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Giovanni Gianelli ISTITUTO DI GEOSCIENZE E GEORISORSE INSTITUTE OF GEOSCIENCES AND EARTH RESOURCES DEEP SEATED UNCONVENTIONAL GEOTHERMAL RESOURCES IN TUSCANY More and m ore steam from Larderello Larderello/ Travale Steam production

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

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DEEP SEATED UNCONVENTIONAL GEOTHERMAL RESOURCES IN TUSCANY

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More and m ore steam from Larderello

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Larderello/ Travale Steam production history

Total production history of the Larderello and Travale/Radicondoli fields

100 200 300 400 500 600 700 800 900 1000 1100 1200 1919 1923 1927 1931 1935 1939 1943 1947 1951 1955 1959 1963 1967 1971 1975 1979 1983 1987 1991 1995 1999 2003

Years Flow-rate [kg/s]

8 new units w ith a capacity in the range 1 0 - 6 0 MW started up in the year 2 0 0 2 8 new units w ith a capacity in the range 1 0 - 6 0 MW started up in the year 2 0 0 2

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Larderello –Travale Geothermal Area

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Features of The Geothermal System

High amplitude reflectors Granite (age 0.7 to 3.8 m.y.)

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GRANITE CAN BE A RESERVOIR

Core sample of 3570 m depth Cataclastic granite with hydrothermal alteration

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Seismic Data

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

Sea level 400 350 300 250 200 150 100 50 500

  • 500
  • 1000
  • 1500
  • 2000
  • 2500
  • 3000
  • 3500
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SLIDE 11

TEMPERATURE vs. DEPTH

Sea level 400 350 300 250 200 150 100 50 500

  • 500
  • 1000
  • 1500
  • 2000
  • 2500
  • 3000
  • 3500
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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 CO2) 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 km2, supports the hypothesis of the presence of a giant reservoir. Rock permeability is due to fracturing.

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Structural Setting 1

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Structural Setting 2

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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

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a)-b) garnet micaschist c) amphibolite

a b c

d) Hydrothermal K-feldspar and epidote e) quartz- tourmaline vein e

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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

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Examples of CMR-1

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Contact Metamorphism of Carbonates-1

Pyroxene, phlogopite anhydrite marble Selva 4A 3370 m

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Contact Metamorphism of Carbonates-2

Kink banding and deformation lamellae in a pyroxene Selva 4A 3370 m anhydrite-rich layer in a silicate marble

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P-T Fluid Evolution

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CONCLUSIONS

  • The fluid evolved from early magmatic-

metamorphic 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.

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Amiata

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Depth of K horizon at Mt Amiata

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

Stratigraphy

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Cross section Amiata

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PC30 PC30A

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

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Hydrothermal alteration

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Hydrothermal and contact metamorphic minerals

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PERMEABILITY

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

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Fluid inclusions and present day fluid

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Model

  • Mt. Amiata

INTERACTION WITH SALINE FLUIDS (EVAPORITES DISSOLUTION?)

METAMORPHIC FLUIDS

HEAT SOURCE MAGMA CHAMBER ? GRANITIC BODY ? 5-6 KM T = 820 °C

?

BOILING

METAMORPHIC FLUIDS

FRAGMENTS TWO-MICA GRANITE

T>500 °C

SALINE FLUIDS

Tmax 160 °C Tmax 220 °C T=300-330 °C T=300-360 °C COOLING

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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.

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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 re-

  • pening (Gianelli, 1994)
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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

  • n salinity
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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.

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  • A well of

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

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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

RIELABORAZIONE DATI

CNR/ENEL

1245 1045 200 MESSA A PUNTO METODOLOGIE DI ANALISI

CNR

1082 1082 GEOLOGIA DI CANTIERE

ENEL

280 ANALISI DI LABORATORIO

ENEL

120

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

MESSA A PUNTO METODOLOGIE MISURA

CNR/ENEL

220 120 100 MISURE DI LABORATORIO(PRE-DRILLING)

ENEL

54 54 MISURE DI LABORATORIO (WHILE-DRILLING)

CNR/ENEL

320 235 85

MISURE IN POZZO

ENEL/ OGS

2422 2422

RIELABORAZIONE DATI

ENEL/OGS

50 50 APPLICAZIONI SPERIMENTALI(PRE-DRILLING)

ENEL/OGS

200 200 MESSA A PUNTO NUOVI SISTEMI MISURA H.T.

ENEL/OGS

300 300 ACQUISIZIONE&ELABORAZIONE LOGS

ENEL/OGS

1872 1872

PERFORAZIONE

ENEL

24000 0 24000

INDAGINI PRELIMINARI DEFINIZIONE ACCORDI DI COLLABORAZIONE RIPRISTINO POZZO DOLMI 4 COSTRUZIONE POZZO PROFONDO

WELL TESTING

ENEL

725 725 MODELLAZIONE DEL SERBATOIO

CNR/ ENEL

400 200 200 INTERPRETAZIONE DATI

CNR/ ENEL/ OGS

1583 723 450 410 TOTALE 38660 4177 30642 3840