Page 247 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 223

2.5.5
Drilling Fluids Program [ ]

PHPA or PAC should be avoided, since they are not compatible with
any divalent cations that will be present in the salt. Viscosity should be
obtained from the use of a biopolymer such as xanthan or wellan gum.
Some forms of starch can also provide viscosity. Starch can be used to
provide filtration control as long as the bottom hole temperature is not
much above 200˚F. At higher temperatures, starch would require to be
stabilized or a variant of starch that is more temperature stable should
be used. At temperatures above 250˚F the usage of CMHEC (carboxy
methyl hydroxy ethyl cellulose) or other temperature stable polymers
should be considered.
There is little point in running a pH in the system much above a
neutral pH unless any deflocculants or any other material used in the
system requires a higher pH. Increasing pH results in increased insolu-
bility for most salts. Any salts that come into solution from the forma-
tion are best kept in solution. For corrosion control, the use of a film
forming amine may be considered.
Seawater polymer mud. If shale is not particularly reactive, it may
be adequately drilled with a seawater polymer system. All the princi-
ples of an encapsulating polymer fluid as previously described in this
section will hold true except that no additional inhibitive chemical
such as potassium chloride would be added to the system. Inhibition
would be obtained from the encapsulating polymers and the chemistry
of the seawater.
Using a seawater polymer system will provide for all the advan-
tages of a nondispersed mud, e.g., the rheology and ability to control
low-gravity solids.
It will be the case that for a seawater nondispersed polymer system,
there will be more inhibition if it is made up on PAC and not on PHPA.
The reasoning behind this is as follows:
In a potassium chloride polymer mud, PHPA is the best available
polymer because of its encapsulating ability but it has to be protected
or otherwise it will break down. The hardness has to be controlled at
below 200 or 300 ppm depending on the pH and the pH must be kept
low and definitely a maximum of 10. However, the potassium ion is
providing substantial inhibition. In using a seawater polymer system as
opposed to KCL PHPA mud, the chemical inhibition from the potassi-
um ion will not be present. Second, any inhibitive effect from calcium
that comes from the seawater will not be present because the calcium

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