Sulfur speciation in soured reservoirs: chemical equilibrium and kinetics

Abstract

Reservoir souring is a widespread phenomenon in reservoirs undergoing seawater injection. Sulfate in the injected seawater promotes the growth of sulfate-reducing bacteria (SRB) and archaea-generating hydrogen sulfide. However, as the reservoir fluid flows from injection well to topside facilities, reactions involving formation of different sulfur species with intermediate valence states such as elemental sulfur, sulfite, polysulfide ions, and polythionates can occur. A predictive reactive model was developed in this study to investigate the chemical reactivity of sulfur species and their partitioning behavior as a function of temperature, pressure, and pH in a seawater-flooded reservoir. The presence of sulfur species with different oxidation states impacts the amount and partitioning behavior of H 2 S and, therefore, the extent of reservoir souring. The injected sulfate is reduced to H 2 S microbially close to the injection well. The generated H 2 S partitions between phases depending on temperature, pressure, and pH. Without considering chemical reactivity and sulfur speciation, the gas phase under test separator conditions on the surface contains 1080 ppm H 2 S which is in equilibrium with the oil phase containing 295.7 ppm H 2 S and water phase with H 2 S content of 8.8 ppm. These values are higher than those obtained based on reactivity analysis, where sulfur speciation and chemical reactions are included. Under these conditions, the H 2 S content of the gas, oil, and aqueous phases are 487 ppm, 134 ppm, and 4 ppm, respectively.

Analysis of Localized Corrosion on Stainless Steel

“…Accordingly, the content of corrosive ions such as chloride and sulfides at the interface between steel and corrosion films would decrease. Eventually, HSand S 2ions play an important role in verifying the sort of corrosion layers formed on 2205 DSS samples [16]. The interface between two phases of brighter and darker has been clearly etched, representing of austenite and ferrite phases, respectively [18].…”

“…In comparison, the sulfidation rates are much higher than oxidation for 2205 DSS due to the higher degree of non-stoichiometry in sulfides compared to the oxides species because of their lattice energies difference [5]. Initially, the corrosion rate was increased with increasing pH₂S, then gradually decreased with further increasing pH₂S, due to different deterioration potential of the wetting flow on 2205 DSS surface [16]. Therefore, H 2 S gas plays dual roles which are increasing a corrosion rate with increased pH₂S (3 bar) by the additional cathodic reaction and the decrease of corrosion rate at 15 bar pH 2 S is due to iron sulfide layer formation, which can protect steels surface from corrosion [17].…”

Tuning Catalyst Selectivity for Ammonia vs Hydrogen: An Investigation into the Coprecipitation of Mo and Fe Sulfides

“…One interesting observation in conditions of direct reduction by only HS – is the high level of ammonium formed at pH 8.0 compared with the results at pH 4.5. Sulfur speciation is highly dependent on the pH; at pH = 8, HS – is the dominant sulfur species (H 2 S ↔ H + + HS – ; p K a = 7.2), in contrast to pH = 4.5, where it is H 2 S. , Thus, the difference in selectivity for the ammonium production may be related to the availability of protons. At high pH, the formation of hydroxides is expected, therefore limiting Fe 2+ available for the reduction processes.…”

Comparative corrosion behaviour of austenitic 316L and duplex 2205 stainless steels: microstructure and property evolution at highly partial pressure of H2S

“…The uniform corrosion rate for both steels slightly increased up to an H 2 S of 3 bar and decreased at 15 bar H 2 S, as shown in 3, the pitting factor, defined as the ratio of the pit penetration rate to the uniform corrosion rate of both steels, shows a value between 2 and 5 at 3 bar H 2 S, representing localised corrosion on its surface [18]. However, at 15 bar, the solubility limit of sulphur dissolved as sulphide (S 2-) in the test solution is reached when the test solution becomes saturated with an immiscible sulphide phase, reducing the uniformity and pit penetration rate on the steel surface [19].…”

“…In Table 3, the pitting factor, defined as the ratio of the pit penetration rate to the uniform corrosion rate of both steels, shows a value between 2 and 5 at 3 bar H 2 S, representing localised corrosion on its surface [18]. However, at 15 bar, the solubility limit of sulphur dissolved as sulphide (S 2– ) in the test solution is reached when the test solution becomes saturated with an immiscible sulphide phase, reducing the uniformity and pit penetration rate on the steel surface [19].

Figure 2 Comparison of uniform corrosion rate and pit penetration rate for both 316L and 2205 stainless steels at different partial pressure of H 2 S.

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