Page 245 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 221

2.5.5
Drilling Fluids Program [ ]

drilling a reactive shale, it should be remembered that there would be
a tendency for the potassium ion to deplete in the system. This is
because of its removal in the clay after it has taken up its “position” in
the clay. It is possible in well planning to use information that might be
derived from analysis of shale samples to determine how much potas-
sium chloride must be added to the mud to satisfy the shales demand
for potassium ion. The ion cation exchange capacity (CEC), or what is
often called the shale factor of the shale sample, will give an idea of the
amount of mixed layer clay present. An XRD analysis that gives a quan-
titative mineralogical composition of the shale is also of benefit.
Sometimes such shale analysis is not available and it is necessary to
proceed by only using the symptoms seen in drilling such a shale with
a less inhibitive mud (e.g., mud rings, bit balling, hole washout).
Potassium chloride is not the only available source of potassium
ions. Potassium polymer muds have also been formulated from other
potassium salts such as potassium carbonate, potassium citrate,
potassium acetate, and potassium carbonate. Each potassium salt has
its own specific chemistry, which will allow or not allow various
chemicals to be used or various parameters to be run. In using these
other potassium salts, it is necessary to study the chemistry of the
specific salt.
As mentioned earlier, the concentration of potassium ion in the
mud should vary according to the clay mineralogy of the shale being
drilled. Sometimes excessive amounts of potassium chloride in a sys-
tem can destabilize a shale that is not particularly reactive. This type of
shale will have very little swelling clay such as montmorillonite pre-
sent. It will contain substantial illite and kaolinite (a clay with a very
small clay structure), probably various micas and other minerals. The
potassium ion can cause this type of shale to slough because it causes
the kaolinite to disperse, and if any exchange of ions is made on the
illite, the net effect would be contraction of the structure.
Drilling ahead with an excessive concentration of potassium ion
for the shale being drilled will normally show no problems. However,
two or three days later the shale may begin to “come in on you.”
Excessive potassium ion can be one cause of time destabilization of
shale (but not the only cause). Examples of this destabilization are seen
in the tertiary sequence of the North Sea. The lower shales in the
Eocene and Paleocene can destabilize with time against the high con-

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