Air and Gas Drilling
Air and Gas Drilling It is a technique in which the more common drilling fluids, (water or mud) are replaced by highly compressible Air or Natural gas. But Compressibility makes pressure and volume calculations difficult. Applicability is limited to Lithological and Pore Pressure conditions.
Significant savings can be achieved with these drilling techniques. Offshoots of Air and Gas drilling: 1. Mist 2. Stable Foam 3. Aerated Mud They are ordered in increasing wet environment.
Advantages 1. Higher ROP (Chip hold down effect) 2. Less drilling time / Lower cost 3. Less formation damage 4. Analyze formation productivity while drilling 5. Minimum lost circulation 6. More footage/ Bit
Disadvantages 1. Limitations of lithologies 2. Water entry -- Balling effect 3. Limitations of pore pressure 4. Significant drill string wear (Sandblasting Effect) 5. No hydraulic dampening of drill string 6. Potential for down-hole fire
Air Drilling Equipment Compressors 1. Positive Displacement 2. Dynamic Boosters 500 to 1200 SCFM Max. 300 psi They take output from compressor and can increase pressure from 100 to 1500 psi
Besides compressors and boosters, an air drilling operation demands specialized equipment not employed in mud drilling. 1. Surface equipment 2. Down-hole equipment
Surface Equipment 1. Mist Pump 2. Soap Pump 3. Standpipe Manifold 4. Rotating Head or Rotating Kelly Packer 5. Blooey Line 6. Sample Catcher 7. De-Duster 8. Pilot Light 9. Burn Pit
Soap Pump In foam drilling operation a soap pump and a small air operated chemical injection pump is installed. This allows for the accurate measurement of volumes on the order of 10 to 20 gal/hr.
Mist Pump Immediately after compressors and boosters there is a mist pump. It is a small positive displacement pump with 10 to 20 bbl suction tank It is used to inject water, foam and corrosion inhibitor. 30 to 50 gal/mint capacity.
Standpipe Manifold Some additions to the standard mud drilling standpipe arrangement are done in air drilling for bleed-off and bypass. Bleed-off Lines: These lines are installed above the main standpipe valve. These are to bleed off the pressure trapped between the upper drill string float valve and the main standpipe valve prior to a connection
Bypass Lines: These lines are below the main standpipe valve. These allow for a connection without the compressors being shut down.
Rotating Head or Rotating Kelly Packer Rotating heads are used to pack-off the annulus, diverting the air/gas flow down the blooie line. Hexagonal kellys are used in air drilling.
Blooie Line The blooie line serves to carry the circulating fluid away from the rig. It must allow for catching of drill cuttings to aid formation evaluation and provide for burning of gas. It carries the stream to a safe distance from the rig (100 to 300 feet). it should be straight.
Sample Catcher It is a device set in the blooie line to recover viewable size rocks for evaluation purpose. It consists of a piece of angle-iron (or tong die) set at an angle against the flow, deflecting rocks for recovery.
De-Duster It is a device to wet the air stream coming from the wellbore, thereby lowering the dust. It consists of a pump, water line and baffles to produce spray. In areas where toxic H2S is drilled, causticised may be introduced to neutralize the gas.
Pilot Light A flame is provided at the end of blooie line to ignite gas, preventing the buildup of an explosive mixture. The flame ideally is provided by a small separate gas line.
Burn Pit A pit should be dug at the end of blooie line for burning of gas coming from the wellbore. This pit will contain any liquid HCs keeping them away from the main reserve pit and hence away from the rig.
Downhole Equipment 1. Float Valves 2. Bottom Hole Assemblies (BHA) 3. Drill Bits