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150 Part II Gas Drilling Systems

5. If the well cannot be controlled, then the annular preventer will need
to be closed, and a liquid pill must be pumped into the annulus to
create a cushion and stop the annular flow. It is possible that the well
will have to be killed.
6. Once the bit has cleared the blind ram cavity, the blind ram should be
closed. The pack-off can be removed after any trapped pressure is bled off.

7.3 PROBLEMS AND SOLUTIONS

Problems frequently encountered in gas drilling include borehole
instability, “mud rings,” water loading, bit balling, crooked holes, corrosion,
and downhole fires and explosions. Special measures are taken to solve these
problems.

7.3.1 Borehole Instability
Owing to the low density of gas, wellbore pressures are much lower in
gas drilling than in conventional drilling operations. Lack of support of
fluids to the borehole frequently causes wellbore instability, often result-
ing in boreholes caving in, collapsing, or getting narrower. These
problems can lead to pipe sticking and failure of the drilling operations.
This type of borehole stability problem may occur when rock formations
are very soft or weak or if they contain significant amounts of water-
sensitive clays. The borehole stability problem can be eased by mixing
the gas with some salt/surfactant solutions. The solution can stabilize the
borehole in two ways: by increasing the fluid pressure to support the
hole wall and by inhibiting the clays from swelling.
Among many chemicals, KCl is widely used for controlling clay swelling.
Anionic surfactants or household detergents are often used as foaming agents
to form foam in the annulus so the inhibiting solution can be brought up to
the intervals where water-sensitive clays exist. Inhibited mist (3‒4% KCl
and soap) can eliminate the hydration of clays. Adding polymer also helps.
To prevent air slugging, use the following formula per barrel of water:
• 1/8 lb polyanionic cellulose polymer (PAC or CMC)
• 1/8 lb xanthan gum polymer (XC or CMC)
• 1% foaming agent by weight water
However, in many cases the formations are too weak or soft to be drilled
with gas even if significant amounts of inhibitors are used. In these