Drilling wells

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Detail of a drill head for pile foundations
Historic well drilling machine on Farm Dabis on the edge of the Namib

A drilled well is a well that is created with the help of a shallow hole . In the case of a cased borehole , steel pipes secure the borehole wall ; in the case of an uncased borehole , this is done by a supporting fluid, for example a bentonite suspension .

Drilled wells are used for general subsoil investigations , for groundwater level monitoring , for closed dewatering for construction pits and for drinking water extraction, for which, however, deep wells are often drilled.

Cased drilling methods

Cased drilling methods are of greatest importance in the production of boreholes, as they are easy to implement and the construction costs are lower than with uncased boreholes. The drilling diameters range from 25 cm to 2 m.

Dry drilling

Large drilling rig with drill pipes

No support fluid is used in this drilling process, the drilling is carried out dry. This method is often used to pre-drill a deeper hole and set a standpipe . A drilling rig brings a steel pipe into the ground by rotating movements or vibrations. You can then begin with the core clearing , i.e. removing the drillings from the pipe. The following tools are suitable for this:

  • Impact and free fall tools
  • Rotating dry drilling tools (driven by rods )
    • Core tubes
    • Drilling bucket
    • Spindle (colloquial: worm)
    • Schappe

While the valve drill can only be used in the water (i.e. below the water level in the pipe), the other impact and free-fall tools are also used above the water table, i.e. in dry soil.

In cohesive soils , such as loam , clay or silt , it makes more sense to use rotating drilling tools.

Rope core drilling

The rope core drilling is a flushing hole for the continuous extraction of cored samples in solid rock. The rope core tube is a special form of the double core tube in which the inner tube (with the drill core) is retrieved from the borehole with the help of a catcher on the rope. The cuttings (core) are then placed in a core box and handed over to the foundation laboratory . This drilling tool is used in subsoil exploration and in the exploration of deposits. A flushing liquid is used in this type of drilling. Drilling is carried out with a special rod that also serves as piping. The drill bits of the cable core tube are very thick-walled and set with hard metal or diamonds .

Uncased drilling methods

Uncased boreholes require a supporting fluid that reliably absorbs the existing earth pressure and thus prevents the wall of the borehole from collapsing.

Smaller well bores are still carried out manually with gravel pumps or, less often, with pocket drills .

Down-the-hole hammer drilling

Down-the-hole hammer before installation

Down-the-hole hammer drilling is a flush drilling method without core extraction that is used for geothermal probe drilling and well drilling in solid rock . The flushing medium (flushing) in this case is compressed air, which is generated by a compressor, the performance of which must be very high with increasing depth and diameter. The air is conveyed through the rod to the hammer, the rock is thus shattered by this with a high number of blows (600–700 blows per minute) and the cuttings are then transported to the surface in the annulus.

A small compressor is sufficient for a geothermal probe, as this can be max. 150 m deep are drilled with a diameter of approx. 150–200 mm. The down the hole hammer with the largest diameter has a diameter of 800 mm, but this is only used for drilling wells. For this diameter you need at least 3 large compressors that can generate a total of around 80 bar air pressure.

Flushing hole

Flushing a well for water extraction

There are two methods of flush drilling, direct and indirect flush drilling. The main feature of a flush well is that the borehole is only cased for the first few meters and the remainder of the borehole is held by the flushing fluid that circulates and creates overpressure. The cuttings are continuously conveyed with the aid of the mud. The flush can be supplemented with special flush additives such as bentonite or carboxymethyl cellulose (CMC). Bentonite supports the borehole wall and produces what is known as the filter cake at the edge of the borehole, which prevents the water from flowing into the ground. Furthermore, the bentonite ensures that when the drilling comes to a standstill (for example to readjust the rods) the cuttings do not fall back onto the bottom of the borehole, but that the mud with the cuttings forms a jelly-like mass. As a protective colloid, CMC ensures that the bentonite particles cannot accumulate any more water and rise more easily. In this way, CMC helps the flush to have a higher load-bearing capacity.

In flush drilling, stabilizers and drill collars are used as tools, as well as various chisels , etc. a. Three-wing chisel, four-wing chisel, eccentric chisel, roller and stepped roller chisel.

With direct flush drilling (also called right flush drilling), the mud is pumped through the rods and the cuttings are conveyed upwards in the annular space of the borehole. The decisive point here is the lift speed in the annulus. In other words, the speed with which the cuttings are transported upwards by the drill bit . In practice, values ​​between 0.3 and 1 m / s are considered necessary here. Since these speeds cannot always be achieved, it may be necessary to switch to left-hand flush drilling (suction, air-lift drilling).

The mud drainage goes to a sink where the cuttings can settle on the ground and the mud fluid is pumped back into the borehole.

In the case of indirect flush drilling (or left flush drilling), on the other hand, there are three different ways of drilling. In all three drilling methods, the cuttings are conveyed through the rods.

Airlift drilling

The classic method, based on the principle of the mammoth pump , is airlift drilling. You also need a compressor for this. In airlift drilling, the fluid is allowed to run into the outer annulus of the borehole (between the rod and the borehole wall). Inside, mostly double-walled rods lead air far down, so that the density inside the rods is reduced. An air inlet is provided in a linkage. This process is based on the density difference in communicating pipes: so much air must be introduced that the three-phase flow in the rods (mud, cuttings and air) has a lower density than the cleaned mud entering the outer annulus. The flush then runs from the outside in and conveys the material upwards. The air does not have to be led to the chisel.

Suction drilling

A pump that can suck in liquid, such as a piston pump, is used for suction drilling . More often, however, a centrifugal pump is used in conjunction with a non-return valve . The cycle is similar to that of airlift drilling, except that suction drilling does not require a double-walled rod or compressor.

Suction jet drilling

Suction jet drilling is technically possible, but is used very rarely. The principle is that the cleaned rinsing liquid is pumped through a double-walled rod to the nozzle at the lower end. The flow rate of the flushing liquid must be high enough to convey the cuttings loosened from the soil upwards. This results in poor efficiency and great energy consumption. Suction jet drilling (counterflash) would only come into play if the use of air is not possible because, for example, the rock drilled should not have any contact with oxygen, as is sometimes the case with exploration drilling for mining.

Unauthorized drilling

Iberian Peninsula

In drought-plagued Spain, in search of aquifers, boreholes are sometimes drilled without permits. In January 2019, Greenpeace published an estimate that there should be over a million such holes. In the same month of January, a two-year-old boy was killed in an accident when he fell into such an (unsecured) borehole near Totalán . In the Spanish vernacular, such a hole is also called pozo lunero .

See also


Individual evidence

  1. Coordinates of the Dabis farm: 25 ° 46'20 "S  016 ° 48'15" E
  2. Is Julen a victim of the "moonlight holes"? Totalán death shaft. In: Spiegel Online . January 26, 2019, accessed January 28, 2019 .
  3. ^ Julen y la trampa de los pozos luneros. In: El Mundo . January 21, 2019, accessed January 28, 2019 (Spanish).