Page 54 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:55 PM Page 30

[ ] Well Design
1.3.5

Rheology and fluid loss properties may have to be controlled.
Nondamaging additives or those that can be easily removed are need-
ed for tailoring these properties. Calcium carbonate, graded by size
depending on the formation pore sizes, makes an effective fluid loss
agent that is acid soluble. Yield point (YP) and gels should be sufficient
to avoid solids settling out unless a solids-free brine is used.

1.3.5. Brines

Brines can offer solids-free systems with densities up to 1.07 psi/ft
(20.5 ppg). Another consideration is using solids-weighting materials
that are acid soluble, such as calcium carbonate and iron carbonate .
While being able to overbalance formation pressures, properly
designed brines do not create formation damage, neither by plugging
the reservoir with unremovable solids nor by causing reactions with
formation fluids or solids. Potential interactions of brines in the reser-
voir include:
Scale from the reaction of a divalent brine with dissolved carbon
dioxide, producing an insoluble carbonate (divalent brines con-
taining calcium or zinc salts, i.e., the metal ion has a valence
of two)
Precipitation of sodium chloride from the formation water when it
is exposed to some brines
Precipitation of iron compounds in the formation resulting from
interaction with soluble iron in the completion fluid (most com-
mon with zinc bromide, ZnBr )
2
Reaction of formation clays with the brine
Corrosion of casings and tubulars (not such a problem with mono-
valent brines)

Consider corrosion and biodegradable properties for completion
fluids that will remain in the well for a long time. Corrosion inhibitors
are available to suit various muds and brines. The pH should also not
be too high or too low to prevent damage to tubulars, cement, and elas-
tomers. Biocides can help control bacterial activity.
Selection of a brine system. There are three main criteria to use in
selecting the brine system for a particular well.

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