Special problems for drilling in large dense BHEfields

In large BHE fields, several problems have to be dealt with. To allow completing of more than 100 BHE in a relatively short time, several drilling rigs have to be on site simultanuosly. The available space, the supply of water, BHE pipes and grout, and the disposal of drilling mud all have to be planned diligently in advance. Fig. 10 shows the drilling for the largest central BHE field in the world at Richard Stockton College in New Jersey, USA. Up to four drilling rigs were in use, and the connections in the field have impressive dimensions. Table 6 lists some of the most interesting examples of large BHE fields.

part

Reel with BHE pipes

Reel with BHE pipes

Ground

Fig. 8: Device for BHE insertion (as suggested by Hess, 1987)

Ground

Fig. 8: Device for BHE insertion (as suggested by Hess, 1987)

Fig. 9: Reel with single-U-pipe in use for the Richard Stockton College wellfield

(photo RSC)

Fig. 9: Reel with single-U-pipe in use for the Richard Stockton College wellfield

(photo RSC)

Richard Stockton Photography
Fig. 10: Drilling on the Richard Stockton college wellfield (left) and main collecting pipes (right); photos: RSC

The problem of borehole deviation becomes serious in large plants. No borehole is exactly straight. With single boreholes, no problem arises, but with a number of boreholes close together like in an Underground Thermal Energy Storage system (e.g. Neckarsulm in table 6) Percussive drilling is particularity perceptible to borehole deviation. Sinkala (1987) reported a field study of borehole deviation in top-hammer blasthole drilling in Northern Scandinavia, and could prove deviations of up to two meters in less than 20 m depth in rocks with dipping layers (fig. 11). This ratio of 10 % would mean a deviaton of 10 m for boreholes of 100 m depth and more!

In order to avoid intersection of boreholes or damage of previously installed BHE, borehole deviation must be minimised. The only way is to limit the pushing force from the top of the hole, and to increase the weight directly above the drill bit. That way the drill string is kept straight.

Tab. 6: Examples of large borehole fields

Name

Location

No. of BHE

Depth of BHE

Total length

Central Valley School

Buxton ND, USA

120

64 m

7680 m

Lulevärme BTES

Luleä S

120

65 m

7800 m

DFS

Langen,D

154

70 m

10780 m

Max-Planck-Institute

Golm, D

160

100 m

16000 m

Corporate Sqare Bldg.

Terre Haute IN, USA

324

52 m

16848 m

Amorbach

Neckarsulm, D

> 600

30 m

18000 m

Sagamore Hotel

Lake George NY, USA

ca. 400

ca. 65 m

20800 m

Beaumont

Bryn Mawr PA, USA

170

ca. 160 m

27200 m

Whitehorse Village

Edgemont PA, USA

236

ca. 160 m

37760 m

Richard Stockton Coll.

Pomona NJ, USA

400

130 m

52000 m

Quary Methods
Fig. 11: Typical unilateral deviation of 19 m deep boreholes in rock with dipping ' layers; wall of quarry in Bulken, Norway (from Sinkala, 1987)

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