Hydrogen sulfide stress cracking
Hydrogen sulfide is a colorless, very poisonous, flammable gas with the characteristic foul odor of rotten eggs. The major hazards of H2S are its ability to cause rapid damage to health or sudden death due to accidental exposure and metal integrity failure due to general corrosion or cracking. There are maximum acceptable H2S concentration limits in sale quality oil and gas, which require any excess H2S to be removed by H2S scavenging process.
H2S in the oilfield typically comes from the original hydrocarbon reservoir itself or activities of sulfate reducing bacteria (SRB) in water-flooded reservoirs or stagnant liquid storage tanks. Steam injection for enhanced oil recovery can also generate H2S possibly due to the hydrolysis of sulfide minerals in the reservoir rocks, i.e. MS + H2O → MO + H2S.
H2S can and does cause metal loss attack which generally tends to be localized due to the tendency for iron sulfide films to be formed on a steel surface (pH dependent) which breakdown or spall locally leading to pitting. However, the primary concern with the presence of H2S is the risk of causing sulfide stress cracking (SSC). This has long been recognized and prompted the development of the NACE Standard MR0175 "Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment".
The cracking caused by H2S can result in catastrophic failure in certain circumstances. It it is generally of greater concern to downhole and topside equipment. It is often also difficult to detect early and monitor in practice, and to subsequently control. Thus emphasis is firmly placed on identifying the risk at the materials selection stage and hence to select a material which is not susceptible to cracking rather than try to control the situation by use of a corrosion inhibitor.
Susceptible alloys, especially steels, react with hydrogen sulfide, forming metal sulfides and atomic hydrogen as corrosion byproducts. The atomic hydrogen under the concentration gradient diffuses into the metal matrix where it can get trapped at inclusions and grain boundaries as well as can pass right through the metal. Higher strength (> ca. 70 ksi yield strength) and hardness steels are most susceptible due to localised hydrogen embrittlement caused by the trapped hydrogen atoms which, once a crack has initiated, will concentrate just ahead of the crack tip and promote its propagation through the metal.
Crack initiation is not dependent on pit formation but can occur at any surface stress raiser or discontinuity in the presence of an applied stress. As with all localized corrosion processes, there is an induction period before a crack initiates which will depend on the stress level, (local) hardness, the material composition/microstructure and hydrogen permeation rate. At low stress levels cracks will tend to be inter-granular whereas at high stress levels they can be trans-granular.
H2S in production fluids are typically removed by H2S scavenging.
Hydrogen sulfide is most commonly obtained by its separation from sour gas, which is natural gas with high content of H2S. It can also be produced by reacting hydrogen gas with molten elemental sulfur at about 450 °C. Hydrocarbons can replace hydrogen in this process.
Sulfate-reducing (resp. sulfur-reducing) bacteria generate usable energy under low-oxygen conditions by using sulfates (resp. elemental sulfur) to oxidize organic compounds or hydrogen; this produces hydrogen sulfide as a waste product.
- Jacques Tournier-Lasserve "Hydrogen Sulfide" in Ullmann's Encyclopedia of Chemical Industry