The most recent estimate of oil in place for the Bakken is in the range of 100 billion barrels to 900 billion barrels. Not the entire oil resource is economically producible which is evident by the number of developed fields. For the current fields, the estimate for primary recovery ranges from 3% to 12% depending on reservoir characteristics. With such low primary recovery factors but a large resource base, small improvements in productivity and recovery factor could lead to millions of barrels of additional oil. The problem with enhanced oil recovery (EOR) in the Bakken is the poor efficiency of injected water to displace oil from the matrix. Similarly, the effectiveness and cost of the EOR injection fluids is also of concern. The EOR fluids could include methane, nitrogen, CO2, and low concentration of polymer, alkali and surfactant aqueous solutions. A specific issue with the EOR fluids is their effectiveness, because injected fluids have to travel long distances through man-made fractures, natural fractures or induced fractures to reach the very low-permeability matrix pores to expel oil from the matrix. Finally, the oil-wet nature of the Bakken reservoirs should reduce the effectiveness of water flooding, but this negative effect could be partially countered with a favorable contribution from the osmotic pressure across the fracture-matrix surfaces. Nonetheless, the CO2 could be the fluid of choice for EOR in the Bakken because it dissolves in oil easily, has a rather low miscibility pressure with Bakken crude, swells the oil, and lowers the mixture viscosity.
While CO2 injection is widely applied in conventional reservoirs, its use for EOR in tight oil reservoirs is a relatively new concept. In conventional reservoirs heterogeneity, wettability, and the degree of pore uniformity have a significant effect on the effectiveness of an EOR scheme while fractures could be detrimental. On the contrary, in tight oil reservoirs fractures are the major contributors to oil productivity; thus, much of the conventional wisdom does not apply.
This paper presents an overview of the geologic characterization of a Bakken reservoir, the Bailey oil field in Dunn County, North Dakota, as a first step to determine its potential for a pilot CO2 EOR test. Geological characterization includes petrographic and petrophysical analyses of core samples, fracture distribution, and core-to-log correlation using multi-mineral petrophysical analysis. These results will be used to create a static model which, in turn, will serve as the foundation for dynamic flow simulation. The flow simulation results should provide insight regarding the effectiveness of CO2 injection as major guide for the pilot test design and operation.