(644b) Research to Investigate the Potential for Enhanced Oil Recovery in the Bakken Petroleum System

In recent years, the greatest expansion in North American oilfield development has been in unconventional tight oil formations (<10 mD permeability), where fluid flow pathways are dominated by natural and artificially induced fractures. The Bakken petroleum system in the Williston Basin, which includes the Bakken and Three Forks Formations, is an unconventional tight oil resource with oil in place estimates ranging from 300 billion barrels (Bbbl) to 900 Bbbl of oil. Despite a challenging price environment, the Bakken petroleum system continues to produce approximately 1 million bbl of oil a day. However, primary recovery is typically below 10%. The magnitude of the Bakken oil resource and its proximity to several large anthropogenic sources of CO₂ makes it an attractive subject of consideration for enhanced oil recovery (EOR). The continued need for reductions in flaring of rich gas (methane, ethane, and other hydrocarbons produced in association with the oil) makes the potential capture and injection of rich gas for EOR an attractive concept as well. The characteristic low porosity and low permeability of the matrix of the Bakken and Three Forks Formations, combined with the fast flow pathways created by hydraulic fracturing, will present challenges to the injection of CO₂ or rich gas for EOR. Application of the U.S. Department of Energy methodology for estimating CO₂ EOR and storage capacity in the Bakken Formation in North Dakota suggests that the Bakken holds a potential storage resource of 1.9 to 3.2 billion tons of CO₂, which could yield 4 to 7 billion barrels of incremental oil. However, the DOE methodology was developed with conventional oil reservoirs in mind, and many of the assumptions upon which that methodology is based (especially with respect to fluid phase behavior and sweep/storage efficiency) may not be as directly applicable to hydraulically fractured tight oil formations because of their unique porosity, permeability, and organic matter characteristics. The widespread exploitation of hydraulically fractured tight oil resources is also a relatively recent development, thus the current level of knowledge of mechanisms and factors affecting both oil production from, and CO₂ behavior in, these unconventional tight formations is relatively low when compared to knowledge of conventional reservoirs. With these issues in mind, since 2014, a research program to evaluate the potential for storage of CO₂ for greenhouse gas (GHG) emission mitigation and attendant CO₂-based EOR in the Bakken has been conducted by the Energy & Environmental Research Center (EERC) at the University of North Dakota. Since 2015, the EERC has also conducted research to evaluate the potential effectiveness of rich gas as a working fluid for EOR in the Bakken. The ultimate goal of these efforts is to provide stakeholders with new knowledge that can be applied toward the design and execution of pilot-scale injection and production tests for EOR, and attendant GHG emission reductions, in a Bakken and/or Three Forks reservoir. Routine and advanced characterization techniques were applied to samples of the Upper and Lower Bakken Shales, as well as the non-shale lithofacies of the Bakken Middle Member and Upper Bench of the Three Forks Formation, to develop a detailed understanding of the pore throat networks of the different lithofacies at scales ranging from macro- to micro- to nanoscale. Laboratory-scale experiments were conducted to estimate the minimum miscibility pressure (MMP) of CO₂ and different rich gas components in Bakken oil under relevant reservoir pressure and temperature conditions. Experiments were also conducted to examine the ability of CO₂ and rich gas components (ethane, methane, and ethane-methane mixtures) to permeate plug samples of the various lithofacies of the Bakken and Three Forks and mobilize hydrocarbons from those samples. Simulation modeling of the plug-scale experiments were used to examine the relative effects of different rock characteristics and mechanisms on hydrocarbon mobilization.