Direct Circulation

Figure 1-6 shows a schematic of a rotary drilling, direct circulation mud system that would be used on a typical double (and triple) drilling rig. Direct circulation requires that the drilling mud (or treated water) flow from the slush pump (or mud pump), through the standpipe on the mast, through the rotary hose, through the swivel and down the inside of the kelly, down the inside of the drill pipe and drill collars, through the drill bit (at the bottom of the borehole) into the annulus space between the outside of the drill string and the inside of the borehole. The drilling mud entrains the rock bit cuttings and then flows with the cuttings up the annulus to the surface where the cuttings are removed from the drilling mud by the shale shaker; the drilling mud is returned to the mud tanks (where the slush pump suction side picks up the drilling mud and recirculates the mud back into the well). The slush pumps used on double (and triple) drilling rigs are positive displacement piston type pumps.

For single drilling rigs, the drilling fluid is often treated fresh water in a pit dug in the ground surface and lined with an impermeable plastic liner. A heavy duty hose is run from the suction side of the on-board mud pump (see Figure 1-5) to the mud pit. The drilling water is pumped from the pit, through the pump, through an on-board pipe system, through the rotary hose, through the hydraulic tophead drive, down the inside of the drill pipe, and through the drill bit to the bottom of the well. The drilling water entrains the rock cuttings from the advance of the bit and carries the cuttings to the surface via the annulus between the outside of the drill pipe and the inside of the borehole. At the surface the drilling fluid (water) from the annulus with entrained cuttings is returned to the pit where the rock cuttings are allowed to settle out to the bottom. The pumps on single drilling rigs are small positive displacement reciprocating piston or centrifugal type.

Oil Rig Bottom Into Ground

Figure 1-7 shows a detailed schematic of a direct circulation compressed air drilling system that would be used on a typical double or triple drilling rig. Direct circulation requires that atmospheric air be compressed by the compressor and then forced through the standpipe on the mast, through the rotary hose, through the swivel and down the inside of the kelly, down the inside of the drill pipe and drill collars, through the drill bit (at the bottom of the borehole) into the annulus space between the outside of the drill string and the inside of the borehole. The compressed air entrains the rock bit cuttings and then flows with the cuttings up the annulus to the surface where the compressed air with the entrained cuttings exit the circulation system via the blooey line. The compressed air and cuttings exit the blooey line into a large pit dug into the ground surface (burn pit). These pits are lined with an impermeable plastic liner.

Figure 1-7 shows a detailed schematic of a direct circulation compressed air drilling system that would be used on a typical double or triple drilling rig. Direct circulation requires that atmospheric air be compressed by the compressor and then forced through the standpipe on the mast, through the rotary hose, through the swivel and down the inside of the kelly, down the inside of the drill pipe and drill collars, through the drill bit (at the bottom of the borehole) into the annulus space between the outside of the drill string and the inside of the borehole. The compressed air entrains the rock bit cuttings and then flows with the cuttings up the annulus to the surface where the compressed air with the entrained cuttings exit the circulation system via the blooey line. The compressed air and cuttings exit the blooey line into a large pit dug into the ground surface (burn pit). These pits are lined with an impermeable plastic liner.

If compressed natural gas is to be used as a drilling fluid, a gas pipeline is run from a main natural gas pipeline to the drilling rig. Often this line is fitted with a booster compressor. This allows the pipeline natural gas pressure to be increased (if higher pressure is needed) before the gas reaches the drilling rig standpipe.

Rig Circulation System
Figure 1-7: Direct circulation air system.

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