Drilling methods for shallow geothermal installations Burkhard Sanner1 and Olof Andersson2 1
2
Institute of Applied Geosciences, Justus-Liebig-University Diezstrasse 15, D-35390 Giessen, Germany
Department of Engineering Geology, Lund University of Technology P.O.Box 118, S-22100 Lund, Sweden
Introduction
Drilling is a very old technique, for water supply and for exploitation of mineral resources. Water wells have been drilled in Egypt more than three millenia ago. Fig. 1 shows an example of a quite sophisticated, hand-operated drill rig used in China to drill several hundred meter deep in a few month.
Fig. 1:
Chinese percussion percussi on boring (after Chugh, 1985)
Today, a variety of mechanised drilling equipment is on the market and in use. The size range is from small augers for post-holes few meters deep to the deepest borehole on Kola peninsula in Russia with a depth of more than 12 km. Economic considerations as well as technical problems restrict the drilling depth for shallow geothermal applications generally to around 100 m. In recent times, also deeper holes have been drilled for borehole heat exchangers (BHE) with ca. 250 m depth, and holes in the 400-m-range are in preparation in Switzerland.
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Although a deeper hole provides access to slightly higher temperature, the increasing problems with insertion of heat exchanger, grouting and static pressure have to be solved. For vertical ground heat exchangers, a distinction has to be made between two basic ways of installation: •
Direct pressing or ramming of the heat exchanger into the ground (in soft ground only)
•
Insertion of the heat exchanger in a borehole drilled beforehand Boreholes can be drilled under almost any subsurface condition From site to site, an optimum choice for type and installation method is necessary according to the geological situation.
Basic drilling methods
For shallow holes down to app. 100 m, not every drilling method is suitable. Hytti (1987) presents a diagram showing the optimal drilling methods in respect to hole diameter and rock strength (Fig. 2). Hard rock Top hammer
DTH
Rotary crushing
Soft rock
Rotary cutting 20
100
60
140
180
220
260
300
340
Borehole diameter [mm] 1
2
3
4
5
6
7
8
9
10 11 12 13 14
Borehole diameter [inches]
Fig. 2:
Applicable drilling methods (revised after Hytti, 1987)
Table 1 lists recommended drilling methods for the various ground conditions. Drilling rates of app. 10 m per hour are realistic when using rotary drilling with drag bits in soft/medium and Down-the-hole- (DTH) or Top-Hammer in hard and very hard rock. The advantage of DTH in hard rock can be seen in data from drilling for Schwalbach GCHP Reserach Station: In the same quarzitic rock, app. 5 m apart, and with the same very light drill rig (~2 metric tons), a 50-m-borehole could be completed using rotary drilling with rock- and button-bit in about 5 days; with DTH, the 50 m were completed after 4½hours. A further restriction applies to the rotary method. Rotary drilling is widely used and can be adopted to almost every drilling problem, but drilling velocity normaly is not very high and can become extremly low in unfavourable conditions. Rotary cutting with drag bits in soft and medium rock can be effective even with light rigs, but rotary crushing with rock- or roller bits and even more with button bits requires heavy load on the bit to crush the rock when rolling over the teeth (fig. 3). In deep holes as in oil well drilling, the drill string alone brings enough load onto the bit; for shallow holes, the required load often exceeds the weight of a light drilling rig, in spite of using heavy tubing. The optimum load increases with borehole diameter; 56
Cambefort (1964) recommends 700-900 kg/in in soft and 1400-1800 kg/in in hard rock. For a standard 115 mm (4½") borehole, suitable for most of the European BHE, in hard rock a load of 6.3 to 8.1 metric tons would result. Even in this optimum condition, the drilling rate in hard rock is only 1-2 m per hour, and can decrease to some 10 cm/h when the load is too small. Thus, for shallow holes in hard rock conventional rotary drilling is not a good choice. Table 1:
Drilling methods
Soil/Rock-type
Method
Remarks
soil, sand/gravel
auger
sometimes temporary casing required
rotary
temporary casing or mud additives required
auger
mostly best choice
rotary
temporary casing or mud additives required
rotary
roller bit, sometimes mud additives required
soil, silty/clayey
rock, medium hard
DTH rock, hard to very hard
1
large compressor required
