Page 238 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 214

[ ] Well Programming
2.5.5

would necessitate a makeup concentration of 1.2 to 1.3 ppb. To get
some amount of satisfactory encapsulation with PAC, a 2.5 ppb mini-
mum excess would be required.
PAC is available in various forms dependent on the amount of vis-
cosity that it will impart. PAC polymers that impart viscosity are longer
molecules than those that provide less viscosity. PHPA, which provides
some viscosity, will provide more anionic charge than PAC, which gives
less viscosity. This is because the degree of polymerization or molecu-
lar weight is different and the longer molecules carry more ionic
charge. It should always be remembered in using PACs that they can
break down at about 1200 ppm calcium with a pH above 9.5, or at
higher levels of calcium if the pH is more neutral. PHPA will provide
encapsulation and viscosity to a fluid but not filtration control. The
amount of viscosity that it will provide will be very substantial at a
concentration of around 0.5 ppb. This is called a “viscosity hump.” The
level of PHPA in the mud should not be run at this concentration.
Some drilling fluid companies have some specialty variants of PHPA,
and you should consult with them on the specifications for their per-
formance and stability.
PAC will provide encapsulation and filtration control to the mud.
It will also provide viscosity, dependent on the type of PAC.
Cationic polymer systems. The majority of polymer systems that
are commonly used are based upon anionic polymers and nonionic
polymers. There are, however, systems available that are based upon
cationic and nonionic polymers. Some of the cationic polymers are
cationic polyacrylamides. These systems are particularly proprietary to
the drilling fluids companies that have developed them. Consequently,
reference should be made directly to these companies. However, some
general comments can be made.
Cationic polymers adsorb onto clays by attachment to anionic sites
on the clay structure. This electrostatic bonding is far more powerful
than the Van der Waal forces that cause an anionic polymer to adsorb
onto clay. The question arises as to whether there is any benefit from
this stronger form of bonding. Properly engineered anionic systems
have stabilized many extremely reactive shales and have demonstrated
that the form of adsorption was sufficient.
Cationic systems are more expensive than anionic systems for two
reasons. First, cationic polymers are overall more expensive to manu-

214