| Use: | Direct air rotary is frequently used when monitoring wells must be installed in consolidated material. Direct air rotary drilling should be given strong consideration for all situations involving consolidated rock and large diameter wells deeper than 15 feet in unsaturated, unconsolidated material where contamination by drilling fluid is not allowed. |
Description:
Air rotary drilling differs from the cable tool method in that it relies on a sharp bit to literally drill through earth and rock layers. A system of cables engines, support mechanisms, lubricating devices, and pulleys control the rotation of the bit below the surface, keep the bit lubricated, and bring debris out of the borehole.
The basic rig setup for air rotary with a tri-cone or roller-cone bit is similar to direct mud rotary, except the circulation medium is air instead of water or drilling mud. Compressed air is circulated down through the drill rods to cool the bit and then carries cuttings backup the borehole (minimum 6-inch diameter hole required) to the surface. A cyclone separator is used to slow the air velocity and allow the cuttings to fall into a container. Several different bits might be used on a single borehole when drilling deep through different layers of rock. A larger diameter bit is generally used at the start of the drilling process with progressively smaller bits used to finish the hole. Use of a downhole hammer in place of a roller-cone bit provides better penetration in hard geologic formations.
Analytes:
| 2. Non-Halogenated SVOCs | 6. Pesticides/Herbicides | 10. Explosives |
| 4. Halogenated SVOCs | 7. Metals | 11. TPHs |
| 5. PAHs | 8. Radionuclides |
Media:
| Soil: | Ground Water: | Surface Water: | Gas/Air: |
| MINIMUM | MODERATE | Not Applicable | MAXIMUM |
No drilling fluid is used, minimizing contamination of formation water. Well suited for highly fractured or cavernous rock because loss of drilling fluid is not a problem. Oil contamination might result from the air compressor if air filters are not operating properly. In dry formations, the cuttings are very fine-grained and a small amount of water and/or foaming surfactant can be added to increase the size of fragments discharged to the surface, allowing good characterization of the surface. However, surfactant foams might react with formation water and affect representativeness of ground water samples. Air can modify chemical and biological conditions in an aquifer.
Major water-bearing zones can be identified when formation water is blown out of the hole along with cuttings. Yields of strong water-producing zones can be estimated with relatively short interruption of drilling. Field analysis of water blown from the hole can provide information on some basic water quality parameters.
| Maximum Depth: | >1000 feet. Depth is limited only by the capacity of the air compressor to deliver enough air down the hole to maintain circulation. |
| Production Rate: | Sample is available after a short amount of time. Drilling is fast and can be used for both consolidated and unconsolidated formations, but is best suited for consolidated rock. |
| Investigation Derived Waste Volume: | Large volume of waste. |
| Technology Status: | Commercially available and routinely used field technology. |
| Certification/Verification: | Technology has not participated in CalEPA certification and/or CSCT verification program. |
| Relative Cost per Sample: | Mid-range expense. Might not be economical for small jobs. The decision to change bits takes time and requires extra equipment to remove the entire drill pipe and bit, and then to reassemble it using a different bit. |
Limitations:
ASTM Standards:
| D 5782- 95 | Guide for Use of Direct Air-Rotary Drilling for Geoenvironmental Exploration and Installation of Subsurface Water-Quality Monitoring Devices. |
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