Objective
To generate 10 MW by exploiting a deep reservoir in the Larderello geothermal field, since the main reservoir (at a depth of 600 to 800 m) is approaching exhaustion. To drill a borehole (San Pompeo 2) to a maximum depth of 5,000 m. The Sasso 22 well (depth: 4,000 m) has already shown signs of a reservoir at around 3,000 m, and temperatures of 350 deg C have been recorded. After the drilling stage, it is planned to carry out a production measurement programme to analyse the physical and chemical characteristics of the fluid: an experimental scheme to generate electricity will also be started. The third stage will involve the design and construction of a 10 Mw unit. There are also plans to drill two more wells and to reinject the effluent. If the three stages are completed, an energy saving of 19,000 TOE/y is expected.
This exploration well encountered two different reservoirs separated by a compact layer (micaschist with plyllites and garnets). This thickness (600 m) represented an efficient cap between the two reservoirs, which are not hydraulically linked. Their characteristics are:
- upper reservoir between 700 and 2,200 m (Trias to Paleozoic) with fluids at 250 deg C and 30-40 kg/cm2
- deep fractured reservoir lower than 2,900 (Paleozoic - Precambrian) with fluids at 400 deg C and more than 240 kg/cm2. Chemical and isotopic analyses of both fluids shows main differences related to:
- high H2 and CH4 content in the deepest
- 18 oxygen
- absolute content of H3BO3 and NH4 which are 3 to 4 times higher in the deepest.
The petrological analyses (thermometamorphic sequence assisted with tourmaline and biotite) indicate the presence (2,900 m) of an aureola limited to the late Alpine thermal event.
The San Pompeo 2 borehole, begun in September 1980, has been drilled to a maximum depth of 2,966 m. Drilling was carried out without returns, using clear water and mud plugs. Loss of stems led to the first side-tracking at 2,600 m. At a depth of 2,937 m several successive explosions threw up a large quantity of fluid and debris at pressure of up to 40 bars. The casing (9 5/8") had been badly damaged between 1,400 and 2,770 m.
Further drilling at 6" diameter after clearing the debris blocking the well caused an explosion followed by blockage at 2,894 m. The loss of 94 stems which cannot be recovered and the poor state of the 9 5/8" casing, called for further side-tracking at 1,280 m. When a depth of 2,869 m had been reached, a 9 5/8" casing was sunk and cemented in down to the 2,850 mark. The temperature recorded at 2,600 m was 364 deg C. Cement drilled into at around 2,791 m. Problems were encountered with regard to resistance of casing and metal debris was recovered indicating that the casing was probably cracked owing to weak cementation at an unknown depth. At around 2,935 m inflows of gas occured with the well shut-off pressure 88 kg/m2. On reopening, the well blew out, throwing up water, gas, steam and debris which caused an obstruction; once the well had been cleared by N2 Lift an explosion at around 2,735 m raised and blocked the drill string, over 200 m of which could not be recovered.
The main difficulties encountered are:
- slurry use even during loss of circulation
- explosion due to instant vaporisation of the liquid phase in slurry at high temperature
- impossibility to foreseeing contact with the formation containing over pressured fluids
- difficulty in maintaining a good stability of the open hole owing to the low hydrostatic level (1,000 m from ground level)
- bad cementation at high temperature in fractured rocks
- rapid corrosion of materials.
Programme(s)
Topic(s)
Call for proposal
Data not availableFunding Scheme
DEM - Demonstration contractsCoordinator
00198 ROMA
Italy