How EGS is investigated in the case of the Larderello geothermal field ? Guido Cappetti Enel – Generation and Energy Management – Geothermal Production Engine Launching Conference Orleans 12-15 February 2006

Contents The first exploitation period of Larderello The Research&Development programs, from mid ’70s: Deep exploration Reinjection Stimulation jobs The reassessment of the geothermal resource New exploitation strategies

Temperature distribution at 3000m depth, in Italy

Geothermal power growth up to 80’s In 1913, at Larderello, first unit in operation (250 kW). Italy was the sole country in the world for electricity generation from geothermal resources up to 1958, when a unit was installed at Wairakey (New Zealand). Development of shallow reservoirs of Larderello, Travale/Radicondoli and Amiata. Beginning of 70’s serious production decline problems Oil crisis induced a renewal of interest towards geothermal energy and new R&D were implemented. 1904 First test in Larderello 1913 First 250 kW unit in Larderello 1904

Top of the shallow Carbonate Reservoir Temperature distribution in the Larderello area Top of the shallow Carbonate Reservoir

The production sustainability The problem of production sustainability was evidenced in the mid-‘70s, when the boundaries of the productive areas related to the shallow reservoirs were reached. The following R&D activities were planned to sustain production. 7000 Deep exploration 3000-4000 m Reinjection / Injection Stimulation of low productivity wells 6000 5000 4000 GWh/yr 3000 2000 1000 1900 1904 1908 1912 1916 1920 1924 1928 1932 1936 1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004 2008 Years

Deep exploration and development

Top of the shallow Carbonate Reservoir Temperature distribution in the Larderello area Top of the shallow Carbonate Reservoir

Conceptual model of the Larderello Geothermal field

Temperature distribution at 3000 m b.s.l. Temperature distribution in the Larderello area Temperature distribution at 3000 m b.s.l.

Reinjection

depleted the fluid inside the reservoir . Intensive exploitation: field depletion Intensive exploitation of the Larderello-Valle Secolo area has gradually depleted the fluid inside the reservoir .

Reinjection results The reinjection has increased the steam production of about 100 kg/s, equivalent to 50 MW. The long reinjection period has not substantially modified the temperature of the produced fluid.

Reinjection results The boiling of injected water produced additional steam devoid of gas which diluted the primary steam and therefore allowed a reduction of the weighted average gas content, with considerable energy savings for gas extraction from the power plant condensers.

Stimulation activities/EGS

Stimulation methodologies Injection of acid mixture (HCl and HF) and water produces self-propping effects in the fractures: Microslippage of the fracture faces Removal of cuttings and mud deposition inside the fractures Acid leaching of hydrothermal mineralizations present in the fractures Contraction and consequent cracking of hot rock formations due to thermal stress

Acid attack Lab Tests Aim: Selection of the best acid mixtures for the acid attack of rock matrices and hydrothermal mineralisations of deep geothermal reservoirs Tested rock samples : Phyllites, micaschists, hornfels,and granites with some fractures filled with siliceous hydrothermal mineralizations (epidotes + K feldspar and chlorides + K feldspar + pyrites) Tested acid mixture : HCl + HF, at different HCl/HF ratios The tests have been carried out by the Enel-Lab and by the Lab of Pisa University - Department of Earth Sciences

Acid attack Lab Tests Tests at batch conditions: temperatures in the range 100 - 150 °C two hours reaction time with an acid/rock ratio = 3 cm³/g Tests in flowing conditions: temperatures of 30°, 50°, 70° 90 °C reaction time = 30’ Tested acid mixture: 9% Hcl - 1% HF; 6% Hcl - 6% HF; 6% Hcl - 3% HF; 12% Hcl - 6% HF; 12% Hcl - 3% HF Performed analyses : - weight of dissolved solids - chemical analyses of reactions products - mineralogic analyses of the leached rock surface by electronic microscope and X ray

Equipment for Lab tests

Lab tests

Lab tests

Field tests Well Bagnore_25: 3397 m bottom hole. Phyllites: fractured zone near the well bottom. Acid job: injectivity improved from 3 to 15 m³/h/bar Well Radicondoli_29: 4826 m bottom hole - hornfels rocks and granite from 4230 m Injectivity improved from 1 to 7 m³/h/bar Well Montieri_4: granite from 2000 to 3721 m (bottom hole), characterized by high hydrothermal alterations, temperatures of about 300 °C and low permeability. Injectivity improved from 0.4 to 5 m³/h/bar The well can produce 50 t/h of steam (6.5 MW) Well Travale Sud_1: 2987 m bottom hole - hornfels rocks Acid job: productivity increased from 30 to 110 t/h (about 15 MW) Well Montieri_1: 2219 m bottom hole - hornfels rocks. Acid job with 200 m³ of 12% HCl - 6% HF. Injectivity improved from 3 to 15 m³/h/bar The well is now producing about 250 t/h of steam (~35 MW)

Field tests Improvement of well production characteristics by the injection of cold water

The results of the R&D in Italy The deep drilling activity pointed out the presence of productive layers up to depths of 3000-4000m, in a very wide area The reinjection/injection strategies proved to be successful for sustaining/increasing the fluid production The stimulation jobs are applied successfully to increase the wells productivity Reassessment of the geothermal resources has been carried out and new exploitation strategies have been adopted, based on: Heat Production Production Sustainability

Larderello/Travale Steam production history

Power development Positive results of deep drillings, stimulations and reinjection 12 new units (314,5 MW) started up in the period 2000-2005 8 units, for a total of 214.5 MW, replaced old units in operation from many years and considered “obsolete”. The new units are characterised by higher efficiency and lower environmental impact. Nuova Larderello (20 MW) 4 units, for a total of 100 MW are new capacity installed both in marginal and central areas

Electricity generation and steam flow-rate in the Larderello geothermal field

USA: The Geyser production trend 500 1.000 1.500 2.000 2.500 1965 1970 1975 1980 1985 1990 1995 2000 2005 Years MW

From hydrothermal systems... Enhanced Geothermal Systems Evolution of the exploitation strategies From hydrothermal systems... Fractured reservoirs with high permeability and fluids inside … to Hot Dry Rocks Tight formations with high temperatures Hydraulic stimulation for the creation of an artificial fracture network Production of the stored fluids  analysis of the field depletion Stimulation jobs and secondary heat recovery through water reinjection/injection Water circulation for the heat extraction Heat Production Enhanced Geothermal Systems

Considerations about EGS What does it means to enhance a geothermal system ? to improve the knowledge of the system to enhance the permeability of the formation to enhance the heat production through water circulation To enhance fluid production and electricity generation

Conclusions Larderello has been the first geothermal system developed and exploited in the world (over 100 years history…) First period of intensive explotation with “conventional” strategies, typical of hydrothermal systems Now Larderello can be considered the largest worldwide example of an Enhanced Geothermal System The exploitation strategies are based on the “Heat Production” and are aimed at the “Production Sustainability”

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