Geothermal fluid circulation in a caldera setting: The Torre Alfina medium enthalpy system (Italy)
Introduction
With an estimated electrical potential of 10–100 times the current production, geothermal energy currently contributes to less than 1% of the world’s primary energy supply (World Energy Resources, Geothermal/World Energy Council, 2016), and has the potential to grow up to 8.3 % of the global electricity production and serve 17 % of the world’s population by 2050 (Bertani, 2016).
Although the heat supply is abundant, the distribution of the geothermal resources is not homogeneous and is localized mainly in active geodynamic regions such as volcanic or intrusive systems and active tectonic zones (Henley and Ellis, 1983; Moeck, 2014).
The majority of the known geothermal systems was localized thanks to the presence of surface manifestations. However, many resources show little or no surface expression and the identification of a new geothermal reservoir can be challenging (Hanson et al., 2014). The amount of “blind” geothermal systems - whose presence gives no signature at the surface (Forson et al., 2014) – remains uncertain. Considering the potential of these undiscovered resources and the high costs of drilling programs, the search for blind systems in diverse geologic environments has been carried out with cost-effective methods including geochemical, geophysical and spatial analysis/structural geology surveys (Bibby et al., 1994; Blackwell and Baag, 1973; Deon et al., 2015; Faulds et al., 2015, 2016; Fowler et al., 2017; Garg et al., 2010; Hanson et al., 2014; Kratt et al., 2009).
Geothermal systems in volcanic regimes are usually characterized by the highest enthalpy and occur in different geological settings such as plate boundaries, continental rifts and hot spot environments (Saemundsson et al., 2009).
Among the volcanic structures, calderas host some of the most consistent geothermal reservoirs. Due to the slow cooling of the magma bodies, the deep heat source can be available for up to 2 million years after the last volcanic activity. Its interaction with groundwater can generate hydrothermal systems developing in permeable rock volumes (e.g. Giordano et al., 2014; Wohletz and Heiken, 1992).
However, despite the high geothermal potential, investigation and exploitation in the interior of these structures can encounter environmental and logistical complications, which can represent an obstacle to the geothermal development itself. Volcanic regions, in fact, are often of historic/environmental significance and/or protected tourist destinations (Newsome and Dowling, 2018). Furthermore, these areas are often sites of intense urbanization with extremely high population density: Campi Flegrei, Italy, hosts more than 550.000 inhabitants within the area of the caldera, and the Kagoshima Bay in Japan has a population of more than 850.000 people. In such densely urbanized settings, any industrial development requires a careful hazard assessment which, in case of seismic areas (McNutt and Roman, 2015), should also include the effects of possible induced seismicity (Carlino et al., 2016; Gaucher et al., 2015; Lermo et al., 2008).
Besides the environmental and geologic issues, there are also social challenges, such as the skepticism of local communities and administrations (Manzella et al., 2018), and technical problems related to the high chemical aggressiveness of hydrothermal fluids on the materials of the plant (Nogara and Zarrouk, 2018).The presence of lakes (e.g. Colli Albani and Monti Vulsini, Italy; Mt Pinatubo, Philippines; Crater Lake, Oregon; Askja, Iceland) and seas submerging the caldera structures in part (e.g. Campi Flegrei, Italy; Santorini, Greece; Aira, Japan) or entirely (Kulo Lasi, Tonga; Kaikata, Japan) (Branney and Acocella, 2015; Wohletz and Heiken, 1992) can also create problems.
Considering these complications, the study of geothermal fluid migrations away from the main volcanic structures can be a successful approach for the identification of new strategic resources. As already mentioned, calderas are areas of intense hydrothermal circulation which is not only influenced by the different permeability of the volcanic deposits (e.g. Heap et al., 2014), but also by the complex geometry of structural discontinuities such as collapse ring faults, fractures, caldera-crossing faults and regional faults which may act as preferential channels for fluid migration (Garden et al., 2017; Giordano et al., 2013, 2014; Urbani et al., 2020; Vignaroli et al., 2013) and promote lateral advection of hot fluids away from the central part of the volcano.
