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When drilling geothermal/geoexchange wellfields/borefields, it is important to pick a methodology that most closely matches the needs of the project and site. The two most common types of drilling are air hammer and rotary drilling. But what is the difference? And how can you be sure which method is right for your site application? Read below to find out.
Air Hammer Drilling
Air hammer drilling, otherwise known as pneumatic percussion drilling, is a technique whereby a compressed gas (typically air or nitrogen) is utilized to cool the drill bit and lift cuttings out of the wellbore, instead of the more conventional use of liquids. In Air Hammer Drilling, compressed air is pumped through the drill pipe to an air hammer bit in the borehole, allowing the pneumatic bit to strike the rock very rapidly. During drilling, the pipe string is rotated by the drilling rig to aid in keeping the borehole straight. Compressed air that escapes at the bottom of the air hammer carries the pulverized cuttings to the surface. Air hammer drilling is very rapid in even the hardest materials.
Pros:
- Efficiency in hard rock: Air hammer drilling is more effective in hard rock formations, providing better penetration rates.
- Faster drilling: Air hammer drilling allows for faster penetration rates, especially in hard rock formations.
- More environmentally friendly: Air hammer requires less water, reducing the environmental footprint of the project.
- Cost effective: Because of its higher drilling efficiency, and lack of a liquid medium, air hammer drilling is often much less expensive than other methods of drilling.
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Simpler equipment: Uses a simpler drilling rig that is easier to operate (no mud pumps or circulation system).
- Straighter bore: Another significant advantage of air hammer drilling is less vertical well deviation.
Cons:
- Limited depth: Air hammer drilling is less suitable for drilling deeper wells compared to rotary drilling, as longer rods will lead to larger energy loss and lower efficiency.
- Less precise: Can result in a less consistent hole diameter and therefore potentially weaker wellbore.
- Noisier operation: Generates significantly more noise pollution compared to rotary drilling.
- Groundwater Management: Does not manage encountered ground water well decreasing productivity and adding substantial cost to handle water discharge.
Rotary Drilling
Rotary drilling consists of a pipe or stem coupled to a drill bit that rotates and cuts through soil. This form of drilling uses drilling fluids to clear out the borehole and cool the drill bit for optimal operation. The cuttings produced from the rotation of the drilling bit are transported to the surface by drilling fluids which generally consist of water, drilling mud, or another medium. The fluid is forced down through the drill pipe and out through the bottom of the drilling bit. Then, the cuttings are lifted to the surface between the borehole wall and the drill pipe. With air rotary drilling, the fluid provides a hydrostatic pressure that reduces the risk of borehole collapse. When considering this method, it is important to evaluate the potential for contamination when fluids and/or air are introduced into the borehole.
Pros:
- Versatility: Rotary drilling can cut through a wider variety of geological formations. Various rotary technologies are available, each with unique advantages based on geology, site conditions, and scope of work. Rotary methods effectively advance through both hard and soft lithology.
- Better control: This drilling method allows for a more precise hole diameter and wellbore integrity.
- Depth capability: Rotary drilling is often suitable for drilling deeper wells, reaching greater depths.
- Quieter operation: Less noise pollution compared to air hammer drilling.
- Ground Water Management: The hydrostatic pressure better contains ground water discharge making it a more efficient drilling method when groundwater exists.
Cons:
- Slower drilling: Rotary drilling generally has slower penetration rates than air hammer drilling.
- Requires more drilling fluid: Can lead to higher water usage and a higher environmental footprint. Due to the introduction of the various circulating fluids, the use of rotary methods requires that the potential for contamination by these fluids be evaluated. The slag flushing medium can be compressed air, air foam, mud, etc.
- More complex equipment: Requires a more complex drilling rig, mud pumps, and a mud circulation system.
- Limited in hard rock: Inefficient for hard rock drilling.
- Lower drilling straightness
In summary:
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Choose air hammer drilling for faster penetration rates, reduced auxiliary equipment requirements, and in hard rock formations.
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Choose rotary drilling for versatility, precise control, noise reduction, in variable soil and rock formations, and when likely to encounter groundwater.
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