Giovanni Gianelli ISTITUTO DI GEOSCIENZE E GEORISORSE INSTITUTE OF GEOSCIENCES AND EARTH RESOURCES

Giovanni Gianelli ISTITUTO DI GEOSCIENZE E GEORISORSE INSTITUTE OF GEOSCIENCES AND EARTH RESOURCES

DEEP SEATED UNCONVENTIONAL GEOTHERMAL RESOURCES IN TUSCANY

More and more steam from Larderello

Larderello/Travale Steam production history Total production history of the Larderello and Travale/Radicondoli fields 1200 1100 1000 900 800 Flow-rate [kg/s] 700 600 500 400 300 200 8 new new units units with with a capacity capacity in in the the range range 10 10 - - 60 60 MW MW started started up up in in the the year year 2002 2002 100 0 1919 1923 1927 1931 19 35 1939 1943 1947 1951 1955 1959 19 63 1967 1971 1975 1979 1983 1987 19 91 19 95 1999 2003 Years

Larderello Travale Geothermal Area

Features of The Geothermal System High amplitude reflectors Granite (age 0.7 to 3.8 m.y.)

GRANITE CAN BE A RESERVOIR Cataclastic granite with hydrothermal alteration Core sample of 3570 m depth

Seismic Data

Deep Reservoir (3-3.5 km depth) producing super-heated steam 400 350 300 250 200 150 100 50 0 500 Sea level -500-1000 -1500-2000 -2500-3000 -3500

TEMPERATURE vs. DEPTH 400 350 300 250 200 150 100 50 0 500 Sea level -500-1000 -1500-2000 -2500-3000 -3500

Same Fluid from the Shallow and Deep Reservoir Rocks The composition of the geothermal fluid is remarkably constant: isotopic imprint characteristic of a meteoric origin, same gas/steam ratio (5wt% of gas, mostly CO 2 ) in all drillholes and different reservoir composition (from Mesozoic dolostone to granite). This almost constant composition of the fluid, over a drilled area of approximately 400 km 2, supports the hypothesis of the presence of a giant reservoir. Rock permeability is due to fracturing.

Structural Setting 1

Structural Setting 2

Contact Metamorphism At Larderello the deepest geologic units (approximately 4 km depth) consist of granite and metamorphic rocks Evidence of hydrothermal alteration in granite and wall rocks indicates fluid circulation at high temperature and pressures

a)-b) garnet micaschist c) amphibolite a d) Hydrothermal K-feldspar and epidote b c e) quartztourmaline vein e

CONTACT METAMORPHIC AUREOLE Pressure conditions at the top of the granite: 80-120 MPa, defined by the presence of late-alpine andalusite, biotite and cordierite at 2.5-4 km depth and an uplift rate of 0.2 mm/y Temperature conditions from 400 to more than 600 C, on the basis of mineral assemblages

Examples of CMR-1

Contact Metamorphism of Carbonates-1 Pyroxene, phlogopite anhydrite marble Selva 4A 3370 m

Contact Metamorphism of Carbonates-2 Kink banding and deformation lamellae in a pyroxene Selva 4A 3370 m anhydrite-rich layer in a silicate marble

P-T Fluid Evolution

CONCLUSIONS The fluid evolved from early magmaticmetamorphic conditions to a late-stage hydrothermal circulation characterised by fluid of meteoric origin (H horizon). A deep-seated fluid, with magmatic to metamorphic connotations and supercritical characteristics likely exists within the K horizon.

Amiata

Depth of K horizon at Mt Amiata

Stratigraphy 0mb.g.l. Volcanites Neogenic Sediments Transgression Surface Ligurian flysch Tectonic Surface Tuscan Nappe 1000 m b.g.l. Verrucano Formation A Formation B 2000 m b.g.l. Formation A Formation C 3000 m b.g.l.

