a few studies investigated 3D Darcy flow coupled with thermal processes (Guillou‐Frottier et al., 2020; Przybycin et al., 2017; ). Compared to conductive static models of the GGB (e.g.…”

Characterization of Southern Peru hydrothermal systems: New perspectives for geothermal exploration along the Andean forearc

“…In the orogenic belt type, no abnormal heat source such as volcanic activity is required. Instead, local geothermal gradients and heat flows, themselves influenced by the radiogenic heat production, the rock rheology, the tectonic regime, and the topography, constitute the heat source and flow path of hot fluids (Guillou-Frottier et al, 2024). The combination of free-convection along permeable faults (Belgrano et al, 2016;Guillou-Frottier et al, 2020) and forced-convection induced by topographic gradients (Volpi et al, 2017;Sutherland et al, 2017;Wanner et al, 2019) ensures fluid circulation between the surface and the great depths in the forearc (Forster & Smith, 1989;López & Smith, 1995).…”