A Comparison of Environmental Drilling and Other Shallow Drilling Technologies: Fitness for Purpose and Project Considerations

 

     There are now several different technologies for environmental drilling, geotechnical drilling, water wells, and shallow geothermal wells, and each drilling method has strengths and weaknesses that make it suitable or unsuitable for different individual projects. Depending on the parameters and goals of the project there will likely be a preferred drilling tech to be employed. Environmental drilling includes the installation of groundwater monitoring wells, boring for soil and/or rock samples, and shallow drilling for soil vapor probing. Groundwater wells, geotechnical bores, and drilling wells for direct use or ground source heat pump geothermal are typically shallow – below about 700ft and usually much lower than that.

Solid Stem Auger

     Solid stem auger drilling uses a claw bit to break the ground and move the auger flights into the ground. The technique works best in clay soils. No circulation fluids are required which makes site cleanup simpler. No casing materials are required in stable formations. Sampling in formations with a semi-consolidated soil is easy. Depth range is down to 400ft or more in semi-consolidated soils. It is inefficient in loose, sandy soil. It does not perform well below the water table after there is water infiltration into the hole. Maximum bore diameter is 24 inches. Solid stem auger drilling is used for shallow soil vapor probes.

Hollow Stem Auger

     Hollow stem auger drilling is similar to solid stem, using a similar claw bit to cut new hole. The main difference is that the stem is hollow rather than solid. The hollow stem serves as a temporary casing. Quality samples can be obtained, including in unconsolidated soils. It can drill rapidly into soild with high clay content. It has problems with cobble stones and boulders. Water infiltration in sandy unconsolidated soils renders sampling difficult. In these cases, a split spoon sampler can be utilized where the casing is split into two halves that can be taken apart to reveal a core of the soil. Depth is limited to 150ft or less. 

  

Roto-Sonic

     Roto-Sonic, aka. Sonic drilling is expensive but is being used more and more. It is used for some water well drilling and increasingly for shallow geothermal drilling. It has a bigger footprint but generates very little waste so if its footprint can be accommodated it can be advantageous. It is very good for collecting quality samples. An article at Geo Drilling International’s website describes roto-sonic drilling:

“Sonic is an advanced form of drilling which employs the use of high-frequency, resonant energy generated inside the Sonic head to advance a core barrel or casing into subsurface formations.

During drilling, the resonant energy is transferred down the drill string to the bit face at various sonic frequencies.

Simultaneously rotating the drill string evenly distributes the energy and impact at the bit face.”

Air-Rotary Casing Hammer

The air-rotary casing hammer (ARCH) drilling method combines air-rotary drilling with the pile driver technique where casing is hammered into the subsurface. There is no secondary waste stream generated by the drilling muds as in mud rotary drilling. This method is very suitable for alluvial formations of unconsolidated sediment. It is a cased-hole method so is good for sampling and hole integrity.

Air Percussion

     Air Percussion drilling, or air drilling, is a technique that uses compressed air instead of fluids to cool the bit and bring cuttings to the surface. It is extensively used in oil and gas drilling where there is not enough formation water to affect drilling. Variations include dust-drilling where holes are dry. Where there is some water mist-drilling may be used which employs water and soap to clean the hole. In foam-drilling the use of surfactants (soaps) is increased. The use of compressed air and fluid together is called aerating. Nitrogen can also be used as the compressed air. The method becomes unsuitable when sufficient formation water enters the hole. Rate of penetration (ROP) decreases as more fluid enters the hole.

Air Drilling Rate of Penetration (ROP). Source: PetroWiki

Mud Rotary

     Mud rotary drilling is preferred for “loose” formations and where wellbore integrity is problematic. The drilling mud can make a “mudcake” that coats the borehole. Thus, hole caving is minimized with this method. It is the preferred method for unconsolidated sediment. Mud rotary is used extensively in oil and gas drilling and in environmental drilling such as groundwater monitoring wells. Drawbacks of mud rotary drilling are cost and footprint. Additional space and cost is required to support vehicles bringing in drilling fluid (mud), storing it onsite, and collecting it after drilling.

 

References

Solid Stem Augers Vs. Hollow Stem Augers in Environmental Drilling. Talon LPE. March 20, 2014. Solid Stem Augers Vs. Hollow Stem Augers In Environmental Drilling (talonlpe.com)

What’s the Best Environmental Drilling Technology for my Project? Greg Cranham. Hargis + Associates. What’s the Best Environmental Drilling Technology for My Project? | Hargis + Associates, Inc.

How sonic drilling works. Sonic Drilling - An advanced form of drilling employing the use of high-frequency, resonant energy. Boart Longyear. March 19, 2019. GeoDrilling International. How sonic drilling works - GeoDrillingInternational

Drilling 103: An Introduction to Air Rotary Casing Hammer (ARCH) Drilling. Cascade Environmental. June 18, 2019. Drilling 103: An Introduction to Air Rotary Casing Hammer (ARCH) Drilling (cascade-env.com)

Air Drilling. PetroWiki. Air drilling - PetroWiki (spe.org)

Geotechnical Drilling Techniques. Central Geotech. Geotechnical Drilling Techniques︱Central Geotechnical Services