Page 568 - Air and Gas Drilling Manual

11-6 Air and Gas Drilling Manual
The horizontal arm on the right side of the power swivel is supported at the far right
end by a tight cable that is run the length of the rig working clearance. This arm and
cable combination resists the internal torque generated by the power swivel as it
rotates the drill string.
The main advantage to using a power swivel on a conventional rig is that the
changes in rotary speed can be made smoothly. Also, the power swivel can be used
to rotate the drill string in either a clockwise or counterclockwise direction. Further,
the power swivel can rotate the drill string while lifting the drill string. These
advantages combine to give very useful characteristics for directional drilling
operations, particularly for long horizontal drilling operations.

Figure 11-5: Typical power swivel (Model S-1) for conventional rotary drill rigs
(courtesy of Bowen Tools, Incorporated).
When drilling long a near horizontal section (of the order of 1,000 ft or greater)
the rock cuttings tend to fall to the low side of the borehole. This is particularly
true when a directional course correction is being made. During course corrections,
the drill string is not rotated and is slid along the low side of the borehole. The
downhole motor rotates the drill bit. This allows the bent sub above the downhole
motor to dictate the direction of the correction. While sliding the drill string, the
rock cuttings entrained in the drilling fluid tend to accumulate between the drill pipe
tool joints. The drilling fluid will seek the least resistance to flow and move
through the high side of the borehole above the drill sting. To clean this
accumulation from the bottom of the borehole, it is necessary to periodically cease
drilling forward and lift and rotate the drill string with the power swivel. It is not
possible to lift and rotate the drill string when drilling with a conventional rotary