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Gas Drilling Operations 151

situations, the open hole needs to be cased before drilling ahead with gas, or
the gas drilling needs to be converted to foam drilling operations to solve
the borehole stability problem. Chemically induced borehole stability pro-
blems, such as clay swelling, get worse as time goes on. It is a good practice
to drill the sensitive intervals fast and then set the casing to isolate them.

7.3.2 Mud Rings
Mud rings are a result of inadequate hole cleaning. As shown in Figure 7.1,
when drill cuttings move to the shoulder of drill collars, a floating bed of
cuttings may form due to the low velocity of in situ gas in the pipe–open
hole annulus. Although the gas density is relatively high at this point
compared to the upper hole section, the kinetic energy of gas at this point is
the lowest. If the gas stream is not powerful enough to keep the cuttings
floating, significant amounts of cuttings may accumulate in this area.
Figure 7.2 illustrates this situation. When the formation is damp from water
or oil, the cuttings form a “mud” that is deposited against the side of the
hole. This tends to form rings of mud that, as they grow larger, restrict air-
flow and cause the pressure to increase. Mud rings cause high friction,
which can result in downhole burn-offs (fires) and stuck pipes.
The most effective means of reducing mud rings is increasing the gas
injection rate. A light mist often does not cut mud rings. If the gas injection
rate cannot be increased due to limited capacity of the compressor or

Low Flow
Floating Velocity
Bed

High Flow
Velocity

Figure 7.1 Floating bed of drill cuttings at the shoulder of drill collars.