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Abstract

Stringent environmental constraints imposed by government regulators upon the oil and gas producing industry has led to the need for new ‘greener’ chemistries, which have less environmental impact to be developed and employed. The environmental impact of a corrosion inhibitor is often defined by three tests, biodegradation, bioaccumulation and toxicity. All three criteria have limits that must be met for a chemical to be permitted for use, with different emphasis on each depending which regulator controls the waters. This has become the motivation for the research into modifying old chemistries and developing new ones.

Quaternary amines (quats) have been known for a long time as excellent corrosion inhibitors for their ability to form a film on the surface of the steel; however, their biotoxicity profile is not particularly acceptable to qualify them as “Green Corrosion Inhibitors”. This work covers the structural modifications that have been imparted on quats to qualify them as green corrosion inhibitors with excellent corrosion protection.

Introduction

Tightening environmental constraints imposed by governmental regulators upon the oil and gas producing industry has led to the necessity for new ‘greener’ chemistries, which have less environmental impact to be developed and employed. Operators now demand identical levels of performance for all chemicals used in offshore production along with the fulfillment of new environmental criteria for any chemicals that may be contained in any offshore discharge water. Corrosion inhibitors are given particular attention due to their inherent design to partition into the aqueous phase. This environmental drive has been spearheaded by Oslo and Paris Commissions (OSPAR) for protection of marine environment of North-East Atlantic and individual contracting parties i.e. Norway, UK and Netherlands interprete policy and implement their own countrywide schemes. Due to its success similar programmes are being considered and implemented in other oil producing regions. This has widened the potential application of any green corrosion inhibiting chemistries.

The environmental impact of a production chemical is typically defined by three tests; biodegradation, bioaccumulation and toxicity. All three criteria have limits that must be achieved in order for a chemical to be permitted for use.  In order for a product to be used without restriction offshore, two of the following three criteria must be satisfied1:

• Biodegradation must be greater than 60% (if less than 20% material is automatically marked for substitution)

• Bioaccumulation as measured by Octanol/Water partitioning coefficient (LogPo/w) must be below 3 (exceptions exist for molecules with molecular weights higher than 600 in UK sector)

• Toxicity to the most sensitive marine species (typically Skeletonema) must be greater than LC50 or EC50 of 10 ppm.

If a product fails to meet at least two of the above three criteria it is put onto the substitution list and the use of the product has a finite life.

Different emphasis is placed on each test depending which regulator controls the waters. This in part, has become the motivation for the research into the modification of old and development of new chemistry to satisfy these environmental demands.

Oil and gas production is typically accompanied by naturally occurring water and acid gases such as carbon dioxide (CO2) and hydrogen sulphide (H2S). CO2 dissolves in water forming carbonic acid and although a weak acid it has been proven to be the dominant form of corrosion in ‘sweet’ or hydrogen sulphide free fields. carbonic acid is able to dissociate into bicarbonate, hydrogen and subsequently carbonate ions. Although it is a weak acid, the extent of corrosion observed under CO2 environments is larger than that for the fully dissociated acid at the same pH. This is probably due to the direct reduction of carbonic acid at the metal surface.  This is thought to catalyse the corrosion reaction and lead to the greater than expected corrosion rates that are observed in CO2 oilfields 2.