rotary DTH
with rock bit or hard-metal insert button bit, very slow
2
large compressor required
top hammer rock under overburden
Fig. 3:
1 2
2
ODEX or similar
special equipment, depth range ca. 70 m in combination with DTH
rock bit (left) and tungsten-carbid button bit (right), from Chugh (1985)
Down-the-Hole-Hammer Overburden Drilling Equipment; ODEX is a trademark of Atlas Copco
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With the hammer techniques, drilling in hard rock is very fast and cost-effective. Problems arise, if intercalations of soft and instable layers are present. For unconsolidated overburden, the ODEX 3-method offers a way to install a casing down to the bedrock while drilling and to proceed drilling in the hard formation with normal DTH-equipment. If the overburden is too thick, or if instable rock is found at considerable depth, the DTH with pneumatic flushing of the cuttings can not be used. In rotary drilling, were a fluid (normally water) is used for flushing the hole, special drilling muds can stabilize the borehole wall. In table 2, some common mud additives are listed. Tab. 2:
Additives for drilling mud in rotary drilling
Name
Properties
Remarks
bentonite
thixotropic, stabilises the hole
possible clogging of aquifers
CMC (cellulose)
stabilises the borehole wall, reduces water losses
growth of bacteria
polyacrylamide
minimises water losses to the formation
baryte, ilmenite etc.
weighting materials, stabilise the hole, keep pressured water down
foam generators
facilitates flushing out of cuttings
possible problems with BHE installation
Two phenomena are used to stabilize the hole with the listed additives. Bentonite and cellulose-products have thixotropic properties, i.e. they build up stable aggregates when stagnant but are fluid in motion. Heavy minerals as baryte (BaSO4), ilmenite (FeTiO3) or hematite (Fe2O3) increase the density of the drilling mud, can counteract the formation pressure and thus stabilize the borehole. They are also used if groundwater under artesian pressure is present. However, thick or heavy muds make insertion of the heat exchanger pipes difficult. The economy of drilling shallow holes is completely different than that of deep oil- or gas wells. Very easy methods, as the tractor-mounted auger (fig. 4) described by Reuss et al. (1990), can be used by a farmer for installing BHE with no external cost; and even drilling with hammer equipment is far from cost and time required for deep holes. Light, mobile rigs, suitable for both rotary and DTH, ensure cost-effective drilling (fig. 5). Equations for calculating drilling cost have been discussed in the literature. Armstead (1983) cites two equations, and Schulz & Jobmann (1989) establish an equation for Germany (here converted into US-$): C = 95,000 . 1.153D US geothermal wells D C = 52,000 . 1.0998 US oil and gas wells C = 112,000 . D . e(0,01 . D) German geothermal wells with
C: D:
Total cost for drilling in US-$ Depth of hole in hundreds of meters
All three equations fit well the data for holes deeper than 500 m, but overestimate cost for shallow holes. A 100-m-hole would cost US-$ 109535, 57189 and 113125 resp., following 3
Overburden Drilling Equipment; ODEX is a trademark of Atlas Copco
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these equations. The realistic drilling cost including BHE and grouting in Europe are from 3550 US-$/m (40-55 €/m) for holes from 50-100 depth, resulting in 3500 to 5000 US-$ (40005500 €) for the 100-m-hole. Only in very unfavourable geologic conditions, where temporary casing etc. may be required, the cost will be higher.
Fig. 4:
Auger principle (left, Atlas Copco) and light auger mounted on agricultural tractor (photo from Beck et al., 1993)
Fig. 5:
Drilling rig optimised for BHE installation, mounted on all-terrain vehicle (Unimog)
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BHE fields with a number of boreholes should yield netterr economy. So it is not unexpected that Garlick (1986) reports US-$ 311,134 for 324 holes each 52 m deep, which means only US$ 18.5 per meter. Rammed or pressed steel earth probes in soft ground cost 22 - 40 US-$ per meter, and for the SIG-system Engvall (1986) calculated 1,640,000 SKr for a 100,000 m3 ground heat store with 54,750 m earth probe length, which would be less than 6 US-$ per meter. With the simple auger shown in fig. 4, Beck et al. (1993) achieved total cost for drilling, installation, ground connections, backfilling in a BHE field with 103 boreholes of 8-12 m depth of 27400 US-$ (30900 €), that is 26,4 US-$/m (30 €/m).
