Page 52 - Air and Gas Drilling Manual

2-2 Air and Gas Drilling Manual
drawworks (hoist system), but not both simultaneously [2]. The development of the
hydraulic top head rotary drive, which replaces the rotary table on most single and
some double drilling rigs, allows the prime mover to simultaneously operate the
rotary action and the hoist system. These smaller hydraulic top head rotary drive
rigs use rig weight (via pull-down systems) to put axial force on the bit.
Figure 2-1 shows the primary compressors (low pressure) that supply
compressed air to a flow line between the compressors and the rig standpipe. In this
example there are two primary compressors supplying the rig. These compressors
intake air from the atmosphere and compress the air in several stages of mechanical
compression. These primary compressors are positive displacement fluid flow
machines, either reciprocating piston, or rotary compressors (see Chapter 4 for more
details). These primary compressors are usually capable of an intake rate of about
1,200 acfm (actual cubic feet per minute) of atmospheric air and output air at
pressures up to approximately 300 psig. These primary compressors expel their
compressed air into the flow line to the standpipe of the drilling rig. This flow line
is usually an API 2 7/8 inch (OD) line pipe (or an ASME equivalent), or larger [3].
Downstream along this flow line from the primary compressors is the booster
compressor. This booster compressor is a reciprocating piston compressor. The
booster compressor is used to increase the flow pressure from the primary
compressors to pressures up to approximately 1,000 psig. In most drilling
operations the injection pressure is less than 300 psig and, therefore, the booster
compressor is commonly used only for special drilling operations such as directional
drilling with a downhole motor.
Downstream from the booster compressor are liquid pump systems that allow
water to be injected into the compressed air flow to the rig. Also solids can be
injected into the compressed air flow. This is accomplished by injecting the solids
into a small water tank, then the water with the entrained solids are injected into the
air flow.
Along the flow line leading from the compressors to the drilling rig standpipe is
an assembly of pressure gauges, temperature gauges, valves, and a volumetric flow
rate meter [3]. This instrumentation is critical in successfully controlling air drilling
operations. Also along this flow line is a safety valve. This flow line safety valve
acts in the similar manner as the safety valves on each of the compressors in
releasing pressure in the event the pressure exceeds safe limits. Also on the flow
line is a valve allowing the compressed air flow to be diverted either to the
atmosphere or to primary and secondary jets in the blooey line.
The blooey line runs from the top of the annulus to the burn pit and allows the
compressed air with the entrained rock cuttings to exit the circulating system to the
atmosphere. The blooey line is about 100 to 200 ft in length. Usually the blooey
line is an API 8 5/8 inch (OD) casing or larger [4]. However, some blooey line
systems are fabricated with two smaller diameter parallel lines. As shown in Figure
2-1, the exit (to the atmosphere) of the blooey line expels the air with the rock
cuttings into a burn pit. For oil and natural gas drilling operations, a pilot flame is
placed at the exit of the blooey line. This ignites any oil or natural gas produced at
the bottom of the well and exiting the blooey line with the circulating air. The mud
tanks (pits) are maintained at air and gas drilling operation locations in the event
high bottomhole pressure forces conversion to mud drilling.