Testing of elastomers for use in sour environments

H₂S is highly toxic and therefore sour testing must be carried out in suitable laboratories equipped to handle the gas. On completion of the test, the laboratory will provide a report detailing the test procedure, media employed and the results.

The most common elastomer sour tests used in the oil & gas Industry are:

API 6A, Annex F.1.13.5.2
To assess elastomer sour resistance, samples are held in a liquid phase under pressure for 160 hours at elevated temperature. A number of different gas mixtures can be used to pressurise the vessel, the most arduous gas mixture is FF/HH which consists of 80% CO₂, 10% CH₄ and 10% H₂S. The liquid phase consists of a hydrocarbon mixture that can vary depending on the customer’s needs. Water is also introduced at 5% of the vessel volume.

Physical properties are measured before and after exposure and changes in these properties are studied. There is no pass/fail criteria specified in API 6A, but it is common for the criteria specified in ISO 23936-2 / Norsok M710 to be adopted.

The methodology adopted in API 6A provides a quick and relatively flexible approach for assessing H₂S resistance of elastomers. Unfortunately, the speed of results and flexibility are also its Achilles heel. As already highlighted, exposure time is a critical variable in elastomer selection, and relying on a 160 hour test to investigate materials for what can be decades of service life can result in poor elastomer selection.

Similarly, care is needed in interpreting the results of testing. It is important to understand the changes in properties caused by the non-sour media compared with the effect of the hydrogen sulphide. Over such short test periods, it is possible for materials to undergo physical changes without suffering from chemical attack, for example, some elastomer types could suffer from swelling induced by the hydrocarbon liquid phase. These can cause changes in physical properties unrelated to the H₂S under examination.

As a result, it is important that the information collected from API 6A is assessed along with other relevant testing and insight, and interpreted by experts to ensure that all results and critical application requirements are understood and considered before a material is selected.

Norsok M710 rev 3 / ISO 23936-2
This test is fundamentally different to the API 6A test in that it studies changes in properties of elastomer samples over an extended time; this can be up to several months of exposure to sour conditions with longer testing periods offering greater insight, accuracy and confidence in the results.

The testing aims to accelerate the rate of chemical interaction by testing at three elevated temperatures. At regular intervals, samples are extracted and their physical properties are assessed. Any changes in the properties are recorded over time for each temperature allowing the assessment of both the rate of chemical interaction and the extent of any physical degradation in the samples. At the conclusion of the test, the results are reviewed using Arrhenius principals and a theoretical service life at a given temperature can be reached.

For samples offering excellent resistance to the sour test conditions, and showing no evidence of chemical deterioration, it is acceptable to conclude that “the material can be described as having indefinite life” at the lower of the three test temperatures.

Customers may require bespoke testing, changing variables such as media composition or temperature, to more closely represent their specific project conditions. Norsok M710 / ISO 23936-2 makes allowances for this, and specific non-standard test conditions can be considered with agreement between customer and the test facility. Norsok M710 / ISO 23936-2 also considers the RGD properties of Elastomers.

Other Testing Protocols
While API 6A F1.13 (an immersion type test procedure) and ISO 23936-2 / NORSOK M710 (a test procedure using Arrhenius principles) are the two most commonly cited methods of evaluating elastomers for use in sour service, there are other test procedures to be aware of. These include:

NACE TM0187 is similar in methodology to API 6A F1.13, exposing samples for 160 hours in a pressurised vessel at elevated temperatures. Differences of note include that the hydrogen sulphide content increases to 20% of the gas phase, while the liquid phase is reduced to 10% of the total volume; half water and half hydrocarbons.

API 16C is also cited regarding sour resistance. API 16C makes reference to the methodology used in API 6A F1.13, however the gas blend is subtly different. API 16C calls for 10% H₂S, 5% CO₂ and 85% methane in the gas phase. There is also an increased amount of water in the liquid phase.