Page 246 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 222

[ ] Well Programming
2.5.5

centration of potassium ion in the mud that is required to avoid
“gumbo problems” at a shallower depth against the Pliocene. These
problems (of “gumbo” and a time-destabilized shale in the same inter-
val) could be avoided by using oil-based mud. Prior to that when using
potassium chloride polymer muds, the most successful operators were
3
those that “slammed” their 13 /8 in casing into the ground when reach-
ing TD on the transition zone in the Paleocene.
Other examples have been seen in the Middle East. In the Malay
basin, better calipers and longer “time envelopes” have been achieved
by controlling the potassium ion concentration to meet the needs of
the shales. In some cases, the concentration of potassium chloride used
has been half that previously employed.
If the clay mineralogy of the shale is available, this can be very
helpful in selecting potassium ion levels. If this is not known, then the
best approach is to start at a predetermined level and increase as the
cuttings condition dictates (but also ensure that there is an excess of at
least 1 ppb of PHPA in the mud because this can be another reason why
the cuttings are soft and sticky). With subsequent wells, depletion lev-
els of potassium chloride can be studied.
If the clays in the shale require a certain concentration of KCl, the
clays will take potassium ion from the mud. To keep a steady concen-
tration of potassium chloride in the mud, more KCl would have to be
added than would be calculated for the KCl concentration being run
(and adjusted for new volume). The depletion rate for KCl should con-
firm that the correct KCl concentration is being run.
Saturated salt polymer mud. In drilling a salt sequence, the mud
downhole must be saturated in order to avoid leaching out of the salt.
If the formation to be drilled comprises only halite (NaCl) and there is
no excessive difference in temperature between surface and bottom
hole temperature, the salt mud can be prepared by saturating it with
sodium chloride. If, however, other salt minerals are present or the
mud that may be saturated at surface will be below saturation down
hole, a mixed salt system should be used. The makeup of a saturated
sodium chloride mud or mixed salt system is more or less the same.
The presence of clay in the mud should be minimized since it will be
flocculated in the presence of the salts, requiring the use of a defloccu-
lant. In some cases, this is unavoidable if the hole section comprises a
shale section above the salt. The usage of anionic polymers such as

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