Drilling Companies’ Classification System & Drilling Methods Used
Drilling methods range from simple digging with hand tools
to high-speed drilling with sophisticated equipment.
The most commonly used methods are briefly described below
for the general information of readers who do not have a drilling background.
See table below for information about drilling methods and their applications.
DRILLING METHODS AND THEIR APPLICATIONS
Cable tool drilling
Cable tool drilling, otherwise known as percussion drilling, is probably the oldest drilling method. Basically it involves lifting and dropping a string of solid steel drilling tools suspended from a wire rope to hit the bottom of the hole. This process drives the cutting bit, fracturing or pulverising the formation.
The crushed material forms a slurry on mixing with water that has been added to, or is naturally present in, the hole. The blow rate varies from 40–60 strokes per minute and, because of the characteristic left lay of the wire rope cable, the bit turns and strikes across a different section of the hole bottom at each blow.
When the bit can no longer fall freely through the water–cuttings mix, the drill tools are withdrawn from the hole. A tubular bailer, which is run on a separate smaller wire rope, is then used to pick up the slurry and cuttings and remove them from the hole before drilling is resumed.
In cable tool or percussion drilling there are basically three major operations:
Drilling the hole by chiselling or crushing the rock, clay, or other material by the impact of the drill bit
Removing the cuttings with a bailer as cuttings accumulate in the hole
Driving or forcing the bore casing down into the hole as the drilling proceeds.
Because of the relatively low initial cost and simplicity of equipment used, the cost per unit drilled is relatively low. However, the technique is slow, and when the increased cost of labour is taken into account, there is usually little advantage over faster rotary drilling methods when drilling new bores.
Cable tool drill plants are used extensively for reconditioning because they:
Are usually lighter than a rotary plant with an equivalent depth capacity
Are easier to establish over a borehole
Can also lower and retrieve tools to probe a bore more quickly than with a rotary plant
Are able to work inside casings
Are able to insert casing liners more quickly because of their better access around casing strings for screwing or welding a joint.
Auger drills are used mainly for soil investigation and for drilling in soils and very soft rock. The mechanical clearing of the hole eliminates any need for pumps or compressors. Types of augers include:
Continuous-flight augers, which can be driven by any top-drive rotary machine provided it has adequate torque rating and slow rotation.
When using continuous-flight augers in deep, small-diameter holes, the cuttings are supported by the hole and carried to the surface by rotation of the helical flights.
Hollow augers, which consist of a continuous-flight auger that has a hollow centre tube. They are normally used with a bit plug held in place by a secondary internal rod string, with the augers used to drill as with a conventional continuous-flight auger to the required depth. At that point the central bit plug and rod string are withdrawn, which permits the casing to be installed before the auger flights are removed.
Short-flight and plate augers, which are loaded with cuttings and then pulled out of the hole. At the surface the cuttings are ‘spun’ off the auger.
Bucket augers, where the cuttings are picked up in a bucket, hoisted to the surface, and dumped through the hinged bottom of the bucket. Extensions are added as the hole gets deeper.
Short-flight, plate, and bucket augers are used for shallow, large-diameter holes.
Rotary drilling techniques
The principle common to all rotary techniques is that a drill bit is attached to the end of a hollow drill pipe and rotated against the bottom of the hole with either a fracturing, digging, or scraping action, depending on the bit type and the nature of the formation.
Rotary drilling techniques are compared in Table above.
Rotary air drilling
The rotary air method is used to drill holes in consolidated or semi-hard formations
such as sandstone or shales that are self-supporting.
The cuttings that this process produces are cleared by circulating air, which is derived from a compressor and fed down the drill pipe to emerge through a bit. The recommended up-hole annular velocity must be maintained to remove cuttings effectively (see table below).
Compressor output, hole diameter, and drill pipe size should be matched
to provide the required velocities.
RECOMMENDED (OPTIMUM) UP-HOLE VELOCITIES 9UHV) (BAILING VELOCITIES)
Holes can be drilled using a large volume of air at high pressure.
However, the equipment usually used is limited in depth once below water level.
A major advantage of the rotary air drilling method is that water is blown to the surface as soon as the water-bearing stratum is encountered. This allows the driller to obtain a progressive indication of the available supply and to monitor any changes in the quality and quantity of water as the drilling progresses.
