Pneumatic (air/gas based) fluids are used for drilling depleted zones or areas where abnormally low formation pressures may be encountered. An advantage of pneumatic fluids over liquid mud systems can be seen in increased penetration rates. Cuttings are literally blown off the cutting surface ahead of the bit as a result of the considerable pressure differential. The high pressure differential also allows formation fluids from permeable zones to flow into the wellbore.
Air/gas based fluids are ineffective in areas where large volumes of formation fluids are encountered. A large influx of formation fluids requires converting the pneumatic fluid to a liquid-based system. As a result, the chances of losing circulation or damaging a productive zone are greatly increased. Another consideration when selecting pneumatic fluids is well depth. They are not recommended for wells below about 10,000 ft because the volume of air required to lift cuttings from the bottom of the hole can become greater than the surface equipment can deliver.