Cross section Amiata

Measured Temperature ( C) 0 0 100 200 300 400 PC30 1000 Depth (m b.g.l.) 2000 3000 PC30A 4000

Hydrothermal alteration

Hydrothermal and contact metamorphic minerals

Hydraulic and tectonic fracturing can enhance permeability of rocks with very low porosity PERMEABILITY

Fluid inclusions and present day fluid

Model Mt. Amiata METAMORPHIC FLUIDS INTERACTION WITH SALINE FLUIDS (EVAPORITES DISSOLUTION?) Tmax 160 C Tmax 220 C COOLING METAMORPHIC FLUIDS T=300-330 C BOILING? T=300-360 C FRAGMENTS TWO-MICA GRANITE SALINE FLUIDS T>500 C HEAT SOURCE MAGMA CHAMBER? GRANITIC BODY? 5-6 KM T=820 C

Future research work? Prediction of the duration of the geothermal resource, and the extension at depth of the reservoir, are the most challenging scientific goals. It is important to characterise the deepest geological units and understand if fluid exists in rocks near a quasi-plastic state, and can be exploited.

High strain rate values (10-12 sec -1 ) for the geothermal areas, Fournier (1991). Such high values can derive from: 1) emplacement of shallow magmatic intrusions, Mt Amiata, Acocella, 2000); 2) fluid overpressures within pre-existing fractures and faults, whose orientation is favorable for their reopening (Gianelli, 1994)

At approximately 100 MPa and 600-650 C (the conditions of the K-horizon at Larderello), the fluid is a L+V saline brine or a supercritical fluid, depending on salinity

CONCLUSIONS Laboratory experiments (Hashida et al., 2001; Tsuchiya et al., 2001) show that, at approximately 25-50 MPa at 400-600 C, granite can still permit the circulation of a supercritical fluid through unhealed microfractures. The main problem is to understand the mechanical behaviour of the rocks at the high temperatures in correspondence of the deep seismic reflector (K). Geophysical data are so far supported by relatively scarce geological data. Collection and analysis of more core samples is necessary to assess the deep-seated, possibly supercritical, geothermal resource.

WELL DESIGN FOR A DEEP SCIENTIFIC DRILLING AT LARDERELLO (June 1999) A well of approximately 5 km to explore the deep seismic reflector K

COST OF THE PROJECT ATTIVITA' ESEC COSTI (ML) ANN0 1 ANN0 2 ANN0 3 ANN0 4 ANN0 5 TOTALE CNR ENEL OGS T1 T2 T3 T4 T1 T2 T3 T4 T1 T2 T3 T4 T1 T2 T3 T4 T1 T2 T3 T4 GEOLOGIA/PETROGRAFIA/GEOCHIMICA CNR/ENEL 2727 2127 600 0 RIELABORAZIONE DATI CNR/ ENEL 1245 1045 200 0 MESSA A PUNTO METODOLOGIE DI ANALISI CNR 1082 1082 0 0 GEOLOGIA DI CANTIERE ENEL 0 0 280 0 ANALISI DI LABORATORIO ENEL 0 0 120 0 GEOFISICA CNR/ENEL/OGS 5908 772 1706 3020 RIELABORAZIONE DATI CNR/ENEL/OGS 923 240 513 170 ACQUISIZIONE NUOVI DATI (FASE PRE- DRILLING CNR/ENEL/OGS 4575 532 1193 2850 PETROFISICA CNR/ENEL 594 355 239 0 MESSA A PUNTO METODOLOGIE MISURA CNR/ ENEL 220 120 100 0 MISURE DI LABORATORIO(PRE-DRILLING) ENEL 54 0 54 0 MISURE DI LABORATORIO (WHILE-DRILLING) CNR/ ENEL 320 235 85 0 MISURE IN POZZO ENEL/ OGS 2422 0 2422 0 RIELABORAZIONE DATI ENEL/OGS 50 0 50 0 APPLICAZIONI SPERIMENTALI(PRE-DRILLING) ENEL/OGS 200 0 200 0 MESSA A PUNTO NUOVI SISTEMI MISURA H.T. ENEL/OGS 300 0 300 0 ACQUISIZIONE&ELABORAZIONE LOGS ENEL/OGS 1872 0 1872 0 PERFORAZIONE ENEL 24000 0 24000 0 INDAGINI PRELIMINARI DEFINIZIONE ACCORDI DI COLLABORAZIONE RIPRISTINO POZZO DOLMI 4 COSTRUZIONE POZZO PROFONDO WELL TESTING ENEL 725 0 725 0 MODELLAZIONE DEL SERBATOIO CNR/ENEL 400 200 200 0 INTERPRETAZIONE DATI CNR/ENEL/OGS 1583 723 450 410 TOTALE 38660 4177 30642 3840