Modeling Highly Buoyant Flows in the Castel Giorgio: Torre Alfina Deep Geothermal Reservoir

Abstract

The Castel Giorgio-Torre Alfina (CG-TA, central Italy) is a geothermal reservoir whose fluids are hosted in a carbonate formation at temperatures ranging between 120 ∘ C and 210 ∘ C. Data from deep wells suggest the existence of convective flow. We present the 3D numerical model of the CG-TA to simulate the undisturbed natural geothermal field and investigate the impacts of the exploitation process. The open source finite-element code OpenGeoSys is applied to solve the coupled systems of partial differential equations. The commercial software FEFLOW5 is also used as additional numerical constraint. Calculated pressure and temperature have been calibrated against data from geothermal wells. The flow field displays multicellular convective patterns that cover the entire geothermal reservoir. The resulting thermal plumes protrude vertically over 3 km at Darcy velocity of about 7 * 10 −8 m/s. The analysis of the exploitation process demonstrated the sustainability of a geothermal doublet for the development of a 5 MW pilot plant. The buoyant circulation within the geothermal system allows the reservoir to sustain a 50-year production at a flow rate of 1050 t/h. The distance of 2 km, between the production and reinjection wells, is sufficient to prevent any thermal breakthrough within the estimated operational lifetime. OGS and FELFOW results are qualitatively very similar with differences in peak velocities and temperatures. The case study provides valuable guidelines for future exploitation of the CG-TA deep geothermal reservoir. IntroductionSince the 1970s, the increasing threat of a worldwide energy crisis has prompted many governments to reduce their dependence on traditional nonrenewable energy sources focusing on renewable ones (e.g., geothermal energy, hydroelectric, wind-energy, and several forms of solar energy, such as bioenergy, biofuel, photovoltaic, and solar-thermal energy [1]). Geothermal energy is expected to play an increasing role to meet future power demand. This is related to its enormous exploiting potential, which is capturing the attention of industries also due to technological advances in the exploration of promising geothermal fields [2,3].Most of the world famous geothermal energy sources exploited today are associated with volcanic and/or recent tectonically active areas. Important examples are the geothermal areas of Yellowstone [4][5][6] northern California [7], the Pannonic Basin [8], and the Rhine Graben [9][10][11]. Italy strongly contributes to the development of geothermal power generation. In 1904, the world first electrical power was produced from a geothermal energy source in the Larderello site (Tuscany, central Italy) [1,[12][13][14][15][16][17][18].Beyond the world famous Larderello system, fossil and active hydrothermal manifestations are distributed all along the Preappennine belt of central Italy, facing the Tyrrhenian coast. This area has undergone both lithospheric extension and upper mantle doming. Such processes have been active since the Miocene [19,20] and are likely...

3D hydrogeological reconstruction of the fault-controlled Euganean Geothermal System (NE Italy)

“…The typical issue in numerical modeling is the representation of discontinuities (i.e., faults, thrusts, fractured zones) through a continuous structure, such as the traditional finite difference grid or the finite element mesh. Recently, the use of unstructured meshes made of tetrahedral elements improved the capability of reproducing the complex geometries and heterogeneities that typically occur in a geothermal system (Blӧcher et al, 2010;Cacace and Blӧcher, 2015;Ingebritsen et al, 2010;Painter et al, 2012;Painter et al, 2016;Passadore et al, 2012;Volpi et al, 2018). This new discretization approach can become particularly profitable when used together with a detailed 3D reconstruction of the geothermal reservoir made with specific tools, such as MOVE (Midland Valley Exploration Ltd.), PETREL (Schlumberger), or SKUA-GOCAD (Paradigm).…”