(736d) 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.

\s

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.