Drilling for exploration purposes
The methods for drilling and data collection vary between countries, but are basically the same as those used for other exploration purposes, like in water well industry, foundation drilling and in the mining industry. The importance of geological exploration for shallow geothermal applications is discussed by Andersson et al. (1997). Exploration boreholes for shallow geothermal purposes would in most cases be performed by conventional rotary drilling. During drilling, samples of the drilled formations (cuttings) can be separated from the drilling fluid. These samples give a basic information for the geological description. By measuring the rate of penetration as a function of rotation speed and thrust or weigth on the drillbit, the relative hardness of the penetrated formations can be described. This information is of value for property descriptions. Furthermore, losses (or gain) of drilling fluid indicates permeable layers or fractures. By measurement of level and amount of losses, important hydraulic properties can be detected (Andersson, 1981). In boreholes for groundwater wells, after drilling, and based on the informations gained during drilling, a screen is properly set in the hole and a testwell is completed. By pumping this well and observation of groundwater level in the well (and in surrounding wells, if available), the hydraulic properties of the aquifer can be determined. Basic methods are described e.g. in Kruseman & De Ridder (1970). Exploration boreholes for BHE in hard rock (consolidated formations, igneous or metamorphic rock) would preferably be carried out by the hammer-drilling method (DTH). In this case compressed air is used to drive the hammer and to flush the hole. Samples of cuttings, separated from the outlet air, are used to describe lithology (type of rock, mineral composition). Similar to rotary drilling, rate of penetration can give information about lithological boundaries , relative hardness, location of fractures etc. (Andersson, 1981). Because no water is added from the surface while drilling, this method furthermore allows a continuous recording of water flowrate out of the borehole, and also water samples for chemical analysis can be taken. If very exact geological data as well as larger samples for determination of thermal and/or hydraulic parameters are required, core drilling is the method to choose. For this method, a hollow pipe with an annular drilling bit ("core barrel") is attached to the drill string, and cylindrical rock or soil samples can be retrieved. Core drilling for the total length of the borehole is expensive, because the drill string has to be completely removed and inserted again after each length of core barrel (1-1.5 m), or very specialized tools (wire-line core barrel) have to be used.
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Groundwater well drilling and completion
Groundwater wells are drilled by the drilling methods mentioned before. For large diameter wells, the reverse rotary method is well suited. While in normal rotary the fluid is pumped downwards inside the drill pipe and rises through the annulus, the sense of motion here is reversed. This allows for reliable removal of drill cuttings through the pipe and helps to keep the borehole wall stable. There are two basic ways for construction of a well: • Natural completion (lost filter completion) • Gravel pack For natural completion, the formation has to be sand and gravel with suitable grain-size distri bution. The well screen is inserted, and then through strong pumping fine material is removed, thus forming a kind of natural gravel filter around the screen. If the formation does not exhibit the composition required for natural completion, a gravel pack (or sand pack) has to be installed in the annulus around the screen, to avoid continuing production of fine material. For a ground source heat pump, the type of well (production or injection) is always the same. In aquifer storage, wells may have to be operated as production well half of the year and as injection well for the other half (fig. 6). Cover Valve Motorvalve
Concrete
Overburden
Standpipe Pipe grouting (e.g. clay) GW-level (ca.) riser pipe injection pipe grouting controlelectrodes well screen submersible pump
