Thermo-Hydro-Mechanical modelling of the Larderello geothermal system, Tuscany, Italy: Emphasis on the role of tectonic structures

Geothermal energy is emerging as a promising renewable power source in the global energetical transition toward a clean energy future. The role of shallow and deep tectonic structures on fluid circulation and heat transfer is of primary importance to better target high-temperature geothermal reservoirs. In extensional context, the complexity of geothermal processes results from the coexistence of multiple structures whose activity evolves through time: normal to oblique faults, detachments and transfer zones. This complex structural setting leads to major questions: What are the main pathways driving fluid flow and heat transfer in high-temperature geothermal systems? What is the plumbing of the system at depth? What is the origin of geothermal fluids and recharge potential of the system? To address these issues, we propose a coupled study of an active system, i.e. the Larderello geothermal system (Tuscany, Italy), and its fossil, exhumed equivalent, i.e. the Elba Island and the Boccheggiano area (Italy). This region is located in a back-arc extensional context, following the eastward retreat of the west-verging Adriatic slab that started about 35 Ma ago. Deformation, that migrates eastward with the slab retreat, has been accommodated by low-angle detachments and associated normal faults. Such structures allowed the exhumation of metamorphic core complexes (MCCs) and the emplacement of plutonic bodies whose ages decrease eastward. Therefore, Elba Island with its exhumed detachments and MCC can be considered as a fossil equivalent of the active Larderello geothermal system. Through this project, we propose a Thermo-Hydro-Mechanical (THM) model of the Larderello geothermal system constrained by the available geophysical and geochemical data. Additional constraints at depth will be then provided by fieldwork observations (structural, mapping, sampling) and laboratory analysis (isotopes, elemental composition, fluid inclusion) on mineralized fault zones mainly from Elba Island. The disclosed part at EGU 2025 will focus on the preliminary modelling activities of Larderello. Firstly, a geological model is built with the PETREL software based on the available borehole data and interpreted seismic lines or cross-sections, which enables to display the 3D lithostratigraphic sequence and the structural geometries at depth. It highlights the complexity of this system related to boudinage favored by low-angle normal faults, which caused important thickness variations (up to disappearance) of some geological units through space, as well as important normal fault offsets, and flat horizons associated to shear zones at depth. Subsequently, the THM numerical model is solved with the COMSOL Multiphysics software by employing the above-mentioned geological model as the main geometrical framework. Presented numerical results focus on the plumbing of the system (different types of faults and their crosscutting relationships) and its role on heat transfer and fluid flow processes. Furthermore, different water recharge scenarios are also investigated. The chosen physical parameters involved in fluid flow, heat transfer and poroelasticity phenomena together with their implications on the numerical solutions are discussed. This on-going and multidisciplinary work participates to assess the geothermal potential, identify new exploitable areas and estimate the lifetime of high-temperature geothermal systems, fitting the global context of carbon-free energy development.