Objective
To drill a pair of directional geothermal wells to a depth of 3,500 m in the sandy layers of the Upper Triassic to the Gassum formation (thickness - 160 m; net pay of aquifer - 65 m; temperature - 100 deg C; salinity - 190 g/l; permeability - 1 Darcy; porosity - 20%, chiefly in the lower part of the Gassum formation). To investigate the aquifers of the Lower Cretaceous and Middle Jurassic in the course of drilling. The geothermal fluid will feed an existing district heating network which supplies flats, private houses and public facilities in the town of Aars. The expected output of the geothermal station is estimated at 5 to 8,5 MW, energy savings of 7,000 TOE/year.
The Gassum formation is not exploitable for geothermal heating, due to low reservoir permeability. As a result the Haldager formation has been tested and appears to be an alternative resource. However the following have to be taken into account:
- water temperature is lower than those expected in Gassum formation;
- possibility of decreasing production as a result of pore plugging with small Kaolinite plates;
- existence of hydraulic barrier. No patents or licences are expected.
The production well was drilled vertically down to 3,400 m between November 1978 and August 1979 after four unsuccessful attempts at directional drilling had been made and many technical problems had been encountered. The Gassum formation, which was traversed for a distance of 180 m (3,205 - 3,385), was equipped with casing, cemented between 3,205 and 3,302 m marks and then perforated over a height of 18 m, while the lower part of Gassum aquifer (between 3,300 and 3,400) which was the main target, was left as an open hole: maximim air-lift-assisted flow, after acidizing of the perforations was 6 m3/h at a temperature of 100 deg C and a salinity of 290 g/l. Gassum formation cannot supply more than 1 m3/h because of low permeability. An overlying formation (Haldager) had been tested from May to June 1981 on a total perforated height of 16 m between the 2,467 and 2,498 m marks (three principal sandy zones tested, first separately and then together over 15 days): temperature - 74 degC; static level - 219 m; salinity - 270 g/l; transmissivity - 7 D.M.; skin + 1,2; average porosity - 22%; net pay - 22 m; gas content - 0,2 litre/litre at 20 deg C. The tested formation produces 10 m3/h for a drawdown of 53 m. Different tests indicated some interconnection between the beds and the presence of permeability barriers not far from the site (faults and thinner beds due to the vicinity of a salt dome). The available geological and seismic data correlated with the pump test show a limited aquifer of 4,5 km2, the outline of which is not precisely known.
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DEM - Demonstration contractsCoordinator
2970 Hoersholm
Denmark