Page 242 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 218

[ ] Well Programming
2.5.5

example, at a pH of 10.5, it will be possible to control the filtrate cal-
cium at around 400-500 ppm depending on how much lime is held in
excess (not in solution). Lime polymer muds are normally run at a pH
of 10.3-11.0. PAC is the only encapsulating polymer that can be used.
PHPA would break down in a lime polymer system due to the pH and
the filtrate calcium that is present.
A lime polymer mud would not provide the inhibitive properties
that a potassium chloride polymer system would against a highly reac-
tive shale. It would be capable of stabilizing most moderately reactive
shales. A lime polymer mud can be prepared from drillwater, hard
water, or seawater.
The relevant principles previously described in this section will
apply. It will be necessary to ensure a sufficient quantity of PAC poly-
mer is kept in the system at all times. This may be in the form of PAC
regular (which will provide viscosity) or a low viscosity PAC.
Additional filtration control should be obtained from stabilized starch.
If additional viscosity is required (to that provided by PAC regular), use
xanthan gum or wellan gum.
The MBT should be controlled in the range of 6 to 12 ppb clay
equivalents (15 ppb maximum). The lower range of MBT is to provide
some filter cake; the upper range is where viscosity problems would
occur in the nondispersed condition. It is unlikely that it will be nec-
essary to add any prehydrated bentonite to the mud unless the MBT
dropped below 6 ppb clay equivalent. This would only be likely after
drilling a long clay-free section (e.g., a long chalk section).
A lime polymer mud would be a relevant mud to use against a
moderately reactive shale that would be drilled with an aerated mud
and require a high pH. It would be applicable for moderately reactive
shales where a persistent carbonate contamination problem existed.
Mixed salt systems. Mixed salt mud is designed for drilling
through salt sequences that include the most soluble ones, such as the
Zechstein formation encountered in the North Sea and northwest
Europe. Evaporite sequences occur as a result of seawater evaporating,
leaving the soluble salts behind. There is a definite order of precipita-
tion as the least soluble salts come out of solution first. If a saltwater
lake evaporated without further influx of saltwater, the order of
precipitation would be calcium carbonate (CaCO ), dolomite
3
(CaMg[CO ] ), gypsum (CaSO .2H O) (which is converted to anhy-
2
3 2
4
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