Aquifer
gravel filter control electrode
Bedrock
Fig. 6:
Well for ATES-applications (production and injection)
Methods for direct installation of ground heat exchangers in soft ground
In unconsolidated sediments, ground heat exchangers can be pressed or driven into the ground. Table 3 shows the most important techniques. The major advantages of direct pressing or ramming the heat exchanger in place are: 61
•
No problems with stability of borehole walls • Good contact to the ground, no grouting required • Complete installation in one single step For an economic configuration of ground heat exchangers, steel pipes can be driven radially from one point as shown in Fig. 7. In this fan-shaped assemblage, the heads of the pipes are close together and connection to a manifold is easy. Tab. 3:
Installation methods for soft ground
Soil type
Method
heat exchanger
Remarks
sand/gravel
ramming/driving
coaxial
only possible with steel pipes, corrosion problems
water jetting
coaxial
steel pipes (corrosion)
ramming/pressing
coaxial
steel pipes (corrosion)
driving with SGI 4 tool
single-U
plastic tubes, with lost tool
silt/clay
The driven pipe remains in the ground and acts as outer wall of the heat exchanger. Because the pipe has to withstand the pressing-/ramming forces, the choice of materials is limited. Steel as heat exchanger material has good thermal properties, is cheap and common in construction work. A major drawback is the lacking resistivity to corrosion by groundwater and soil humidity, which makes active electric corrosion protection necessary. In tests in a groundwater well over two years (Sanner & Knoblich, 1991b), steel proved to be badly eroded after some decades, and the risk for leakage is high within few years (Tab. 4). The use of stainless steel, which shows very good anticorrosive properties, is limited due to the very high price. The values in Tab. 4 have been measured in ground water with low chloride content; chloride ions can initiate pit-hole corrosion even in stainless steel. Tab. 4:
Some results of In-Situ Corrosion Test in Schwalbach
Material
Mass loss per year (average)
extrapolated time for loss of half of the sample mass
Steel St37, tube
2.15 %
32 years
Copper, tube
1.74 %
40 years
0.0 %
---
Inox-steel, corrugated tube
To avoid problems with corrosion, some methods for using plastic pipes have been investigated. A summary of installation techniques is given in Bouma & Koppenol (1983). In Sweden, a method had been developed around 1990 (Engvall, 1986; Lehtmets, 1991), which also is shown in Fig. 7. A 32 mm (1¼") polyethylene pipe is pressed down into soft ground by using a guidance tool. Engvall (1986) reports tests with the first design of the tool. In clayey subsoil, in general a depth similar to that attainable with a standard penetrometer can be achieved. The data in Tab. 5 summarise the logs of these tests. Although the depth for direct installation of ground heat exchangers is limited to some 10 m, the easy, fast and cheap methods are well suited for cost-effective installation under certain 4
Swedish Geotechnical Institute, Linköping, Sweden
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conditions. The thickness of unconsolidated sediments must exceed at least 10 m, and no large boulders or coarse gravel should be in the sediment. In other places, drilling is unavoidable for heat exchanger installation. force reel with PE-pipe ditch ground
ground
installation tool of SGI, Sweden
ramming of steel tubes
Fig. 7:
Pressing and ramming in soft ground
Tab. 5:
Field tests for plastic pipe installation rig (after data from Engvall, 1986)
Location (Sweden) Kista
Depth [m] 0-1 1-10 10-14 14-20
Soil material dry crust clay silt, clayey silt
Consist. index Ic
Pressing force [kN]
Remarks
1.0 0.25-0.75 1.0 >1.0
18 9-18 18-22 22-26
pre-perforation
rig anchored
Upplands Väsby
0-1.5 1.5-5.5 5.5-11 11-14.5
dry crust clay clay clay
>1.0 0.5-0.75 1.0 >1.0
18 10-13 13-19 19-20
pre-perforation
Uppsala
0-1 1-2.5 2.5-8.5
dry crust org. clay org. clay
>1.0 0.75-1.0 >1.0
22 11 13-26
pre-perforation pre-perforation rig anchored
Drilling for borehole heat exchangers
