Dual Rotary Drilling a Concept
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Cable Tool (Percussion/Drilling and Driving)
Percussion drilling is still used effectively in many parts of the world. Cable tool drills are also commonly known as ‘churn’ drills or ‘spudders’. A weighted string of drilling tools attached to hoisting cable is repeatedly lifted and dropped. The force of the impact crushes the material at the bottom of the borehole. Periodically, the crushed material is cleared using a bailer.
When drilling in unconsolidated formations with a cable tool drill, a casing is often advanced to keep the hole open. The casing is advanced via repetitive percussive blows on top of the casing. Typically the cable tool can only perform one function at a time and the driller alternates between drilling the hole and advancing the casing, switching at various intervals.
– Low capital investment
– Low maintenance and operational costs
– Minimal cross-contamination
– Water is the only media required for cuttings removal
– Large diameter holes can be drilled
– Drilling is slow in hard formations
– Boulders are difficult to drive casing through, often requiring the use of dynamite
– Casing penetration rates decrease with depth in a given formation
– Casing retrieval is slow
– Noise and vibration can be significant and of special concern when drilling in populated
areas or near sensitive structures, and can have longer term negative impact on
operator hearing
– Shortage of experienced cable tool drillers

Rotary Drilling (Open-Hole Method with ‘Mud’)
It is possible, in some situations, to keep the hole open while drilling without the benefit of casing. Rotary Drilling with Bentonites (Mud) Bentonites and synthetic stabilizers are mixed with water and circulated in the borehole. The resultant fluid, commonly referred to as mud or drilling mud, is used to cake and stabilize the borehole wall. The mass of the fluid also provides pressure in the hole, which helps to keep it open.
The drilling fluid is circulated down the hole through the drill pipe, where it exits through ports in the bit. The fluid (mud) flushes the cuttings away from the face of the bit and carries them up the annulus to the surface. Reverse circulation with mud is also possible. In either case, once the mud reaches the surface, it feeds into a settling tank where the cuttings are separated from the mud before it is circulated down the hole again.
– Hole penetration is very fast is some clay, sand and shale formations
– No temporary casing is required
– Fluid pressure in the hole can help control heaving sands
– Low horsepower requirement
– Requires mud mixing equipment and dug pits or metal tanks for circulation
– Requires significant amounts of water on location to mix initially and maintain circulation
– Requires a fundamental knowledge of bentonites and additives needed to achieve adequate
penetration rates and stabilize formations
– More difficult to identify water bearing zones, especially in low flow formations
– Loss circulation zones can cause aquifer contamination and dramatically increase
bentonite costs
– Mud may plug the aquifer and cause decreased production
– Driller still bears the risk of hole collapse or swell, resulting in possible loss of drill string
or jamming of casing during installation
– Disposal of mud after hole is completed can be inconvenient and costly
– Freezing temperatures make working with mud more difficult

Air Rotary Drilling with use of Foams and Polymers
This method involves rotary drilling with the injection of stabilizing polymers into the air to seal off
potential ground hazards that would disrupt the stability of the hole. The foam also helps lift the
cuttings, allowing proper evacuation at lower uphole velocities.
– Hole penetration is very quick in suitable formations
– Inexpensive under appropriate conditions
– Simple set-up
– Limited to relatively stable formations