Hydrogen sulfide formation in oil and gas

Abstract

Hydrogen sulfide (H2S) can be a significant component of oil and gas upstream production, where H2S can be naturally generated in situ from reservoir biomass and from sulfate-containing minerals through microbial sulfate reduction and (or) thermochemical sulfate reduction. On the other hand, the technologies employed in oil and gas production, especially from unconventional resources, also can contribute to generation or delay of appearance of H2S. Steam-assisted gravity drainage and hydraulic fracturing used in production of oil sands and shale oil/gas, respectively, can potentially convert the sulfur content of the petroleum into H2S or contribute excess amounts of H2S during production. A brief overview of the different classes of chemical reactions involved in the in situ generation and release of H2S is provided in this work. Speciation calculations and reaction mechanisms are presented to explain why thermochemical sulfate reduction progresses at faster rates under low pH. New studies regarding the degradation of a hydraulic fracture fluid additive (sodium dodecly sulfate) are reported for T = 200 °C, p = 17 MPa, and high ionic strengths. The absence of an ionic strength effect on the reaction rate suggests that the rate-limiting step involves the reaction of neutral species, such as elemental sulfur. This is not the case with other thermochemical sulfate reduction studies at T > 300 °C. These two different kinetic regimes complicate the goal of extrapolating laboratory results for field-specific models for H2S production.