In table 1 possible drilling and installation methods for vertical ground heat exchangers are listed. As can be seen from that table, for the same rock or soil types often more than one suitable method exists. Beside the auger method, where the material from inside the borehole is brought to the surface mechanically, all methods listed use flushing of the borehole. Water or compressed air (in DTH) is used to remove the cuttings in a constant flow. Problems can arise when inserting the heat exchanger pipes into the completed borehole. In stable rocks, with clear water as drilling fluid, the heat exchanger (filled with water) slides into the borehole under its own weight. Contrary to common perception, hard, stable rocks are very 63
well suited for BHEs, with fast DTH-drilling, now stability problems and easy installation. These rocks, in particular if they contain a good amount of quartz, also exhibit very good thermal conductivity. If the borehole is filled with thick mud, and if the mud cannot be flushed out in order to keep the hole stable, additional techniques have to be used to get the pipes into the hole. It is of little use to push hard on the part of the pipes still out of the hole, as this would result in bending the pipes, pushing them towards the borehole wall, and increase friction (not to mention the danger of damaging a plastic pipe). A force has to be applied to the bottom of the pipes: •
hanging a weight to the bottom part of the heat exchanger or tying one just above the bottom part
•
applying pressure to the bottom part by a steel rod running between the pipes and fitting into a receptable on the bottom part of the heat exchanger
•
applying pressure to the bottom part by using the drill pipes fitting to a rim around the bottom part of the heat exchanger
Helpful is a reel-like device to hold the pipes while inserting (fig. 8 and 9). This method today is standard in particular in Switzerland, where reels for 400 m deep double-U-tube-BHE and tremie pipe have been built in 2001. Reel with BHE pipes
Device to straighten the BHE pipes Ground
Fig. 8:
Device for BHE insertion (as suggested by Hess, 1987)
Special problems for drilling in large, dense BHE-fields
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.
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Fig. 9:
Reel with single-U-pipe in use for the Richard Stockton College wellfield (photo RSC)
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 particularily 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.
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Tab. 6:
Examples of large borehole fields
Name
Location
Central Valley School
Buxton ND, USA
120
64 m
7680 m
Lulevärme BTES
Luleå, 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
Fig. 11:
No. of BHE
Depth of BHE
Total length
Typical unilateral deviation of 19 m deep boreholes in rock with dipping ‘ layers; wall of quarry in Bulken, Norway (from Sinkala, 1987)
Grouting of borehole heat exchangers
A good thermal contact between the pipes and the ground is paramount for the performance of the BHE. In Scandinavia, in hard, crystalline rock the borehole often is just filled with water. With increasing temperature (e.g. for borehole heat storage), the water shows convection, and heat transport is increased, as measured during thermal response tests (Gehlin & Hellström, 2000). In most other cases, and in particular if water authorities demand plugging the hole for groundwater protection, the borehole annulus has to be filled („grouted“) with a suitable material. This grouting material has to be pumpable and is pressed through a tremie pipe to the bottom of the hole, from where it rises to the borehole mouth. Grouting from bottom to the top that way is the only method to guarantee perfect filling of the hole. Grouting procedures are dealt with in the German guideline VDI 4640, part 2 (VDI, 2001). Some data of grouts are given in table 7. In the end, the BHE looks like shown in fig. 12, and is ready to be laid in a connecting trench or hooked to a manifold.
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Since some time, thermally enhanced grouts have been used in USA to improve heat transfer. In 2000 also in Germany two brands of thermally enhanced grouts did hit the market. They use very fine-grained quartz or graphite to improve thermal conductivity. Reasonable ranges of thermal conductivity of the finished grout are λ = 1.6 - 2.0 W/m/K. Tab. 7:
