Sulfur corrosion is usually associated with high H2S systems but sulfur can also be deposited in systems containing small amounts of H2S under conditions where oxygen is allowed to enter the gas stream. Oxygen ingress may be due to air drawn in by elements such as compressors or vapour recovery units on oil tanks or may be carried in by methanol injected for hydrate control. The solubility of oxygen in methanol is significantly higher than in water and sufficient amounts can be present to react with H2S and deposit elemental sulfur. Case histories are presented where this has led to failures in pipelines and downhole equipment
Oil and gas operators in Western Canada have learned to be cautious when operating some of the very high H2S content natural gas reservoirs that have been exploited. Of particular concern are those reservoirs where no liquid hydrocarbon phase is being produced. Under certain operating conditions, these producing streams can deposit elemental sulfur which can lead to rapid failure due to severe pitting attack. Most sulfur deposition has been ascribed to oversaturation conditions resulting from changes in pressure and temperature as the gas stream moves from the reservoir to the tubing string and into the pipeline. Models have been developed for predicting where such deposition may occur. When such predictable conditions are anticipated, operating strategies involving the addition of sulfur solvents and aggressive pigging operations are implemented The production of gas streams containing sulfanes and ionic polysulfides have engendered less predictable difficulties. Ionic polysulfides are stable under alkaline conditions, but will precipitate elemental sulfur when the water in which it is dissolved becomes more acidic.
This can result in unpredictable deposition conditions and unanticipated corrosion difficulties.
Sulfur chemistry is complex owing to the multiple valences available to the atom. When in solution in the form of hydrogen sulfide, it may be present as a dissolved gas, as a sulfide, or a hydrosulfide, depending on the pH of the solution. It may also be present as a polysulfide in various forms. Reactions with oxygen will take place at varying rates depending on the sulfur species present and the presence of catalysts such as metal ions. The straightforward reaction with H2S is described as follows:
The equilibrium constant is large (Log10K0 = 68.41) and the Gibbs Free Energy (= -387.98 kJ/mol) suggests the reaction will be favored. The kinetics of the reaction will vary considerably with the conditions within the system. The reaction is known to be slow in distilled water unless some process accelerates it. In brines, the reaction is much more rapid with a sulfide half-life of less than one hour. The reaction of sulfide with oxygen is complex. It may proceed with a metal catalyst or as a self catalyzed reaction.
