Oxygen scavenger

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Example of oxygen corrosion in water injection flowline

Dissolved oxygen in water can cause destructive oxygen corrosion to metal pipes and process equipment. Oxygen scavenger is a class of chemicals added to react and remove oxygen in water to protect against oxygen induced corrosion.

The most common uses of oxygen scavenger in oil and gas production are for seawater injection facilities, hydrotesting (the process of using water under pressure to test the integrity of pipelines and vessels), and acid stimulation. For water injection systems, primary oxygen removal is normally either by use of a vacuum tower or a gas-stripping tower. Oxygen removal to <10 ppb (preferably less than ~5 ppb to avoid significant oxygen corrosion) can be achieved by the addition of an oxygen scavenger downstream of the deaeration vessel.

Chemistry

The most common oxygen scavengers in oilfield production are sodium bisulfite (SBS) and ammonium bisulfite (ABS). Ammonium bisulfite is more water soluble than sodium bisulfite and typically react with oxygen faster[1]. However, ammonium in the ammonium bisulfite can promote bacteria growth.

Being a salt of a weak acid and a strong base, sodium bisulfite forms a significantly basic aqueous solution whereas ammonium bisulfite is only slightly basic at around pH 8. As a rule of thumb, 60ppm of 65wt% ammonium bisulfite is required to reduce the oxygen concentration in water from 9ppm to 50ppb.

The reaction of bisulfite with oxygen is typically faster at higher temperatures and can be facilitated by transition metal ion catalysts, e.g. Cobalt (II).

Applications

The most common applications of oxygen scavenger in oil and gas production are for seawater injection facilities, hydro-testing, and acid stimulation.

Drawbacks of bisulfite oxygen scavengers

  1. Nitrogen in ammonium bisulfite provides food for bacteria.
  2. They can react with biocide to have decreased efficiencies of both chemicals.
  3. The oxygen scavenging ability of bisulfites is compromised at low pH. It prefers a pH range of 7.5-9[2] and therefore it does not work in acid stimulation packages.
  4. The oxygen scavenging rate is measurably slower in very cold seawater, close to 32F.

References

  1. ↑ N.Matsuka, Y.Nakagawa, M.Kurihara, and T.Tonomura, "Reaction kinetics of sodium bisulfite and dissolved oxygen in seawater and their applications to seawater reverse osmosis", Desalination 51(2) (1984): 163-171
  2. ↑ R.W.Mitchell, "The forties field seawater injection system," SPE6677, SPE Journal (1978):877


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