The main reason for using air for drilling purposes is to reduce the hydrostatic pressure in the wellbore. The “lightest” form of drilling, using only gas (air, nitrogen, or natural gas) to circulate with results in an equivalent specific gravity (s.g.) of close to zero. This can be then be “weighted” to about 0.2 by injecting small amounts of fluid, a method called “Mist Drilling”. Conversely, a fluid (water, oil-based mud, etc) can be lightened down to an s.g. of about 0.6 by injecting gas, a method also called “Aerated Drilling”. By injecting small amounts of surfactant the gas/fluid mixture can also be foamed to create a circulating fluid with an s.g. in the range 0.2 – 0.8. These circulating systems are shown in the graph below:
Air Drilling is a technique used in areas where formations are "Dry", i.e. there is no influx of water or liquid hydrocarbons. High volumes of low-pressure air are used in place of conventional drilling fluids to circulate the well bore clean of cuttings. This method is also called “Dusting” due to the clouds of finely ground rock cuttings that blow away from the return line, also called the “Blooie Line”.
The main advantages of Air Drilling are as follows:
The disadvantages of Air Drilling are its inability to handle formation fluids or to contain sloughing shales, thus reducing its application to consolidated “hard rock” country applications where there is little to no fluid influx.
Mist Drilling is used when the rock formations begin to produce small amounts of water (10 to 100 bbls per hour) during air drilling operations. Air volumes are increased and a Mist Pump is used to inject small quantities of a solution of water and foaming agent. This solution entraps the water influx and enables the air phase to lift the cuttings and influx to surface. Without the addition of surfactant, wet cuttings may coalesce and form a “Mud Ring” in the annulus, usually just above the drill collars where velocities are reduced, resulting in lost circulation, stuck pipe, and/or downhole combustion. The same advantages as for Air drilling also apply to Mist Drilling, though the increased air volume requirement for mist drilling will involve the 30-40% additional compression equipment.
Foam Drilling is especially suitable for drilling large holes in formations that are prone to lost circulation. Foam is generated at the surface by mixing the air from the compressors with a foaming solution from the mist pump. The consistency of this “fluid” is much like shaving cream with an interlocking bubble structure of encapsulated air providing a lifting capacity superior to that to any drilling fluid. The adjustment of the Foam Quality (gas/fluid ratio) in conjunction with a back pressure valve allows the operator to produce a bottom hole pressure equivalent to a circulating fluid weight in the range 0.2 to 0.8 s.g.