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.

Horizontal drilling in combination with hydraulic fracturing has dramatically changed the energy landscape as it allows for the more efficient extraction of natural gas from less accessible reservoirs. An issue being explored in greater detail is the increase of hydrogen sulfide (H2S) and mercaptan (C x H y -SH) content during the early production from hot shale gas reservoirs (T > 100 °C). Hydraulic fracturing technologies rely on the use of chemical additives for modifying the physical and chemical properties of fracturing fluids to drag proppant into the reservoir. Under downhole conditions, native H2S or metal sulfides can be partially oxidized by dissolved oxygen or other aqueous species, thus producing elemental sulfur. Over time, this elemental sulfur can slowly oxidize the chemical additives, thus regenerating H2S and other organosulfur species. In this work, we focus on the reaction kinetics of sulfur and alcohol reaction under downhole conditions. Rates and reactions are presented and discussed as an alternative mechanism for the delayed production of mercaptans and H2S.

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North American production of liquefied natural gas is at an all-time high in large part due to shale gas extraction from unconventional reserves. In producing shale gas, hydraulic fracturing in combination with horizontal drilling is often used to create a path to free the hydrocarbons embedded in the reservoir. Biocides are incorporated into the fluid during drilling and fracturing for preventing activity from both native and non-native bacteria and potential reservoir souring (biogenic generation of H 2 S). Certain sulfur-containing biocides have been reported to be used as part of the fracturing fluid in several reservoirs. Our research has shown that at high-temperature downhole conditions, some of these compounds can decompose and generate unwanted H 2 S and organosulfur compounds as byproducts. In the case of dazomet, a sulfurcontaining biocide, the decomposition products underwent hydrolysis under downhole conditions to produce undesirable H 2 S, CS 2 , and CH 3 SH. A second biocide tested, methylisothiazolinone, eliminated sulfur and generated H 2 S by sulfur dehydrogenation of the reaction intermediates. Our findings highlight the chemical transformation that sulfur-containing biocides could undergo under hydraulic fracturing conditions. In these circumstances H 2 S and organosulfur compounds can be generated.

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