Selecting elastomers for sour service

Hydrogen sulphide may be present in a well stream but there will be other factors and chemicals that can interact with elastomers that will need to be considered. Examples include:
H₂S resistance: Typically established via testing, there are a number of testing protocols used, API 6A F1.13 (an immersion type test procedure) and ISO 23936-2 / NORSOK M710 (a test procedure using Arrhenius principles) being the two most commonly cited methods of evaluating elastomers for use in sour service.
RGD resistance: Quite apart from its aggressive chemical properties, H₂S is also considered a high risk gas in blow down events, causing as much damage as high concentrations of CO₂. This aspect should be considered when the H₂S levels exceed 10%.
Hydrocarbons: Some types can induce reversible physical changes without chemically degrading some materials. It’s important to understand and differentiate the impact these have on a seal when compared with the effects induced by chemical attack from aggressive media such as hydrogen sulphide.
Metallic components: In an aqueous environment, H₂S is liable to cause corrosion in the form of cracking and pitting of susceptible metals leading to structural failure, even in ppm levels. H₂S chemical attack of metals is quite different to that of elastomers/ polymers and standards and test procedures for assessing suitability of metals in a sour environment should not be confused with standards and test methods of assessing elastomer sour resistance.

Many high pressure sealing products in the oil and gas industry employ metal anti extrusion devises to withstand the high pressures. Selection of the metals for these anti extrusion devices must take into consideration the presence of H₂S, even in ppm levels and often involves the selection of Corrosion Resistant Alloys (CRAs). Standards such as NACE MR0175 can provide further insight and guidance on the identification and selection of appropriate metallic components.

Corrosion inhibitors: often used in conditions where H₂S is likely to be present. Corrosion inhibitors can reduce the level of interaction between H₂S and any metallic components extending their service life. However, certain corrosion inhibitors can also aggressively chemically attack some elastomers resulting in much shorter service lives or the requirement for much lower maximum operating temperatures.

Gas sweetening: If hydrogen sulphide is present in a gas stream, it must be removed before the gas can be sold onto the consumer. This is called gas sweetening and a number of processes are utilised to carry out this operation and all involve solvents and chemicals such as amines in high concentration. Seals will be required in this process but the addition of other highly aggressive chemicals complicate the selection of an appropriate elastomer.

Prior to making any material selection, a full study of the media and service conditions should be undertaken. This should be done in conjunction with the seal supplier to review any available and relevant test data in both sour media and other relevant test fluids, to ensure appropriate elastomer selection. They will be able to evaluate the available information and highlight any risks in both the short term over the extended service life of the seal.

James Walker has extensive sour test data available covering both API6A testing procures and those in Norsok M710/ISO23936-2. This data is further supported by elastomer compatibility analysis and evaluation with a range of other oilfield chemicals which can be interpreted by highly experienced materials technologists and engineers. This information can be used support material selection, to provide confidence on a compound's suitability for a given environment, and to ensure that sealing products are able to perform in extreme service conditions. Our team are always happy to review your specific requirements.