Page 170 - Drilling Technology in Nontechnical Language

Chapter 7 – DRILLING FLUIDS 161

Solids-free brines

Brines are used when working within the reservoir so as to minimize

damage to the formation that would slow down the flow of oil or gas to the
well. Sometimes a solids-free or a brine system is used for drilling through
the reservoir. Later on during completion or workover operations, these
systems are again useful to reduce or eliminate damage. (As previously
discussed, a workover is the process of repairing damage to a well, often
by removing the completion and running a new one.)
Brines can be formulated as solids-free systems with density gradients
up to 1.07 psi/ft (2.47 SG). Solids weighting materials and fluid loss additives

that are acid soluble can also be added, such as calcium carbonate and iron
carbonate. While being able to overbalance formation pressures, properly
designed brines do not create formation damage, whether by plugging the
reservoir with irremovable solids or by causing reactions with formation

fluids or solids. Potential interactions of brines in the reservoir include
the following:
▪ Scale from the reaction of a divalent brine with dissolved carbon
dioxide, producing an insoluble carbonate. (Divalent brines are
those containing calcium or zinc salts.)
▪ Precipitation of sodium chloride from the formation water when it
is exposed to certain brines.
▪ Precipitation of iron compounds in the formation resulting
from interaction with soluble iron in the completion fluid (most

common with zinc bromide, ZnBr ).
2
▪ Reaction of formation clays with the brine.
▪ Corrosion of casings and tubulars.

Oil muds and invert oil emulsion muds

An oil mud is comprised of various solids and additives mixed into
an all-oil continuous phase, with little or no water (10% or less by volume
of liquid).
With an invert oil emulsion mud, water is present at more than 10% by
volume within the continuous oil phase as an emulsion. The water (brine
would be more accurate because it will contain dissolved salts) forms tiny

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