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Geological and numerical modeling to support electric production in geothermal fields: a case test in Central Italy

pubblicazioni - Presentazione

Geological and numerical modeling to support electric production in geothermal fields: a case test in Central Italy

Per l’area di Castel Giorgio – Torre Alfina situata nel centro Italia, a Nord del Lago di Bolsena, sono state realizzate delle accurate simulazioni numeriche 3D di dettaglio al fine di verificare la sostenibilità del campo indagato per lo sviluppo di un impianto pilota da 5 MWe basato sulla tecnologia dei cicli binari ORC (Organic Rankine Cycle).

This study describes the geological and numerical modeling strategies that can be adopted to support electric production in a medium-enthalpy geothermal field located in Castel Giorgio-Torre Alfina area, close to Bolsena Lake in Central Italy. The simulations have been carried out in order to verify the sustainability for geothermal electric production by a 5 MWe nominal power pilot plant that foresees a total fluid re-injection in the same original reservoir, and which process is based on the binary ORC (Organic Rankine Cycle) because it allows the geothermal field exploitation at temperatures lower than those of an high enthalpy field.

The considered geothermal reservoir is located in permeable and fractured carbonate formations belonging to the Complex of Tuscan facies while the caprock has been identified in the overlying Complexes of Ligurian and internal Austro-alpine facies. The top reservoir depth ranges from about -800 m to -1700 m a.s.l. while reservoir temperature ranges from 125°C to 150°C.An accurate 3D numerical model including geological and spatial discretisation has been realized in order to perform simulations devoted both to reproduce the natural state of the geothermal system and to perform predictive analyses in order to support the electricity production by the ORC industrial process.

The accuracy of numerical simulation related to the natural state of the field has been verified by comparing simulated and measured temperatures in correspondence to a number of exploration wells drilled in the interesting area. The correctness of the natural state simulation allowed to analyze a number of different operating scenarios, all considering a production period of 50 years being the geothermal fluid extracted from five wells and totally re-injected into four different wells, with the aim of optimizing the production flow rate. Also the time necessary to recovery the natural state conditions after the end of production/reinjection activity has been estimated by numerical simulation.

As the result, a constant flow rate of 1050 t/h has been considered as reference value, since in this case a very efficient convective circulation inside the geothermal system has been observed to be kept, and production sustainability is guaranteed for all the period. Furthermore, the over pressure field around the re-injection wells is limited to 2% of pre-existing one and no interference effect has been highlighted between the production and re-injection wells, which locations are such to ensure that the re-injected fluids are heated before reaching the production wells.

Finally, the original natural pressure state is recovered in less than a century, while temperature seems to reach more slowly its natural equilibrium.

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