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Resource Abstract

Investigating H2S Occurrence in Bakken Oil-Producing Wells Using Sulfur Isotopes in Gas, Water, and Rock Samples

Date Published: 6/18/2024
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Resource Abstract

This presentation was given at the Unconventional Resources Technology Conference (URTeC) in June of 2024. The present study aimed to shed light on the potential mechanisms of hydrogen sulfide (H2S) generation in the Bakken petroleum system (Bakken) through a combination of laboratory pyrolysis and exposure experiments, sulfur isotope analysis, and wellsite measurements, with a focus on the five primary H2S-generating mechanisms. Unlike earlier local efforts, this study considered a broader area of the Bakken. Samples of H2S from wells were collected and analyzed, revealing a wide range of sulfur isotope (d³4S) values, though all positive, suggesting nonbacterial sources. Screening-level tests indicated that harsh reservoir conditions made it unlikely for sulfate-reducing bacteria to thrive, ruling out the bacterial sulfate reduction (BSR) mechanism. Thermochemical sulfate reduction (TSR) was similarly discounted, as the sulfur isotope composition of Three Forks (TF) anhydrite—a possible sulfate source for TSR—differed from that of the well-produced H2S. Additionally, laboratory TSR simulations using TF anhydrite, Bakken oil, brine, hydrochloric acid (HCl), and fracture (frac) fluids failed to produce detectable H2S, which could be due to the slow kinetics of TSR that often requires thousands of years.

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