Studies performed in non-caldera settings showed that faults can act both as high permeability conducts or as hydraulic barriers, depending on mineralization or the comminution of fault core and fracturation of damage zone (e.g. Faulkner and Rutter, 2001; Giordano et al., 2013; Rowland and Sibson, 2004; Vignaroli et al., 2013). However, only a few studies deal with the permeability of caldera collapse faults (Carlino, 2018; Garden et al., 2017; Jasim et al., 2015), because exposed outcrops of active structures are rare (Garden et al., 2017; Geyer and Martí, 2014).
The connection between caldera structural discontinuities and the anisotropic permeability and consequent fluids channelization in the internal margin of the volcano is now confirmed by drilling data (Carlino, 2018), numerical modelling (Jasim et al., 2015) and field structural surveys (Garden et al., 2017). The role of these structures in influencing hydrothermal fluids circuits towards peripheral areas outside the caldera margin remains unexplored.
The main aim of this study is to investigate the hydrothermal circulation where a caldera collapse structure intersects a regional reservoir, to define the role of caldera faults in the deep hydrothermal circuits and to assess their contribution in promoting lateral advection to peripheral areas.
To this purpose, numerical modelling was applied to the Torre Alfina geothermal system, for which a rich database is available from previous geologic studies and whose characteristics make it a suitable case study for modelling the paths of fluid migrations.
Results contribute to the comprehension of the mechanisms of fluid circulation and heat supply from the caldera to peripheral geothermal systems, with relevant implications for geothermal exploration planning and for targeting new distal resources.
Section snippets
Geological framework
The Torre Alfina geothermal area is located in Central Italy, along the Tyrrhenian margin of the Apennine Mts (Fig. 1a), and at the northernmost tip of the Quaternary alkali potassic volcanoes of the Vulsini Volcanic District. The evolution of the Apennine chain was characterized, at first, by Miocene-Pliocene compressional tectonic phases, with the eastward migration of thrust fronts and foredeep basins in a classical piggyback sequence; then, post-orogenic extension followed and is still
Previous conceptual model of Torre Alfina geothermal system
The geothermal reservoir of Torre Alfina was discovered after some preliminary regional studies aimed at identifying potential geothermal resources. Since 1973, the area has been investigated by geological, geophysical, geochemical and hydrogeological surveys (e.g. Barberi et al., 1994; Buonasorte et al., 1988, 1991; Chiarabba et al., 1995; Chiodini et al., 1995; Costantini et al., 1984; Doveri et al., 2010; Duchi et al., 1992).
The drilling of deep boreholes (Fig. 1) allowed the definition of
Methodology
We tested different conceptual models of the Torre Alfina geothermal system by applying the TOUGH2 geothermal simulator (Pruess et al., 1999). This numerical simulation program, based on an integral finite difference method, describes the coupled flow of heat and fluid through heterogeneous porous media. The fluid propagation is described according to a multi-phase version of Darcy’s law that accounts for water phase changes with latent heat effects. Phase interference is taken into account
Discussion
The first outcome of the simulations is the influence of the geometry of the permeable domain in hydrothermal fluid circulations: in SIM1 Case A: Closed system (Fig. 3), despite the absence of external perturbations, convective cells of variable extensions form and the asymmetry of the domain produces asymmetric circuits. This highlights the role of the structural setting of the geothermal reservoir as well as the contrasting permeabilities of the different rock units; the geometry of these
Conclusions
This study allowed us to understand some aspects of Torre Alfina geothermal system with implications also for processes that can take place in geothermal reservoirs.
The main considerations about geothermal system dynamics are:
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the presence of structural highs of reservoir rocks can promote the development of thermal anomalies, even in areas with no heat sources, because they act as a “trap” of deeper and warmer groundwater in shallower areas of the crust;
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the domain geometry and the distribution
Author statement
G.G. designed the research; I.T. and M.T. performed the numerical modelling; all the authors discussed the data and wrote the paper. All together revised and approved the final form of the manuscript.
Declaration of Competing Interest
The authors report no declarations of interest.
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