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146 Part II Gas Drilling Systems

Drill string data
Drill pipe OD: 4.5 in
Drill pipe ID: 3.643 in
Drill collar length: 288 ft
Drill collar OD: 6.25 in
Drill collar ID: 2.5 in
Material properties
Specific gravity of rock: 2.75 (water = 1)
Specific gravity of gas: 1 (air = 1)
Gas specific heat ratio (k): 1.25 (water = 1)
Specific gravity of misting fluid: 1 (water = 1)
Specific gravity of formation fluid: 1 in
Pipe roughness: 0.002 in
Borehole roughness: 0.25
Environment
Site elevation (above mean sea level): 2000 ft
Ambient pressure: 13.67 psia
Ambient temperature: 65°F
Relative humidity: 0.7 fraction
Geothermal gradient: 0.01 F/ft
Minimum velocity under standard conditions: 50 ft/sec
Operating condition
Surface choke/flow line pressure: 14.7 psia
Rate of penetration: 90 ft/hour
Rotary speed: 50 rpm
Misting rate: 5 bbl/hour
Formation fluid influx rate: 15 bbl/hour
Dewatering efficiency: 0.90 fraction
Bit orifices: 20-1/32nd in
20-1/32nd in
20-1/32nd in

1. Assume that the pressure loss in the surface injection line is
negligible, and predict the minimum required gas injection
pressure using a gas flow rate safety factor of 1.15 for the design
gas injection rate.
2. Plot the profiles for pressure, velocity, gas density, and kinetic
energy index inside the drill string and in the annulus.
6.3 For the air drilling conditions specified in problem 6.2, determine
the required compressor horsepower. Assume that the efficiency of
the compressor is 0.75.