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

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. [ABSTRACT FROM AUTHOR]