Nearly all air and gas drilling operations are land operations. Figure 2-1 shows a typical air drilling location plan for the drilling rig and the other important surface equipment . The plan in this figure shows the location of the drilling rig (borehole directly below the rotary table). This is a typical triple drilling rig configuration. The drilling rig floor is larger and, therefore, it is easier to show the important features of an air drilling operation with this type of rig. This rig is a typical mud rotary drilling rig that has been set up to drill with compressed air as the circulating fluid. The rig is powered by two prime movers on the rig floor. These prime movers provide their power to the rig equipment through the compound (a chain drive transmission system). The prime mover on a triple rotary drilling rig like that shown in Figure 2-1 is limited to operating either the rotary table or the
drawworks (hoist system), but not both simultaneously . 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 . 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 . 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 . 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.
Note that Figure 2-1 shows a drilling location that is oriented so that the blooey line exit is downwind of the prevailing wind over the site. This keeps dust or smoke from blowing across the location.
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