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).

Borehole Diameter

Borehole diameter [inches]

Fig. 2: Applicable drilling methods (revised after Hytti, 1987)

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 4Mhours.

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;

Cambefort (1964) recommends 700-900 kg/in in soft and 1400-1800 kg/in in hard rock. For a standard 115 mm (4 V") 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 rotary

sometimes temporary casing required temporary casing or mud additives required

soil, silty/clayey

auger rotary

mostly best choice temporary casing or mud additives required

rock, medium hard

rotary DTH 1

roller bit, sometimes mud additives required large compressor required

DTH 2 top hammer

with rock bit or hard-metal insert button bit, very slow large compressor required special equipment, depth range ca. 70 m

rock under overburden

ODEX 2 or similar

in combination with DTH

Drilling Rig Bit Diagram

rock bit (left) and tungsten-carbid button bit (right), from Chugh (1985)

rock bit (left) and tungsten-carbid button bit (right), from Chugh (1985)

1 Down-the-Hole-Hammer

Overburden Drilling Equipment; ODEX is a trademark of Atlas Copco

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

possible problems with BHE installation

foam generators

facilitates flushing out of cuttings

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

C = 112,000 . D . e(0,01 . D) German geothermal wells with C: Total cost for drilling in US-$

D: 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 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.

Roller Bit Drilling Action Diagram
  1. 4: Auger principle (left, Atlas Copco) and light auger mounted on agricultural tractor (photo from Beck et al., 1993)
  2. 4: Auger principle (left, Atlas Copco) and light auger mounted on agricultural tractor (photo from Beck et al., 1993)
Tractor Mounted Augers

Fig. 5: Drilling rig optimised for BHE installation, mounted on all-terrain vehicle

(Unimog)

Fig. 5: Drilling rig optimised for BHE installation, mounted on all-terrain vehicle

(Unimog)

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).

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Responses

  • bill cosby
    drilling is how you get the energy from the ground... to our homes and cars you see theo.. rudyyyyyy
    7 years ago
  • temesgen
    What methods may be used to stabilize a rotary mud hole during drilling?
    6 years ago
  • giulio
    What is the cost per metre for 4" dth hole?
    5 years ago
  • Kelsi
    What are the basic offshore drilling techniques?
    5 years ago
  • stephen
    How you can choice drilling methods?
    2 years ago
  • yolanda
    What are two basic drilling methoda?
    1 year ago

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