O2 Information
Oxygen dissolved in water is one of the primary causes of corrosion in the oil field.
When oxygen is present, the most common types of corrosion include pitting corrosion and
uniform corrosion. For more on the theory and mechanisms of these types of corrosion,
go to the theory page.
Oxygen is a strong oxidant and reacts quickly with metal. The maximum amount
of oxygen in water is only 8 ppm, so the mass transport of oxygen is the rate limiting step
in oxygenated non-acidic environments. Controlling the rate of oxygen transport (often by
controlling flow velocity) is thus critical to corrosion control.
O2 corrosion products include iron oxides,
including FeO(OH) - goethite, Fe2O3 - hematite, Fe3O3 -
magnetite, and FeO(OH) - ferrous hydroxide.
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Differential Aeration
Corrosion may occur in oilfield applications due to the existence of differential
aeration. In these cases, one section of the metal is exposed to oxygen while
the other is not. The section with no aeration becomes anodic, and is subject
to preferential corrosion. This can occur with pipelines and other metals near
the surface. The first figure shows an example of how a corrosion cell can
form when a pipe is buried below the surface. The soil above the pipe can become
aerated due to the digging and backfilling process, so the top of the pipe
is exposed to more oxygen than the bottom. The bottom becomes anodic and the
top becomes cathodic. In the second figure, a section of pavement restricts
oxygen from reaching the pipe in the part of the pipe under the pavement. That
part of the pipe becomes anodic and corrodes preferentially.
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Where Found
Although it is not normally present at depths below around 330 ft (100 m), oxygen
is often introduced in oil production through leaking pump seals, casing and process vents,
open hatches, and open handling. In addition, oxygen removal processes such as gas stripping
and chemical scavenging often fail, allowing oxygen contamination in waterflood systems.
Oxygen corrosion occurs commonly in drilling fluid, primary production in
rod pumped wells, outdoor rod storage (rusting), oxygen entry into wellbore
through annulus, lower part of well including casing, pump,
tubing, lower part of rod string
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Prevention / Mitigation
Oxygen removal may be done by mechanical and chemical means. Mechanical means include gas
stripping and vacuum deaeration; chemical means include sodium sulfite, ammonium
bisulfite and sulfur dioxide. Mechanical means of oxygen removal are usually employed when large quantities
of oxygen need to be removed, while chemical means are used to remove small quantities of
oxygen and may be used to remove residual oxygen after mechanical means have been used.
It is often more economical to exclude oxygen from oilfield equipment than to remove it
after it has entered the system. The most common way of excluding oxygen is through the use
of gas blankets, composed of oxygen free gas such as natural gas (methane) or nitrogen.
Gas blankets may be used on water supply wells and water storage tanks, supply wells
and producing wells, and pumps. Most tanks only require a few ounces of pressure.
The regulator should supply gas at a rate adequate to maintain pressure when the fluid
level drops. Maintenance of valve stems and pump packing is also important.
To reduce or prevent corrosion in an O2 environment:
Drilling - oxygen scavengers
Producing wells - corrosion inhibitors, oxygen scavengers, elimination of O2 sources
Flowlines - corrosion inhibitors, oxygen scavengers, elimination of O2 sources
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Pictures - Click on thumbnail to see larger picture
