Sour media conditions in upstream oil & gas operations

Hydrogen sulphide (H₂S) is a foul smelling, toxic gas found in some oil and gas reservoirs and oxygen depleted environments such as subsea sediment. H₂S can be present in small amounts from parts per million (PPM) up to very high concentration levels (60% or more). H₂S concentrations can also vary over the life of a reservoir, often with higher concentrations found towards the end of a reservoirs life. Environments containing H₂S are commonly referred to as ‘sour’ while those without H₂S can be referred to as ‘sweet’. Where H₂S, present in large amounts, it is often deemed ‘lethal service’ and can be extremely dangerous to anyone nearby.

H₂S can chemically attack many types of polymers including those used in some elastomers. This can lead to significant changes in physical and mechanical properties, adversely affecting their long term serviceability. The rate and degree of chemical attack will depend on several factors including:

Temperature; at higher temperatures the rate of chemical attack increases, potentially reducing the maximum service life of seals.
H₂S concentration; Small amounts (ppm levels) have little effect on many elastomer types but as concentrations increase, so does the rate and degree of chemical attack, with high percentage levels of H₂S drastically increasing the rate of degradation.
Exposure time; Over time the elastomer will begin to degrade as a result of H₂S. Some materials may offer resistance and even test data showing suitability for short term service, but they may offer limited resistance over months, years or even decades of exposure.
Polymer/elastomer type; Some elastomer compounds offer higher resistance to chemical attack from H₂S compared to others. Careful selection can result in excellent performance in service, while poor selection as the potential to result in catastrophic failures.

Elastomer Types and the effects of H₂S
Many types and grades of elastomer are used to manufacture seals for the oil & gas Industry. Their resistance to sour environments differ greatly and careful considerations should be given to material selection before proposing seals for sour applications.
Butadiene- Acrylonitrile (ACN), also known as ‘nitrile’, is not resistant to H₂S and should not be considered for sour streams above ppm levels.
Hydrogenated Nitrile (HNBR, HSN) has some H₂S resistance but limited to a few percent up to 120ºC. For short term exposure, higher levels of H₂S and temperature can be considered.
Fluoroelastomer(FKM, FPM) can have excellent H₂S resistance to high concentrations and temperature, although not all FKMs offer the same high performance.
Aflas (FEPM) has greater resistance than fluoroelastomers and may be used with high levels of H₂S and higher temperatures.
Perfluoroelastomer (FFKM), have outstanding chemical resistance and may be used with the highest H₂S concentration and temperature.

The above observations are generic to the elastomer types but there is likely to be significant variability in H₂S resistance between different grades of the same elastomer type. As a result, a good understanding of the polymer technology and the compounding of the elastomer material is essential, followed by appropriate testing to understand and demonstrate the degree of resistance.