Page 208 - Well Control for Completions and Interventions

Completion, Workover, and Intervention Fluids 201

Gases such as oxygen, carbon dioxide, and hydrogen sulfide are solu-
ble in some brines. The combination of dissolved gas and salinity can lead
to very high rates of corrosion. Since corrosion could ultimately cause the
failure of completion tubulars or components, resulting in a possible loss
of containment, selecting an appropriate brine, compatible with the com-
pletion tubulars, is essential. Costly corrosion resistant stainless steels with
a high chrome and nickel content are still susceptible to corrosion in the
form of chloride stress corrosion. This is a type of intergranular corrosion
and occurs in austenitic stainless steel when they are under tensile stress in
the presence of oxygen, chloride ions, and high temperature.

5.6.2 Fluid compatibility with completion materials
(elastomers)
Many completion components that are critical to well integrity rely on
elastomers seals; production packers, seal assemblies, and tubing hangers,
for example. Some of the commonly used elastomers material degrade
and can fail when exposed to particular types of brine, especially some of
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the heavier brines. Some of the additives used with packer fluid can also
lead to elastomer degradation. Elastomer degradation, and ultimately fail-
ure, is a function of the concentration of the aggressive species, pressure
differential and absolute pressure, seal movement, and temperature.
In general, large seal sections, for example, packer seals, have better
chemical resistance than small components, such as T seals and O rings
(Table 5.6).

Table 5.6 Fluid compatibility with completion elastomers
Elastomer material Poor resistance to
NBR (nitrile) Brines containing zinc bromide.
Small section seals in sodium bromide (NaBr) H 2 S.
Amine based corrosion inhibitor.
HNBR (hydrogenated Brines containing zinc bromide, H 2 S. Amine based
nitrile) corrosion inhibitor.
FKM fluoroelastomers Some formate brines (pH . 8). Amine based
(viton) corrosion inhibitor.
FEPM fluoroelastomers Organic acids.
FFKM perfluoroelastomers Amine based corrosion inhibitor.
(kalrez, chemraz)