Numerical modelling of fluids related thermal anomalies

Among the renewable and sustainable energy sources, geothermal energy has been recognized as “the choice” to meet the future electricity demand, economically and environmentally speaking. The increasing threat of a worldwide energy crisis and the growing interest in geothermal systems require further development and application of advanced software and numerical modelling approaches to facilitate geothermal exploration and exploitation. In this regard, the present study is aimed to perform accurate three-dimensional simulations of three high to low-enthalpy type geothermal systems, for which no model was previously available. The selected sites are : i) the Bormio hydrothermal system and ii) the Castel Giorgio – Torre Alfina geothermal reservoir in Italy, and iii) the Tiberian Basin between Israel, Jordan and Syria. The historical Italian thermal site of Bormio (Central Italian Alps) is a typical alpine low enthalpy geothermal site, whose waters are currently exploited by two thermal establishments. Thermal waters from ten springs are heated at a temperature of about 40°C in deep circulation systems and ascend vigorously along the regional permeable Zebrù thrust. A hydrochemical characterization of the discharged thermal waters has been performed to validate the assumptions formulated in the regional numerical model built with the finite element code Feflow®. Results correctly simulate the observed discharge rate of ca. 2400 l/min and the spring temperatures, showing a complete cooling of the aquifer within a period of approximately 50,000 years. Groundwater flow and temperature patterns suggest that thermal water flows through a deep system along a fracture network associated with the thrust system. The Castel Giorgio - Torre Alfina geothermal field (Central Italy) is a promising, early explored and so far not exploited medium enthalpy reservoir. The involved fluids are hosted in a carbonate formation at temperatures ranging between 120-210 °C. Detailed hydro-geothermal data recognized a strong thermal anomaly associated with a vigorous convective regime. The 3D reservoir-scale numerical model has been developed, via the open source finite element code OpenGeoSys, to simulate the undisturbed natural geothermal field and investigate the impacts of a possible exploitation process. The commercial software Feflow® is also used as additional numerical constraint. 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 years production at a flow rate of 1050 t/h and prevents any thermal breakthrough within the estimated operational lifetime. The Tiberian Basin, within in the Jordan Rift Valley, is characterized by a shallow heat anomaly making that field a potential site for production of electricity through geothermal methods. The Jordan Rift Valley hosts the Lake Tiberias, the main freshwater resource of the entire Middle East. The sustainability of this resource is endangered by the occurrence of clusters of hot (20 - 60 °C) and salty springs, along the lake shore and the Lower Yarmuk Gorge. This deep depression is supposed to act as the mixing zone of different flow paths responsible for the ascent of thermal waters. The first regional 3D model of the Tiberian Basin has been developed with the commercial finite element software Feflow®, accounting for major aquifers, aquicludes and deep-cutting faults. Available water levels for wells at different depths, allowed the correct calibration of the groundwater flow model. Results show that the discharge of thermal waters is tied to the coexistence of free convection in permeable units, and additional advective flow fields induced by topography gradients. These simulations identify the mixed convection as the dominant heat flow process driving thermal waters below the Lower Yarmuk Gorge.