The reverse-air rotary drilling method operates by the same general principles as direct-mud rotary, except that compressed air is pumped down the drill rods and returns with the drill cuttings up through the annulus. The reverse-air rotary method is best suited to drilling in relatively stable to consolidated formations. Casing sometimes is used to prevent caving in poorly consolidated formations.
Reverse-air rotary drilling is a very fast and efficient means of drilling. Rigs that are properly equipped and staffed can drill several hundred feet of hole per day. The reverse-air rotary method can reach to several thousand feet in depth and create hole diameters up to approximately 17 inches. Reverse-air rotary rigs are unrestrained by karst (cavernous) terrain.
Sediment sampling is supported both in poorly lithified materials (by split-barrel samplers) and in consolidated rock (by coring). Reverse-air rotary drilling supports the telescoping of casings to successively smaller sizes to isolate drilled intervals and protect lower geologic units from contamination by previously drilled contaminated upper sediments. Reverse-air rotary rigs sometimes are fitted with a casing driver to overcome borehole instability problems in unconsolidated sediments. When so equipped, reverse-air rotary rigs minimize the potential for interaquifer contamination.
Reverse-air rotary drilling can present some disadvantages. In contaminated formations, the use of high-pressure air may pose a significant hazard to the drill crew due to rapid transport of contaminated material up the borehole during drilling. Large volumes of hazardous gases may be discharged at the surface, posing an immediate hazard to the drill crew and others in the vicinity. Introduction of air to ground water could interfere with chemical analyses primarily by oxidation and by vigorous agitation and mixing.
Concentrations of volatile contaminants are very likely to be reduced in ground water adjacent to holes drilled using the reverse-air rotary method. The air discharged from air compressors normally contains finely atomized lubricating oil. To help prevent this oil from contaminating monitoring well drill holes, compressor discharge filters must be installed – and maintained during regular intervals – on rigs used to drill monitoring wells. Air-discharge samples should be collected as reference samples for future comparison where hydrocarbon contamination is being studied. These samples are a necessity in applications where lubrication of down-the-hole hammers or other tools is essential. The use of foam additives to aid cuttings removal also can introduce organic contaminants into the monitoring system. These should be avoided, but where necessary, samples of the foaming agent must be taken as reference samples.
Cuttings above the water table usually are very fine and hard to interpret. Also, the drying effect of the air in the annulus may reduce or eliminate any natural moisture in the cuttings, thereby masking low-yield water producing zones. Conversely, when high-yield aquifers are encountered, large volumes of water may be produced during drilling – a definite disadvantage if the water is contaminated and requires special handling and disposal.
When reverse-air rotary methods are used, hole diameters should be 3 inches to 5 inches larger than the outer diameter of the well casings to allow effective placement of filter and sealing materials. Two-inch diameter monitoring wells should therefore be installed within 5.5-inch diameter or larger holes.
ND
Reverse-air rotary drilling is a very fast and efficient means of drilling. Rigs that are properly equipped and staffed can drill several hundred feet of hole per day. The reverse-air rotary method can reach to several thousand feet in depth and create hole diameters up to approximately 17 inches. Reverse-air rotary rigs are unrestrained by karst (cavernous) terrain.
Sediment sampling is supported both in poorly lithified materials (by split-barrel samplers) and in consolidated rock (by coring). Reverse-air rotary drilling supports the telescoping of casings to successively smaller sizes to isolate drilled intervals and protect lower geologic units from contamination by previously drilled contaminated upper sediments. Reverse-air rotary rigs sometimes are fitted with a casing driver to overcome borehole instability problems in unconsolidated sediments. When so equipped, reverse-air rotary rigs minimize the potential for interaquifer contamination.
Reverse-air rotary drilling can present some disadvantages. In contaminated formations, the use of high-pressure air may pose a significant hazard to the drill crew due to rapid transport of contaminated material up the borehole during drilling. Large volumes of hazardous gases may be discharged at the surface, posing an immediate hazard to the drill crew and others in the vicinity. Introduction of air to ground water could interfere with chemical analyses primarily by oxidation and by vigorous agitation and mixing.
Concentrations of volatile contaminants are very likely to be reduced in ground water adjacent to holes drilled using the reverse-air rotary method. The air discharged from air compressors normally contains finely atomized lubricating oil. To help prevent this oil from contaminating monitoring well drill holes, compressor discharge filters must be installed – and maintained during regular intervals – on rigs used to drill monitoring wells. Air-discharge samples should be collected as reference samples for future comparison where hydrocarbon contamination is being studied. These samples are a necessity in applications where lubrication of down-the-hole hammers or other tools is essential. The use of foam additives to aid cuttings removal also can introduce organic contaminants into the monitoring system. These should be avoided, but where necessary, samples of the foaming agent must be taken as reference samples.
Cuttings above the water table usually are very fine and hard to interpret. Also, the drying effect of the air in the annulus may reduce or eliminate any natural moisture in the cuttings, thereby masking low-yield water producing zones. Conversely, when high-yield aquifers are encountered, large volumes of water may be produced during drilling – a definite disadvantage if the water is contaminated and requires special handling and disposal.
When reverse-air rotary methods are used, hole diameters should be 3 inches to 5 inches larger than the outer diameter of the well casings to allow effective placement of filter and sealing materials. Two-inch diameter monitoring wells should therefore be installed within 5.5-inch diameter or larger holes.
ND
Air Supply Matters
Air has no density or viscosity, so cuttings are blown out of the hole at high velocity. The up-hole air-flow is turbulent and more effective in lifting the cuttings. The lack of density and viscosity increases the particle slip, so a continuous and high velocity up-hole flow must be maintained to keep the hole clean. An upward velocity of about 4,000 fpm is sufficient to clean cuttings out of the hole. Cutting removal also depends somewhat on size, density, and amount of cuttings.Air compressors are rated on the following items:
- Intake air volume.
- Condition. Compressors wear with use, causing capacity to decrease.
- Sea level. Output volume capacity is reduced about 3.5 percent for every 1,000 feet above sea level.
- Temperature. Efficiency is reduced when temperatures are above 60 degrees F; it is increased when temperatures are below 60 degrees F.
- Rotation speed. Output from the compressor is directly proportional to the motor’s RPM. Do not operate a compressor at lower RPM to reduce wear.
The Pros and Cons
Be aware of these attributes of the reverse-air rotary drilling method:Advantages
- Quite fast – several hundred feet of borehole advancement per day is possible.
- Capable of drilling to full range of depths and diameters necessary for monitoring well installation. Direct-mud rotary is limited only by large poorly supported boulders and cavernous formations; reverse-air rotary requires casing in poorly cohesive materials.
- Rotary drilling easily supports the telescoping of casing to isolate drilled intervals and prevent cross contamination of strata encountered during drilling.
- Supports a broad range of sampling (disturbed and undisturbed) in all types of geologic materials.
- Geophysical logs may be run before well installation.
Disadvantages
- Rotary drilling produces relatively large volumes of cuttings and well-development residue. If contaminated, these materials may potentially cause a severe disposal problem.
- Complex equipment may introduce lubricants into the monitoring system.
- A large volume of potentially contaminated air is discharged at the surface, a potential threat to drill operators and surrounding area.
- The introduction of air to ground water may change ground water chemistry.
This article is provided through the courtesy of the U.S. Environmental Protection Agency’s Environmental Response Team.
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