Download deep electrical resistivity model of the larderello geothermal field (italy)

GNGTS 2016
Sessione 3.1
Deep electrical resistivity model of the Larderello geothermal
field (Italy): preliminary results of the FP7 IMAGE experiment
L. Capozzoli1, G. De Martino1, V. Giampaolo1, A. Godio2,3, A. Manzella3, F. Perciante1, E. Rizzo1,
A. Santilano2,3
CNR-IMAA, Potenza, Italy
Politecnico di Torino, Italy
3
CNR-IGG, Pisa, Italy
1
2
The paper describes the exploration of a deep geothermal systems based on integration of
different electrical resistivity surveys (DC and MT). The integrated approach applied in an
area of the Larderello site, where there is the oldest field in the world under exploitation for
power production. It is located in southern Tuscany (Italy) and the actual installed capacity is
about 795 MWe (Bertani, 2015). A vapour-dominated system is exploited to depth over 3500
m, with temperatures exceeding 350°C, from two different reservoirs. The Larderello test site
has been investigated by many geological and geophysical data of previous exploration projects
but nowadays several critical issues on deep features of the field are still matter of debate, e.g.,
permeability distribution in the hydrothermal reservoir and the presence of fluids at supercritical
condition at depth. Previous MT studies were carried out in southern Tuscany in the frame
of exploration and research projects (Fiordelisi et al., 1998; Manzella, 2004; Manzella et al.,
2006). Recently, Santilano et al. (2015) proposed a re-analysis of old MT data acquired by
Enel in the ’90 by integrating geological and geophysical data in order to set a well-constrained
a-priori model and to check inversion results supporting the interpretation. The analysis of
the data integration confirmed the effects of strong resistivity inhomogeneity at the depth of
the crystalline basement, where a resistive behaviour is expected. Despite of the lithological
features of the reservoirs and the vapour state of the geothermal fluids, low resistivity anomalies
recognized.
This study regards new electromagnetic acquisition in the area. First, a new broadband
magnetotelluric (MT) survey was carried out in the SW sector of the Larderello field near
Monterotondo Marittimo town (Fig. 1). In order to improve the quality of the MT data potentially
affected by near-field noise effect of the electrified railways, we installed a permanent remote
station in the Capraia Island. We carried out 22 MT station, using high-resolution, multichannel 32-bit receivers able to record broadband time-series from 0.0001 to 1 kHz, using
a Zonge International Inc equipment. The two perpendicular horizontal components of the
electric and magnetic fields on surface were measured with a L-shaped configuration of 100
meters electrical dipoles and two Zonge Ant/4 magnetometers. For each site, we recorded at
least 17 hours using the sampling rate of 256 Hz. In addition, we acquired over one hour with a
sampling rate of 4096 and 1024 Hz. The preliminary results of MT data processing shows that,
although some of the soundings resulted too affected by noise, most of them are suitable for
imaging the resistivity distribution at depth.
Furthermore, we designed a new experimental high resolution Surface-Hole Deep Electrical
Resistivity Tomography (SHDERT) in the Venelle 2 well (kindly provided by Enel GP), that is
located in the central sector of the MT survey area. The aim of the SHDERT acquisition was to
obtain a detailed deep electrical resistivity distribution (until 2 km), in order to constrain and
improve the MT inversion. The Venelle 2 well was accessible down to 1.6 km with a temperature
up to 250°C and a metallic casing down to 1 km. We designed an in-hole electrical cable both to
stand high temperature and to have flexible metallic electrodes to be located along the open-hole
portion (1050 m-1600 m). In details, we carried out the SHDERT by using a dipole transmitting
station that injected a direct current (5-10 A) into the ground and a multichannel receiver to
record the drop of potential. The injected dipoles were located only on the surface and the
receiver ones were distributed both in the Venelle 2 well and on the surface to cover an area
around 16 km2. Therefore, the described system permitted to record several drops of potential
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GNGTS 2016
Sessione 3.1
Fig. 1 – Schematic geological map of the Larderello geothermal area, simplified from the geological maps at scale
1:10000 of Tuscany Region. 1) Quaternary deposits; 2) Neoautochthonous terrigenous deposits (Miocene-Pliocene);
3) Ligurian and sub-Ligurian Flysch complex (Jurassic-Eocene); 4) Tuscan Nappe formations (Upper Trias-Miocene);
5) Metamorphic Units (Paleozoic); 6) Remote site on Capraia Island; 7) Study area; 8) MT site acquired in IMAGE;
9) MT site previously acquired; 10) Position of surface current electrodes for the acquisition of the DERT-SHDERT;
11) Venelle 2 area.
simultaneously for each current injection. The crucial task was the data processing, considering
the large distance between the Tx and Rx systems that strongly reduces the signal to-noise ratio.
To overcome this drawback, for each quadripole position the corresponding voltage signal was
filtered, stored and processed with advanced statistical packages (Colella et al., 2004; Rizzo et
al., 2004, Tamburiello et al., 2008).
The integration of MT model and experimental DC resistivity measurements improved the
knowledge on the deep structures of the Larderello field.
Acknowledgements. This study is part of the EU FP7-funded Integrated Methods for Advanced Geothermal
Exploration (IMAGE) Project under grant agreement n° 608553. We thank the colleagues that supported the fieldwork
during the MT and DC surveys. We thank Enel Green Power for the precious technical and logistical support on
carrying out the borehole experiment.
References
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Tamburriello, G., Balasco, M., Rizzo, E., Harabaglia, P., Lapenna, V. and Siniscalchi, A. Deep electrical resistivity
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