Page 239 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 215

2.5.5
Drilling Fluids Program [ ]

facture and second, the polymers are proprietary. There is a reason as
to why they should remain proprietary. Major drilling fluids companies
have completed substantial research in the applications of polymers of
all types. In the case of the anionic polymers, these have become more
or less commodity products. The mud company does not always recov-
er the benefit of its research. The field of cationic systems allows, at the
present, a mud company to market its (cationic) polymers as propri-
etary products.
Cationic polymer systems have mostly been applied for clay stabi-
lization. They do have the benefit of being more stable in make up
waters with high concentrations of divalent cations being present.
However, the cases where such high concentrations of cations are
encountered are not in the drilling of reactive shale but more in respect
of evaporite sequences. It is the case that many of these evaporite
sequences have been successfully drilled using mixed salt systems with
nonionic polymers.
Engineering of a polymer system. In putting a polymer system
together, the viscosity provided by the polymers has to be reconciled.
XC polymer may be required to provide low shear rate rheology, but
there could not be enough room for it in respect of the apparent vis-
cosity of the total fluid if the polymer selection is not optimized. PHPA
will provide viscosity in itself. It also depends on which mud company
is used because the PHPAs will vary with company.
Potassium chloride is often used in conjunction with encapsulat-
ing polymer systems because it provides clay stabilization by base ion
exchange. KCl or any other inhibiting electrolyte such as NaCl is ben-
eficial to the performance of an encapsulating polymer system because
it affects the adsorption characteristics of PHPA. A clay when it is “free”
in water is not immediately encapsulated by PHPA. It will tend to
expand all the time until the encapsulation is complete. Expansion
causes increase of surface area to be encapsulated. Potassium ions will
slow the rate of expansion.
KCl will also inhibit the development of viscosity provided by the
PHPA. It will minimize the extent to which the molecule will “unrav-
el” in water and thus provide a higher charge density per surface area
of polymer. For polymer encapsulation, the best results are seen when
the polymer has the highest charge per surface area and is adsorbing
onto a clay whose surface area has been minimized.

215