Naslov
The Euganean geothermal field (NE Italy): a new hydrothermal structural model

Autori
Pola, Marco ; Fabbri, Paolo ; Zampieri Dario

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, prošireni sažetak, znanstveni

Izvornik
2nd European Geothermal PhD Day - Collection of Abstracts / - , 2011, 66-67

Skup
2nd European Geothermal PhD day

Mjesto i datum
Reykjavík, Island, 02.03.2011

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
thermal waters ; fault-controlled geothermal system ; numerical simulations ; Euganean Geothermal Field

Sažetak
The Euganean Geothermal Field (EGF) is the most important thermal field in the northern Italy. It is located in Veneto, east of the Euganei Hills and southwest of Padova. The EGF extends on a plain band of 36 km² and comprises Abano Terme and Montegrotto Terme, two famous spa towns. At present about 250 wells are active and the total average flow rate of exploited thermal fluids is 15 Mm³/year. Physical and chemical parameters of the Euganean thermal waters were statistically analyzed by several authors: the temperature ranges from 60°C to 86°C, and their T.D.S. is 6 g/L with a primary presence of Cl and Na (70%) and secondary of SO4, Ca, Mg, HCO3, SiO2. ³H and ¹⁴C measurements suggest a residence time greater than 60 years, probably a few thousand years. The analyses of the Oxygen isotopes show that the thermal waters are of meteoric origin and infiltrate in an area up to 1500 m a.s.l. . The previous conceptual model [Piccoli et al., 1976] located the recharge zone of the thermal circuit 80 Km northwest of the EGF. The meteoric waters infiltrate in Mesozoic carbonate formations, uplifted in the footwall of a normal fault separating the chain from the foredeep, and flow inside it. They warm up by a normal geothermal gradient and then rise quickly in the EGF, thanks to the high fracturing of the rocks. Due to a misunderstanding of the structural setting (the uplifted block is the hanging wall of a south-verging thrust) and the use of an idealised (wrong) cross section, this model cannot work. The thrust uplifts the crystalline basement that crops out downstream of the recharge zone. Therefore, the low permeable metamorphic rocks of the basement hydrogeologically isolate the recharge area from the outflow parts of the thermal circuit. More recently, it has been proposed that the EGF is located above a left stepover structure (relay zone) of the Schio–Vicenza fault system (SVFS) covered hundred meters beneath the alluvial cover [Zampieri et al., 2009]. Given the Neogene to Quaternary sinistral strike-slip kinematics superimposed on the fault system, the relay zone has accommodated along-strike local extension and may be responsible for rock fracturing and permeability development. The presence of a 5 meter-high hill of travertine in Abano Terme strongly supports the existence of a releasing structure that controls 1) the outflow of thermal waters in the EGF and 2) ongoing activity of the SVFS that keeps open the fractures. The hill is affected by a network of fractures (mainly oriented WNW-ESE and NNE-SSW), which allow us to refer to the travertine deposit as a travertine fissure mound. The fracture network is interpreted as a fault/fracture mesh developing in a dilational stepover between strike-slip or transtensional fault segments of the SVFS. Geostatistical analyses on transmissivity of the thermal aquifer show a WNW-ESE anisotropy [Fabbri, 1997] that parallels the direction of fissures. A 3D model of the EGF subsurface and a cross section of the thermal circuit is constructed using seismic sections and the stratigraphy of deep wells (Villaverla 1 and Vicenza 1 in the northern part, Due Torri in Abano). The new conceptual model locates the thermal circuit east of the SVFS, instead of west like in the previous model. The meteoric waters infiltrate in an area 60 Km north of EGF thanks to the high secondary permeability of the outcropping rocks. They flow to the south inside a carbonate reservoir (mainly composed of the Dolomia Principale formation), which in the EGF is structurally displaced at depths between 2000 and 3000 m, and warm up by a normal geothermal gradient. The damage zone of the SVFS acts as a conduit for the hot waters, because of the higher permeability of the rocks than the protolith. In EGF area the local extensional regime keeps open the fractures and permits the quick rising of hot waters. A preliminary mathematical hydrothermal model of the EGF is developed using the software Hydrotherm [Kipp et al., 2008]. Hydrotherm simulates thermal energy transport in three-dimensional, two-phase, hydrothermal, ground-water flow systems.

Izvorni jezik
Engleski

Znanstvena područja
Rudarstvo, nafta i geološko inženjerstvo

POVEZANOST RADA