Down-the-Hole Drilling Method Explained

Down-the-hole (DTH) drilling has made it easier for contractors to drill wells faster and more efficiently, and to transition from dirt boring to rock boring just by adding a compressor and hammer to the drill bit.

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Down-the-hole (DTH) drilling has made it easier for contractors to drill wells faster and more efficiently, and to transition from dirt boring to rock boring just by adding a compressor and hammer to the drill bit.

Usually, contractors working on a dirt bore would pack up and move on to the next dirt bore when they encountered rock, leaving the rock boring for another contractor to complete.

With DTH drilling, contractors can change the drill that they were using for the current directional bore to a rock drill when they encounter rock by connecting the drill with a compressor and a steerable hammer.

Well drilling also needed changes to be made to rotary drilling when the well needed to take a turn.

A Brief History of Down-The-Hole Drilling

DTH technique was used in well drilling exclusively to dig, accurate, vertical bores, and rotary drilling was used to turn the hole. The Directional DTH system, patented by Atlas Copco, was first used successfully in by drilling crews in the Marcellus Shale region of the United States to steer percussive drilling. Though the technology already existed, Atlas Copco succeeded in overcoming engineering challenges by modifying the design, such as:

• Shortening the directional motors bit box to accommodate the hammer between the motor and the bit.
• Adjustment to the blow-down sequence of the hammer to prevent its over-rotation when lifted off the bottom.
• Jet subs were placed along the drill string that re-directed airflow away from the bottom-hole-assembly (BTH) to enable chip removal up the annulus.
• Introduction of an hydro-cyclone to manage fluids that control the integrity of the hole.

Drilling in the gas plays of Appalachia in is a classic example where DTH pneumatic percussion drilling tools reduced the time and cost of a project in half. The original estimate for completion of a 12,000-foot hole was 30 days using the conventional rotary technique, which was reduced to 13 days using the DTH method.

Before DTH was used, drillers would typically case the vertical portion of the well and convert to rotary drilling to drill at a tangent. With DTH, it was now possible to turn 2-4 degrees curves and drill the well&#;s tangent up to 30 degrees or greater averaging 300 feet per hour, where rotary drilling would progress 30-40 feet per hour.

DTH Equipment & Method Explained

The DTH system is easy and quick to adapt to a directional drill and is considered one of the best and most effective methods to drill high-quality holes that are stable and straight. It can be done on both hard and soft rock and is extensively used in the construction, oil and gas, and water well industries.

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DTH equipment consists of a drilling hammer and a piston-powered by compressed air. As the drill string rotates, the drilling hammer strikes down on the rock. The drill bit receives its striking power from a piston inside the hammer that is powered by compressed air.

This action along with the rotational movement of the drill string crushes the rock efficiently. Since the piston strikes directly on the bit, energy transfer takes place down the hole with minimum loss of energy, allowing drilling to greater depths.

The driving medium i.e. compressed air is also the flushing medium. The flushing medium (in some cases, water is also used) is pressed down through the drill pipes, down-the-hole hammer and the drill bit. It is then forced back out of the borehole along with the cuttings through the annular gap between the drill pipe and the borehole.

Benefits of Down-The-Hole Drilling

Below are few of the many benefits associated with DTH drilling:

• The DTH system is easily adaptable to most directional drills in a short period of time.
• Lower operating costs, as the same drill will be used for drilling through dirt and rock.
• The use of air makes this method very cost-effective as air does not have to be sourced, transported or treated.
• Less amount of drilling fluid and additives are required.
• Less weight-on-bit allows for lighter rigs that are easier to set up and takedown.
• Since cutting force is supplied by reciprocal force, the bit encounters lesser deflection and therefore drills more precisely.
• Rate of penetration of DTH is 2-5 times greater than the rotary drilling technique.
• Lesser time consumed compared to when the contractor has to be changed when geology changes.
• Less wear and tear on the drill bit during drilling operation as the hammer is doing most of the work.
• DTH requires a minimal amount of rotational torque to cut rock that also allows the hammer to drill straight.
• Vibrations from the DTH percussive hammer can stimulate the formation to enhance production capability, reducing the need for hydraulic fracturing.

What We've Learned

The DTH method is the most productive and cost-effective method. It is environmentally friendly, as it produces lesser noise and vibration compared to other drilling methods making it ideal for cities.

It also leaves a lesser carbon footprint as it uses low amounts of drilling mud and additives, reducing the amount of drilling fluid that needs to be recycled and disposed of, in turn reducing the number of trips required for disposal of used mud.

This technology will help contractors do more in lesser time at lesser cost and help keep ahead in a very competitive market.

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