Properties of BHE grouting materials (after Sanner, 1992, revised)
Material
thermal conductivity [W/m/K]
hydraulic conductivity
pumpability
impact due to freezing
sand, water saturated
1.7-2.5
good
-
-
sand, dry
0.3-0.6
good
-
-
clay
0.9-1.4
low
poor
exists
3
0.7
very low
good
high
bentonite with sand
1.4-1.8
very low
poor
medium
bentonite/cement
0.6-1.0
very low
good
low
therm. enhanced grout
1.6-2.0
very low
good
low
bentonite 1.3 g/cm
for comparison: air water
Fig. 12:
0.03 0.6
left: BHE field for a school in Northern New Jersey, with single-U-tuber; right: BHEs for a residential house in Germany, with excess BHE-length to be coupled without further connection to the manifold at the house
References
Andersson, O. (1981): Borrning och Dokumentation, Borrningsteknik jämte Metodik för Geologisk Datainsamling under Borrnings Gång. - 281 pp., Diss. Thesis 8, Dept. Quartern. Geol. Lund Univ., Lund Andersson, O., Mirza, C. & Sanner, B. (1997): Relevance of geology, hydrogeology and geotechnique for UTES. - Proc. MEGASTOCK 97, p. 241-246, Sapporo Armstead, H.C.H. (1983): Geothermal Energy. - 404 p., 2nd ed., E. & F. N. Spon, London Beck, M., Müller, J., Reuss, M., Schulz, H., & Wagner, B. (1993): Untersuchungen zur saisonalen Speicherung von Niedertemperaturwärme im Erdboden - Erd-Wärmespeicher. Landtechnik Weihenstephan, Landtechnik-Bericht 7, 443 p., Freising
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Bouma, J.W.J. & Koppenol, A.D. (1983): Investigation into a complete earth-to-water heat pump system in a single family dwelling focussing on the application of a vertical subsoil heat exchanger. - Report EUR 8077 BF, Brussels Cambefort, H. (1964). Bohrtechnik. - 420 p., Bauverlag, Wiesbaden Chugh, C.P. (1985): Manual of Drilling Technology. - 567 p., Balkema, Rotterdam Engvall, L. (1986): Energilagring i lera, ny metod för installation av värmeväxlarrör. - 17 p., SCBR R92:1986, Stockholm Garlick, E.A. (1986): Digging into the commercial market. - Engineered Systems, Jan/Feb 1986, p. 51-57, Troy Gehlin, S. & Hellström, G. (2000): Recent Status of In-Situ Thermal Response Tests for BTES Applications in Sweden. - Proc. TERRASTOCK 2000, pp. 159-164, Stuttgart Hess, K. (1987): Ground-Coupled Heat Pumps. - Proc. WS on GSHP Albany, Rep. HPC-WR2, pp. 209-217, Karlsruhe Hytti, P. (1987): Improved drilling performance with hydraulic top-hammer trackdrills. - Proc. DRILLEX 87, pp. 65-73, IMM, London Kruseman, G.P. & De Ridder, N.A. (1970): Analysis and evaluation of pumping test data. Bull. Int. Inst. Land Reclamation and Improvement, 11, 200 pp., Wageningen Lehtmets, M. (1991): Technical and economic basis for Swedish R&D in high temperature storage in ground and water. - Proc. 5th int. Conf. Energy Storage THERMASTOCK 91, pp. 4.6.1-4.6.6, NOVEM, Utrecht Reuss, M., Schulz, H. & Wagner, B. (1990): Solar assisted heat pump with duct storage in Donauwoerth. - Z. Angew. Geowiss. 9, pp. 79-91, Giessen Sanner, B. & Knoblich, K. (1991a): Advances in drilling and installation for vertical ground heat exchangers. - Proc. Workshop Ground Source Heat Pumps, Montreal, IEA Heat Pump Center Report HPC-WR-8, p. 105-116, Sittard Sanner, B. & Knoblich, K. (1991b): In-Situ Corrosion Tests for Ground Heat Exchanger Materials in Schwalbach GCHP Research Station. - Newsletter IEA Heat Pump Center 9/3, p. 27-29, Sittard Sanner, B. (1992): Erdgekoppelte Wärmepumpen, Geschichte, Systeme, Auslegung, Installation. - 328 S., Ber. IZW 2/92, Karlsruhe Schulz, R. & Jobmann, M. (1989): Hydrogeothermische Energiebilanz und Grundwasserhaushalt des Malmkarstes im süddeutschen Molassebecken. - 56 p., Ber. 105 040, NLfB, Hannover Schunnesson, H. (1985): Borrning av Värmelager. - Högskolan i Luleå, Teknisk Rapport 1985:24T, 61 p., Luleå Schunnesson, H. (1987): Longhole drilling with the top-hammer technique, its potential application in thermal heat storage. - Proc. DRILLEX 87, p. 151-160, IMM, London Sinkala, T. (1987): Rock and hole pattern influences on percussion hole deviations: a field study. - Proc. DRILLEX 87, p. 161-174, IMM, London VDI (2001): Thermal Use of the Underground - Ground Source Heat Pumps. - Guideline VDI 4640, part 2, Beuth Verlag, Berlin
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