CO2-Based Enhanced Oil Recovery from Unconventional Reservoirs: A Case Study of the Bakken Formation

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Abstract As one of the largest unconventional resources in the world, the Bakken petroleum system has approximately 7.4 billion barrels of recoverable oil as estimated by the U.S. Geological Survey in 2013. These resources lie in a 200,000-square-mile area in North America. However, primary recovery rates are low, typically 3%–5%. The use of carbon dioxide (CO2) as an injection fluid may reduce the hydrocarbon viscosities in the reservoir and allow additional trapped oil to be produced. Another benefit of CO2 injection is to mitigate climate change. In this study, detailed petrophysical and geological field models focusing on the Middle and Lower Members of the Bakken Formation in the Bailey and Grenora areas of western North Dakota were developed based on field characterization, well log interpretation, and laboratory core analysis by scanning electron microscopy, ultraviolet fluorescence, and standard optical microscopy techniques. A fine-scale model with a pair of horizontal wells was extracted from the initial Bailey model to examine the potential effectiveness of CO2-based oil recovery techniques. Based on characterization results for the simulation investigation, two three-stage hydraulic fractures were incorporated into the model for CO2 injection and oil production wells, respectively, as well as the characterized oil properties such as swelling test and minimum miscibility pressure measurement from selected reservoirs. A total of four cases for simulation were designed to enhance oil recovery by CO2 injection. These cases were run on a single porosity–permeability model to test the basic concepts and later incorporate these results into a dual porosity–permeability model. The results show that CO2 injection may play a significant role in increasing oil production. Results of this study indicate that production can be enhanced by 43% to 58% compared to cases without CO2 injection. Since a single-porosity, singlepermeability model was used, relative permeability curves that represent the fractures and matrix were both tested to determine the effects of this variable on the outcome. The results indicated that CO2 enhanced oil recovery (EOR) is very sensitive to the relative permeability, and that result varied over 100% between the cases. This case study for CO2 storage in the Bakken Formation provides a basic guideline to address CO2-based EOR. Specifically, the efforts provide modeling input for future investigations that could lead to the design and implementation of a possible pilot-scale field test.