| Title | Caldera-related geothermal resources: implementation of the volume method and implications for exploration at Colli Albani caldera and the Central Italy geothermal province |
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| Authors | Giordano, De Benedetti, Bonamico, Ramazzotti, Mattei |
| Year | 2013 |
| Conference | European Geothermal Conference |
| Keywords | geothermics, caldera-related resources, Colli Albani, fractured reservoirs |
| Abstract | The Quaternary Roman Volcanic Province extends for over 200 km along the Tyrrhenian margin of the Italian peninsula and is composed of several caldera complexes with significant associated geothermal potential. In spite of the massive programs of explorations conducted by the then state-owned ENEL and AGIP companies between the 1970s and 1990s, and the identification of several high enthalpy fields, this resource remains so far unexploited, although it occurs right below the densely populated metropolitan area of Roma capital city. The main reason for this failures is that deep geothermal reservoirs are associated with fractured rocks, the secondary permeability of which is difficult to be predicted thus making it uncertain the identification of the most productive volumes of the reservoirs and the localization of productive wells. In facts, almost half of the many deep bore-holes that were drilled in the area reached a dry target. This work proposes a review of available data and a re-assessment of the geothermal potential in caldera-related systems in Central Italy, by analysing in detail the case of the Colli Albani caldera system, the closest to Roma capital city. A GIS based approach is implemented for the identification of areas corresponding to reservoir volumes most promising in terms of geothermal exploitation, to improve the well established volume method approach to the exploration of geothermal resources. The identification is based on a three dimensional matrix of georeferenced spatial data; the A axis accounts for the modeling of the depth of the top of the reservoirs based on geophysical and direct data; the B axis accounts for the thermal modeling of the crust (i.e. T with depth) based on measured thermal gradients. Both A and B data are necessary but not sufficient to identify rock volumes actually permeated by geothermal fluids in fractured reservoirs. We therefore discuss the implementation of a C axis that aims at evaluating all surface data that are evidence of actual geothermal fluid circulation in the geothermal reservoir. We considered datasets on: i) distribution and density of tectonic lineaments; ii) temperature and iii) electric conductivity of shallow groundwaters; iv) partial pressure of dissolved CO2 in shallow groundwaters. The geothermal potential of Colli Albani is then discussed, and implications for caldera-related geothermal systems in Central Italy are discussed based on the role of the geometry and structure of reservoirs in relationship with volcanotectonic structures associated with calderas and deep geothermal fluid migration paths. |