Hydrocarbon Mobilization and Potential CO2 Storage Mechanisms in the Middle Bakken, Bakken Shales, and Three Forks

Recent estimates of the original oil in place (OOIP) in the Bakken range from 160 to 900 billion barrels (Nordeng and Helms, 2010; Continental Resources, 2013, Kurtoglu et al., 2013).  However, since recoveries are typically low (3 to 6% according to Bohrer et al. 2008 and Nordeng and Helms, 2010), even a small incremental increase in recoveries could yield many billions of barrels of additional produced oil. In addition, there are large organic rich members of these formations which contain substantial oil that is not currently considered to be amenable to production.  Efforts to increase oil recovery in these unconventional formations could include carbon dioxide (CO₂) enhanced oil recovery (EOR) with incidental storage of large quantities of CO₂. 

The processes and mechanisms which enhance oil production and trap CO₂ in conventional oil reservoirs are expected to be very different from those in tight unconventional reservoirs (Hawthorne et al., 2013). In conventional reservoirs, CO₂ flows through the permeable rock, and oil is mobilized by a combination of oil swelling, reduced viscosity, hydrocarbon stripping, and CO₂ flushing, especially when above the minimum miscibility pressure. In tight unconventional oil reservoirs, CO₂ flow will be dominated by fracture flow, and not significantly through the rock matrix. Fracture-dominated CO₂ flow could essentially eliminate the "flushing" mechanisms responsible for increased recovery in conventional reservoirs. As such, other mechanisms must be optimized in these unconventional oil reservoirs.

Conceptual  mechanisms that may occur when CO₂ interacts with these tight formations include: (1) CO₂ flows through the fractures, (2)  unfractured rock is exposed to CO₂ at fracture surfaces, (3) CO₂ permeates the rock driven by pressure, carrying some hydrocarbon inward; however, the oil is also swelling, which forces oil out of the pores,  (4) oil migrates to the bulk CO₂ in the fractures via swelling and reduced viscosity, (5) as the CO₂ pressure gradient gets smaller, oil production is driven by concentration gradient diffusion from pores into the bulk CO₂ in the fractures, and (6) some fraction of the injected CO₂ is trapped in the irreducible fluids that remain in the reservoir after the production phase.

To investigate these concepts, rock samples from the Bakken Middle Member (low permeability, oil-saturated siltstone), Bakken Upper and Lower Shale Members (very low permeability, oil-saturated shale), Three Forks (low permeability, oil-saturated muddy dolostone), and a conventional reservoir (high permeability, oil-saturated sandstone) were exposed to CO₂ at typical Bakken conditions of 110 C and 5000 psi (230 F, 34.5 MPa) to determine the effects of CO₂ exposure time on hydrocarbon removal.  Varying geometries of each rock ranging from small (mm) "chips" to 1 cm-diameter rods were exposed for up to one week, and mobilized hydrocarbons were collected for analysis. Nearly complete (>95%) hydrocarbon recovery occurs in hours with the more permeable matrices, while several days of exposure are required for the upper and lower Bakken shale samples.  For example, while oil recoveries from a 1-cm round rod of middle Bakken and Three Forks rock were >95% after 24 hours, the oil recoveries from 1-cm diameter rods of upper and lower Bakken shales were only ca. 55 % after 24 hours (as shown in Figure 1). However, the recoveries from the same shales crushed to pass a 3.5 mm screen approached 95% after 24 hours, demonstrating that there is sufficient pore connectivity in middle Bakken and Three Forks rock, and even in upper and lower Bakken shales, to achieve increased CO₂-enhanced production of oil as well as to achieve CO₂ storage. While these laboratory results are encouraging, a better understanding of the controlling mechanisms is needed to allow exploitation in the Bakken play.  We are currently performing additional rock/CO₂ exposure scenarios and developing models to describe the processes that control oil recovery and potential CO₂ storage.  The results and implications for CO₂ EOR processes in unconventional reservoirs will be presented.

References

Bohrer, M., Fried, S., Helms, L., Hicks, B., Juenker, B., McCusker, D., Anderson, F., LeFever, J., Murphy, E., and Nordeng, S., 2008, State of North Dakota Bakken Resource Study Project, North Dakota Department of Mineral Resources, 23 p.

Nordeng, S.H., and Helms, L.D., 2010. Bakken Source System – Three Forks Formation Assessment, North Dakota Dept. of Mineral Resources, April, 2010.

Continental Resources, Inc., 2012. Bakken and Three Forks, website http://www.contres.com/operations/bakken-and-three forks, accessed May 30, 2013.

Kurtoglu, B., Sorensen, J., Braunberger, J., Smith, S., and Kazemi, H., 2013. Geologic Characterization of a Bakken Reservoir for Potential CO₂ EOR. Paper URTeC 1619698 presented at 2013 Unconventional Resources Technology Conference, Denver, Colorado, USA, 12-14 August 2013.

Hawthorne, S., Gorecki, C., Sorensen, J., Steadman, E., Harju, J., Melzer, S.,  FILLIN "What is your paper title?" \* CHARFORMAT Hydrocarbon Mobilization Mechanisms from Upper, Middle, and Lower Bakken Reservoir Rocks Exposed to CO₂, Paper presented at the 2013 Unconventional Resources Conference, Calgary, Alberta, Canada, 5-7 November, 2013.

Acknowledgments

Financial support from the U.S. Department of Energy, National Energy Technology Laboratories (NETL) Cooperative Agreement No. DE-FE0024454, and the North Dakota Oil and Gas Research Council are also gratefully acknowledged.

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Figure 1:  Recovery of crude oil hydrocarbons from 11-mm diameter rock core samples from a McKenzie County (North Dakota) well with CO2 at 110 C and 5000 psi.