Definition - What does Air Drill mean?
Air drilling is an under balanced drilling (UBD) technique in which compressed air or nitrogen is used as a circulating medium in place of conventional liquids, so as to cool the drill bit and to lift drill cuttings from the wellbore.
This technique is also referred as pneumatic percussion drilling.
Petropedia explains Air Drill
Drilling fluid or simply mud is a prerequisite for any drilling operation. The main function of this system is to lift drill cuttings, cool drill bit and act as a primary barrier for well control operations.
In air drilling, compressed air or nitrogen is employed instead of mud. With the help of large Compressors air is piped through the top of a Kelly into a drill string which passes through the bit at the bottom and sweeps away all the drill cuttings from the annulus. There are different types of air drilling which are classified based on the drill site conditions, they include dust, mist, foam, aerated drilling, and nitrogen membrane.
Applications of air drilling:
- In drilling hard rock where penetration rate is less than 15 ft/hr.
- For drilling mud loss, soft or sensitive to formation damage zones.
- Mostly employed in highly deviated wells.
Advantages of air drilling:
Air has lower density values as compared to mud medium. So, using this method has following advantages.
- Eliminates lost circulation.
- Rate of penetration is higher and this leads to lower drilling time.
- Formation damage is minimal
- Drill stem test provides accurate results.
- Extended bit life.
Disadvantages of air drilling:
- As the hydrostatic head of air column is less so there are chances of formation fluid to enter the wellbore.
- Efficiency will be reduced when formation fluids enter into the wellbore.
- High pressure formations may destabilize/collapse as there is huge pressure differential with air when compared to fluid system.
- Huge volume of air is needed for this operation.
- Hole cleaning is inefficient when the angle of well is above 50 deg.
- Not suitable for drilling hydrogen sulfide zones.
- There are Chances to catch fire in the down hole.