Page 240 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 216

[ ] Well Programming
2.5.5

The performance of the polymer system must be continuously
monitored and any potential problem must be addressed proactively.
Emphasis must be put on looking at all the parameters and not on any
one alone.
The indicators for performance of the system will be:

1. The buildup rate of the cation exchange capacity. It is necessary to
keep the CEC below 10. The figure of 15 ppb equivalent clay is an
absolute maximum because with such a CEC, the rheology will be
difficult to control (and the hole could be washed out). The build
rate of the CEC must be addressed long before the CEC ever gets
to 15.
2. The condition of the cuttings at the shaker.
3. If PHPA is used, the concentration of PHPA can be measured. It
could show trends but could not accurately provide a definitive
concentration. It is a difficult test to do as it involves measuring the
rate of ammonia generation in a gas train. It is easier just to make
sure all polymer additions are by measured premix.
4. The buildup of low gravity solids.
5. The lagged in and out rheologies in the lower shear rates.
6. The pH of the fluid would normally be run at around 9.0 to 9.5.

Some considerations to make in respect of selection of the best pH
to run would be as follows. A pH of 9.3 will have removed most of the
magnesium ion in the system if seawater is used as the makeup fluid.
The pH must be run within a range that will allow for stability of the
polymers present in the mud. Most water dispersible polymers disperse
best in a pH of 9.0 to 9.5. Minimum dispersion of clays occurs at a neu-
tral pH (when water has its least polar nature), but the KCl and the
polymer, if run properly, will minimize dispersion at a higher pH.
The hardness must be controlled below 200 ppm if PHPA is in the
system. If PHPA is not in the system, the hardness will be controlled
below 500 ppm (a safe level for PAC) since there is no need to keep it low
unless it needs to protect the PHPA. Allowing some calcium ion to be pre-
sent in the filtrate is a good protection against carbonate contamination.
Gel CMC mud. In a way, this is not a true nondispersed polymer
mud, as any clay present would tend to become dispersed due to the
effect of pH. However, this type of mud is normally used for drilling

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