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160 Part II Gas Drilling Systems

cold gas from the bit orifice can cause failure of the borehole wall due to
local thermal stress at the bottomhole; and high-velocity gas in the upper
hole sections can cause borehole erosion in soft formation intervals. All of
these effects can be minimized by reducing the gas injection rate. If this is
not an option due to hole cleaning concerns, using large bit orifices can
remedy the first two problems. A flow diverging joint (FDJ) can be installed
at the drill collar shoulder to reduce gas flow through the bit without affect-
ing hole cleaning in the borehole above it. For the third problem, sometimes
it is necessary to set the casing deeper to protect soft formations from erosion.
A crooked hole is usually not a problem in gas drilling operations when
air hammers with flat-bottom bits are used. Air hammers require a very low
weight on bit (WOB) to drill even very hard formations. A low WOB
allows for a straight bottomhole assembly (BHA) while drilling, resulting in
straight holes. However, crooked holes are occasionally reported from gas
drilling operations. This usually occurs when conventional rock bits are used
along with high injection rates of gases. In these situations, bottomhole
washout is believed to be responsible for hole deviations. Reducing the gas
injection rate and/or using large bit orifices should ease the problem. Use of
FDJ at the drill collar shoulder is another option.

7.3.6 Corrosion
Corrosion occurs in wet systems, such as mists and foams. If misting is
implemented for lifting large volumes of water, the misting agent needs to
be added in a much higher concentration. The pH and corrosion control
must be considered on a case-by-case basis. pH control is important to avoid
corrosion in air drilling and in operations using a nitrogen membrane system
(which introduces small levels of oxygen downhole). The pH of the mixture
must be kept above 9 at the blooey line. Lime and cement have been used
in injection water, but they both leave scale and can damage foamers. On
the other hand, the presence of calcium ions reduces shale swelling. KCl
appears to be a better choice, since it does not leave scale in the pipe and it
has a buffer effect. NaOH or caustic soda is not buffered enough to maintain
its pH. Soda ash (Na 2 CO 3 ) has been used in some areas.
Corrosion inhibitors with air drilling or in the presence of oxygen from
a nitrogen membrane include phosphates and filming amines. Different
compounds have been found to work satisfactorily in various areas. Corro-
sion in freshwater appears to be minimized, although it is worse when
using saline water. With produced water, corrosion can be significant, and