Understanding the Souring at Bakken Oil Reservoirs
- Yevhen Holubnyak (Energy & Environmental Research Center) | Jordan M. Bremer (Energy & Environmental Research Center) | John A. Hamling (EERC at UND) | Benjamin L. Huffman (Energy & Environmental Research Center) | Blaise Mibeck (Energy & Environmental Research Center) | Ryan J. Klapperich (Energy & Environmental Research Center) | Steven Alan Smith (Energy & Environmental Research Center) | James Alan Sorensen (U. of North Dakota) | John A. Harju (Gas Technology Institute GTI)
- Document ID
- Society of Petroleum Engineers
- SPE International Symposium on Oilfield Chemistry, 11-13 April, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 2.5.2 Fracturing Materials (Fluids, Proppant), 5.2 Reservoir Fluid Dynamics, 4.2.3 Materials and Corrosion, 5.5.2 Core Analysis, 4.3.4 Scale, 5.5 Reservoir Simulation, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.2.2 Geomechanics, 4.1.2 Separation and Treating, 5.3.4 Integration of geomechanics in models, 4.2 Pipelines, Flowlines and Risers
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The souring of oil (increasing concentrations of hydrogen sulfide [H2S] gas) from reservoirs in the Bakken Formation has been observed in the field. Souring of oil presents challenges including but not limited to health and environmental risks, corrosion of wellbore, added expense with regard to materials handling and pipeline equipment, and additional refinement requirements. As such, sour oil and gas have lower profit margin (~10% lower price) than traditional sweet Bakken crude.
The understanding of causes for souring in the Bakken Formation and its timely identification are essential for determining the best operational practices and mitigation procedures at this formation. This paper will present an outline of the research goals, a current understanding of souring at the Bakken, and initial findings. Over the course of this project, the series of case-oriented uncoupled compositional reservoir simulations were developed to research the most probable mechanism of H2S generation in the Bakken Formation. The results of this investigation will be correlated in the future with field data from the Bakken oil field operator and laboratory experiments.
The Bakken Formation is rapidly emerging as an important source of oil in the Williston Basin. The formation typically consists of three members, with the upper and lower members made up of shale and the middle member composed of dolomitic siltstone and sandstone. Total organic carbon within the shales may be as high as 40%, with estimates of total hydrocarbon generation across the entire Bakken Formation ranging from 200 to 400 billion barrels. Crude oil quality from the area is generally characterized as light (31° to 45° API) and sweet, exhibiting low concentrations of hydrogen sulfide (H2S). It is the naturally sweet nature of the crude oil coupled with an increasing concern that the Bakken may become soured through current oil field practices that formulate the hypotheses of this research.
Oil field reservoir souring is defined as occurring when increasing concentrations of H2S are observed in production fluids. This is a relatively well-known problem in the contemporary oil industry. However, the identification of the source of H2S is site-specific and requires a rigorous analysis. The general causes of souring are geomechanical (fracturing and intrusion into another formation), thermochemical (e.g., mineral dissolution), biogenic (sulfur-reducing bacteria activity), or combinations thereof. In all cases, the causes of excessive H2S production in previously nonsour environments are primarily anthropogenic and caused by certain operational practices.
The souring of oil (increasing concentrations of H2S gas) from reservoirs in the Bakken Formation has been observed in the field. Souring of oil presents challenges including but not limited to health and environmental risks, corrosion of wellbore, added expense with regard to materials handling and pipeline equipment, and additional refinement requirements. As such, sour oil and gas have lower profit margin (~10% lower price [Semcrude, 2011]) than traditional sweet Bakken crude.
Based on anecdotal evidence provided by oil field management personnel working in the Bakken play and a preliminary literature review, the proposed project will focus primarily on the possibility of geochemical, geomechanical, or a combination of two scenarios for the origination of sour gas.
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