Air is used principally in consolidated formations. Foaming additives
are occasionally used to increase the up-hole carrying capacity of the return air.
Down-hole hammer method
The down-hole hammer method involves a pneumatically operated drill bit that efficiently combines the percussion action of cable tool drilling with the turning action of rotary drilling.
A pneumatic drill bit can be used on a standard rotary rig with a high-pressure air compressor of sufficient capacity. It is used for fast and economical drilling of medium to extremely hard formations. Fast penetration results from the blows transmitted directly to the bit by the air piston. Continuous hole cleaning exposes new formation
to the bit and practically no energy is wasted in re-drilling old cuttings.
Down-hole hammer drilling is generally the fastest method of penetrating hard rock. Foaming additives are occasionally used to increase the up-hole carrying capacity of the return air. Down-hole hammers are used for hard rock drilling and enable water
bores to be established from fractured hard rock aquifers.
This method is not effective for drilling loose, unconsolidated materials.
Reverse circulation drilling — air (dual tube rotary air and down-hole hammer)
For this drilling method, air is introduced through a dual swivel head on a top drive rotary rig and pumped down the annulus in the dual drill pipe to the bit or hammer being used. Cuttings are returned to the surface through the inner tube.
This method is commonly used for mineral sampling to obtain an uncontaminated strata sample. It is not a common method for water bores; however,
it is sometimes used for water sampling programs.
Large-diameter dual tube rotary air drill strings permit the insertion of up to 50 mm PVC-U casing through the inner tube for the construction of monitoring bores.
Rotary mud drilling
Rotary mud drilling uses drilling mud (mixes of water, bentonite clays, polymers, and additives) as the circulation medium. In the rotary mud system, drilling fluid or mud is pumped down through the drill pipe and out through nozzles in the bit.
As the bit penetrates the formation material, the drilling fluid circulates continuously and removes the cuttings. The fluid also serves to cool and lubricate the bit. The mud fluid then flows upwards in the annular space around the drill pipe to the surface, carrying the cuttings with it in suspension.
At the surface, the drilling fluid is conditioned before being recirculated down the hole. Properly conditioning the mud helps to prevent down-hole problems.
The basic fluid normally used for rotary drilling is water, to which specific chemicals and other additives can be added to increase the density or viscosity to improve hole support. The fluid can also be weighted to control artesian pressures.
The mud forms a membrane that inhibits flow through the walls of the hole, and the internal pressure of the mud provides structural support to the hole wall. Drilling fluids are also used for drilling deep bores that are beyond the capacity of air compressors.
The technique is useful for drilling operations in soft,
unconsolidated formations, deep bores, and pressure bores.
Reverse circulation drilling — mud
In the reverse circulation drilling method, instead of circulating the drilling fluid through and up the outside of the pipe, the process is reversed. Fluid is fed down through the space between the wall of the hole and the drill pipe where it is then pumped up, together with the cuttings, through the hollow part of the pipe and then out through a discharge pipe.
Of particular importance is the possible use of a light (nearly clear) drilling fluid for large diameter holes, rather than a viscous and heavy drilling mud (as used in conventional rotary mud drilling), which sometimes tends to seal-off water-bearing formations. However, a substantial quantity of fluid must be present to maintain an open hole.
This method is used for rapid drilling of large-diameter holes
in soft formations where gravels are encountered.
It is possible to bring gravel to the surface through the hollow drill pipe because of the extremely high velocity of the fluid as it is drawn up by the suction pump. The walls of the hole are held in place by the pressure of the fluid against the sides of the hole.
Sonic drilling is a relatively new technique, where a high-frequency vibration is combined with rotation to advance the drill stem. The core barrel is retrieved and the sample vibrated into a plastic sleeve or core trays. This technique is relatively continuous and undisturbed geological samples are obtained without the use of drilling fluids or other potential contaminants.
This technique is best suited to drilling unconsolidated formations,
but its depth capability is somewhat limited by current technology.
Choice of Drilling Method
Each of the common drilling methods has its advantages and disadvantages (see Table 4.1 for guidance). The choice of drilling method should be based on the
expected geological conditions and the type of bore to be constructed.
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