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

Transcription

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

2 DEEP SEATED UNCONVENTIONAL GEOTHERMAL RESOURCES IN TUSCANY

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4 More and more steam from Larderello

5 Larderello/Travale Steam production history Total production history of the Larderello and Travale/Radicondoli fields Flow-rate [kg/s] new new units units with with a capacity capacity in in the the range range MW MW started started up up in in the the year year Years

6 Larderello Travale Geothermal Area

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

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

9 Seismic Data

10 Deep Reservoir (3-3.5 km depth) producing super-heated steam Sea level

11 TEMPERATURE vs. DEPTH Sea level

12 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.

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14 Structural Setting 1

15 Structural Setting 2

16 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

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

18 CONTACT METAMORPHIC AUREOLE Pressure conditions at the top of the granite: MPa, defined by the presence of late-alpine andalusite, biotite and cordierite at 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

19 Examples of CMR-1

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

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

22 P-T Fluid Evolution

23 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.

24 Amiata

25 Depth of K horizon at Mt Amiata

26 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.

27 Cross section Amiata

28 Measured Temperature ( C) PC Depth (m b.g.l.) PC30A 4000

29 Hydrothermal alteration

30 Hydrothermal and contact metamorphic minerals

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

32 Fluid inclusions and present day fluid

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

34 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.

35 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)

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

37 CONCLUSIONS Laboratory experiments (Hashida et al., 2001; Tsuchiya et al., 2001) show that, at approximately MPa at 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.

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

39 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 RIELABORAZIONE DATI CNR/ ENEL MESSA A PUNTO METODOLOGIE DI ANALISI CNR GEOLOGIA DI CANTIERE ENEL ANALISI DI LABORATORIO ENEL GEOFISICA CNR/ENEL/OGS RIELABORAZIONE DATI CNR/ENEL/OGS ACQUISIZIONE NUOVI DATI (FASE PRE- DRILLING CNR/ENEL/OGS PETROFISICA CNR/ENEL MESSA A PUNTO METODOLOGIE MISURA CNR/ ENEL MISURE DI LABORATORIO(PRE-DRILLING) ENEL MISURE DI LABORATORIO (WHILE-DRILLING) CNR/ ENEL MISURE IN POZZO ENEL/ OGS RIELABORAZIONE DATI ENEL/OGS APPLICAZIONI SPERIMENTALI(PRE-DRILLING) ENEL/OGS MESSA A PUNTO NUOVI SISTEMI MISURA H.T. ENEL/OGS ACQUISIZIONE&ELABORAZIONE LOGS ENEL/OGS PERFORAZIONE ENEL INDAGINI PRELIMINARI DEFINIZIONE ACCORDI DI COLLABORAZIONE RIPRISTINO POZZO DOLMI 4 COSTRUZIONE POZZO PROFONDO WELL TESTING ENEL MODELLAZIONE DEL SERBATOIO CNR/ENEL INTERPRETAZIONE DATI CNR/ENEL/OGS TOTALE