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ELECTRICITY GENERATION BY GEOTHERMAL POWER [STAGE 1]

Ziel

The use of geothermal energy to operate a 20 MW electricity generating unit following preliminary trials with a 3,5 MW mobile generator. Projected energy savings of 19,000 TOE/yr are anticipated. Problems of progressive scaling, which contribute to declining production in reservoirs of this type, are also to be solved. In addition, due both to high salinity, which prescribes reinjection of effluents and pressure drawdown in the reservoir, several production and injection wells were planned.
Both M1 and M2 showed sufficient potential for electricity production (4 MW each). Pre-injection treatment was necessary due to a high CO2 content and silicate deposition in the sludge. Tests indicate two reservoirs (possibly more) present in the Phlegraean Fields, proving the recharge of the reservoir by sea water. The 10 MW objective seems to be reasonable, in the view of the results of the further well drilled in the field and completion of pit test.
The first well (M1) was completed in January and February 1979 to a final depth of 1,606 m. A 5" liner was installed between the 1,223 and 1,467 m mark. Drilling stems (3,3") not recovered between 1,467 and 1,606 m. Second well (M2) drilled between May and July 1979 down to a depth of 1,987 m. Liner with filter screen installed between the 1,272 and 1,987 m. marks (sidetrack). The upper part of the reservoir is capped by 600 to 1000 m of impermeable volcanic strata which have become impermeable as a result of hydrothermal alteration. Below 1,100 m the reservoir consists of volcanic rocks (tuff) and sedimentary rock (marl and silt) hardened by various deposits, all permeable through fractures at some levels. Static conditions at bottom of well are: M1: 295 deg C and 119 atm., m2: 338 deg C and 159 atm. Test on M1 at atmospheric pressure showed the following characteristics: flow-rate: between 18 and 33 t/h (35% steam), temperature: 150 deg C; salinity: 13 g/litre; well-head pressure: between 7 and 4 kg/cm2; gas/steam ratio: 45NL/kg. Production test on M2 showed the following characteristics: flow-rate between 47 and 56 t/h (50% steam); temperature: 150 deg C; salinity: 24 g/litre; well-head pressure: between 28 and 4 kg/cm2; gas/steam ratio: 50 to 100 NL/kg. Since the well characteristics had not stabilised when they are tested (1979 and 1980), dynamic testing with a pressurised separator was carried out in August 1981 on M1 and in April 1982 on M2. Stabilised production from M1: 23,1 t/h at a well-head pressure and temperature of 3 kg/cm2 and 144 deg C (7 t/h of steam). Stabilised production from M2: 75 t/h at well-head pressure and temperature of 5,5 kg/cm2 and 170 deg C.
Flash at reservoir level and consequent deposit and mud-clogging problems made it necessary to clean the well prior to testing. The reservoir consists of three permeable zones which display a large number of fractures and could be interconnected. The reinjection trials revealed permeability between 3,6 mD and 13 mD, depending on the degree of fracturing, and a skin of between +3 and +7. In 1982 well N1 casing was perfored at the level 550-896 m. Testing was completed successfully indicating a well head flowrate and temperature of 210T/h (31 T/h steam) and 220 deg. C.

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