Page 255 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 231

2.5.6
Drilling Fluids Program [ ]

polymer mud. A field in northern central Siberia increased production
per well from 40 tons per day to above 120 tons per day. The change
was that, instead of the previously used mud with dispersants (lignin
liquors such as cmad, kssb) and poor control of solids, a 3% KCl poly-
mer system was used.
Another factor to consider is pH. High pH might also cause dis-
persion of interstitial clays. It may also cause emulsions dependent on
the crude in the reservoir.
The occurrence of precipitates should be avoided. Potential to gen-
erate precipitate scales because of reaction of filtrate with connate
water should be investigated. It should also be remembered that in
some cases scaling can occur due to changes in wellbore pressure. If
there is a possibility that scaling may occur, a scaling tendency for the
fluid planned to be used should be made against the chemistry of the
connate water. It should take into consideration chemical concentra-
tions as well as temperatures and pressures. Most major mud compa-
nies can do this as well as operators and independent laboratories.
Without going into complete detail on scaling, an idea of the potential
causes of scaling can be derived from the type of scales experienced in
the oilfield. In many cases (but not all) one ion will come from the fil-
trate and another from the connate water. The reader is directed to fur-
ther reading in production operations manuals for further detail. Types
of scales are:

Carbonate scale; the carbonate or bicarbonate ion with the calcium
ion
Gypsum or anhydrite scale; the sulfate ion with the calcium ion
Barium or strontium scale; the barium or strontium ion with the
sulfate ion
Iron scales; ferric or ferrous ions, usually as corrosion by-products
with oxygen or sulfides
Sodium chloride precipitates; changes in pressure and temperature

Carbonate ion or sulfate ion can cause scaling if present in suffi-
cient or certain quantities in the connate water (or filtrate). Often min-
imizing the concentration of divalent cations such as calcium is
enough to avoid this type of scaling. The use of monovalent brines
such as sodium chloride, sodium bromide instead of calcium chloride,
and calcium bromide might be considered.

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