|Title||The subsurface sulfur system following hydraulic stimulation of unconventional hydrocarbon reservoirs: assessing anthropogenic influences on microbial sulfate reduction in the deep subsurface,
|Licence||Please note the adoption of the Open Government Licence - Canada
supersedes any previous licences.|
|Author||Kingston, A W;
Ardakani, O H; Scheffer, G; Nightingale, M; Hubert, C; Meyer,
|Source||Geological Survey of Canada, Scientific Presentation 142, 2022, 23 pages, https://doi.org/10.4095/330712 Open Access|
|Publisher||Natural Resources Canada|
|Province||Alberta; British Columbia|
|NTS||83E; 83F; 83K; 83L; 83M; 83N; 84C; 84D; 84E; 84F; 84K; 84L; 93G/01; 93G/02; 93G/07; 93G/08; 93G/09; 93G/10; 93G/15; 93G/16; 93I; 93K/01; 93K/02; 93K/07; 93K/08; 93K/09; 93K/10; 93K/15; 93K/16; 93J; 93O/01;
93O/02; 93O/07; 93O/08; 93O/09; 93O/10; 93O/15; 93O/16; 93P; 94B/01; 94B/02; 94B/07; 94B/08; 94B/09; 94B/10; 94B/15; 94B/16; 94A; 94G/01; 94G/02; 94G/07; 94G/08; 94G/09; 94G/10; 94G/15; 94G/16; 94H; 94J/01; 94J/02; 94J/07; 94J/08; 94J/09; 94J/10;
94J/15; 94J/16; 94I|
|Lat/Long WENS||-123.0000 -116.0000 59.0000 53.0000|
|Subjects||fossil fuels; hydrologic environment; hydrocarbons; subsurface geology; sulphur isotope ratios; sulphur; Montney Formation|
|Illustrations||location maps; photographs; time series; plots; graphs|
|Program||Energy Geoscience Clean Energy Resources - Decreasing Environmental Risk|
|Released||2022 10 31|
Hydraulic fracturing is a reservoir stimulation technique that involves the injection of high-pressure fluids to enhance recovery from unconventional hydrocarbon reservoirs.
Often this involves the injection of surface waters (along with additives such as biocides) into formational fluids significantly different isotopic and geochemical compositions facilitating geochemical fingerprinting of these fluid sources. In some
instances, the produced fluids experience an increase in hydrogen sulfide (H2S) concentration over the course of production resulting in an increased risk to health and safety, the environment, and infrastructure due to the toxic and corrosive nature
of H2S. However, questions remain as to the origin and processes leading to H2S formation following hydraulic fracturing. In this study, we analyzed a series of produced waters following hydraulic fracturing of a horizontal well completed in the
Montney Formation, Western Canada to evaluate variations in geochemical and microbiological composition over time and characterize potential sulfur species involved in the production of H2S. Initially, sulfur isotope ratios (d34S, VCDT) of dissolved
sulfate in produced water had a baseline value of 27per mil similar to the d34S value of 25per mil for solid anhydrite derived from core material. Subsequently, d34S values of sulfate in produced fluids sequentially increased to 35per mil coincident
with the appearance of sulfides in produced waters with a d34SH2S value of 18per mil. Oxygen isotope values of dissolved sulfate exhibited a synchronous increase from 13.2per mil to 15.8per mil VSMOW suggesting sulfate reduction commenced in the
subsurface following hydraulic fracturing. Formation temperatures are <100°C precluding thermochemical sulfate reduction as a potential mechanism for H2S production. We suggest that microbial reduction of anhydrite-derived sulfate within the
formation is likely responsible for the increase in H2S within produced waters despite the use of biocides within the hydraulic fracturing fluids. Initial assessments of microbial communities indicate a shift in community diversity over time and
interactions between in situ communities and those introduced during the hydraulic fracturing process. This study indicates that biocides may not be fully effective in inhibiting microbial sulfate reduction and highlights the role anthropogenic
influences such as hydraulic fracturing can have on the generation of H2S in the subsurface.
|Summary||(Plain Language Summary, not published)|
Hydraulic fracturing involves injecting large amounts of surface water into geologic formations, which has significant implications for subsurface
chemistry and microbiological communities. In the Montney Formation many wells which undergo hydraulic fracturing result in the formation of hydrogen sulfide over the course of production. Here we show that: a) the effectiveness of biocides is
negatively impacted by immediate dilution with formation waters; b) hydrogen sulfide is produced via microbial sulfate reduction; c) the source of sulfate is anhydrite cements; and d) there are large shifts in microbial community diversity over time.
These results aid in the understanding of hydrogen sulfide formation in the subsurface associated with hydraulic fracturing operations.