So why use air while drilling? With air drilling bits, 20 to 30 percent of the air supplied for flushing the drilled rock out of the borehole is actually diverted around the nozzles in the drill bit to cool the bearings of the drill bit.
Each roller cone has its own bearing that heats up under pressure and friction loads while drilling the rock.
The nozzles in the bit are designed as exhaust ports to jet the air into the borehole for cleaning the rock and drilled material out of the path of the next tooth or insert from the rolling cones. The nozzles are designed to provide an adequate pressure drop across the drill bit. This insures plenty of velocity for cleaning the face of the drill bit.
Pressure drop across the bit should never be allowed to drop below 30 PSI (pounds per square inch). For optimum bearing life, the nozzles should be selected to hold pressures near the maximum allowable on the compressor.
Air Compressor and Pressure Capabilities
Next consideration during air drilling are both the volume or flow available for the air compressor and pressure capabilities and requirements of the borehole.
Let us first address pressure by itself. Air pressure must overcome piping, valves, drill pipe, cross over and connecting subs, nozzles in the drill bit and any water and material in the borehole. Air is your main drilling fluid. It is the energy or lifting force to move all of the material out of your drilled borehole.
Water is present in the ground most everywhere you are drilling. As you begin to drill, water will start to seep into the borehole. The water will need to be lifted out to proceed drilling forward.
Water exerts a pressure of 0.48 PSI per foot of column height. Therefore a column of 500 ft of water would require 0.48 X 500 or 240 PSI of air pressure to unload the hole. Often times you might be asked what pressure does a static column of water exert or represent. Every 2.2 FT represents 1 PSI of water pressure to overcome. These numbers can be organized into a table or graphical chart to provide an easy reference for well drillers.
When we started in this industry, Ingersoll-Rand provided a handy slide-rule type calculator for well drillers to use. Now all of this data and reference material is provided in an app downloaded to your smart device.
Commercial available air compressors usually operate in around 3 specific pressure bands. The first tier is around 125-150 psi, the second tier is around 175-200 psi and the third tier is 350 – 500 psi. These pressure ranges can also be displayed in metric standard units of “BAR”. 1 bar is equivalent to 1 standard atmosphere or standard air pressure at seal level, which is 14.7 PSI. Therefore. the pressure tiers for commercial available air compressors are:
- 7-10 bar for the first tier
- 12-14 bar for the second tier
- 24-34 bar for the high pressure requirements
When drilling with air, the cuttings are carried up the annular space around the drill pipe by the bouyant effect of the air rushing to the surface. The benefits of drilling with air over mud is the faster speed of lifting the cuttings to the surface. Recommended bailing velocity ofr air drilling is 3,000 – 7,000 feet per minute.
This is determined by two things. The first is the amount of air available. The second is the ratio of the area of the annular area to the volume of air.
BAILING VELOCITY = (CFM of Air X 144 in2/ft2) / Area of the Annulus
Sizing The Compressor
In sizing the compressor for the job consider first the volume of air delivered for cleaning the face of the drill bit. The following are recommended minimums for adequate bit face cleaning:
6″ bit = 475 cfm of air
6-1/2″ bit = 525 cfm of air
7-1/2″ bit = 650 cfm of air
8″ bit = 750 cfm of air
In practical applications, the formula gets you close enough to start your drilling project.
While drilling, you should check the performance of the entire circulation system. This involves watching the cuttings at the collar of the hole. You should have a steady flow of cuttings coming back up the annulus of the borehole and hitting the underside of the centralizer. In practice you will want to use some type of diverter box or diverter rubbers to change the direction of these high velocity cuttings to a horizontal discharge pipe commonly known as a flow line or discharge line.
If you see an unsteady flow or “belching” of cuttings this indicates circulation problems. The size of the cuttings also is a good indication on how the entire system is working. You want to see rock chips not rock dust. These chips should be close to the size of your small finger nail. The smaller dust sized particles can indicate that you are regrinding a lot of your cuttings and thus dulling your drill bit more quickly. It also means that you are experiencing recirculation problems where the drill cuttings are falling back down to the bottom of the borehole before exiting out the collar or top of the hole.
If you are limited in the amount of air volume you have available, you may now want to introduce a drilling foam agent to help add additional lift properties to your circulation system.