American Oil and Gas Reporter - February 2016 - 60

judged accordingly.
The findings strongly suggest that a conventional huff-andpuff approach will not be effective in unconventional formations.
However, some important lessons have been learned.
Water and gas injectivity into various lithofacies, including the
Upper Bakken, has been demonstrated. Production responses to
injection were observed, and while those responses were not necessarily related to higher oil production, they do suggest that fluid mobilization can be influenced. If CO2 or other gases that enhance oil mobility can be injected, and fluid mobilization can be
influenced, developing an effective means of EOR in unconventional formations is possible.
That said, developing an effective EOR approach will require
more field work, and another key lesson learned from the Bakken
tests is that detailed pre- and post-test data on reservoir conditions and fluids production are essential for test and offset wells.
Robust reservoir characterization provides information that is crucial to creating realistic geomodels and conducting valid dynamic simulations of potential EOR scenarios.
This knowledge is essential to designing the operational parameters of injectivity tests and interpreting the results. This is
true for any EOR effort, whether in a conventional or unconventional reservoir.
In the case of unconventional reservoirs, fracture networks are
overwhelmingly the dominant means of fluid movement. As such,
it is clear that fracture complexity and density within the target
reservoir will strongly influence CO2 permeation and hydrocarbon extraction. Fracture systems must be well understood and taken into account when designing an EOR scheme. This can be
achieved by analyzing core, specialized downhole logging, and
microseismic survey data.

Because fracture networks are so important in tight oil formations, core studies should focus on quantifying macro-, microand nanoscale fracture and pore networks, which in turn, will support multifluid permeability analysis. Core studies also can yield
data on CO2 penetration and hydrocarbon extraction rates in the
matrix of the targeted injection zone rock.
A wide variety of log data also should be a major component
of the characterization process. Resistivity microimage, ultrasonic, and dipole sonic logs provide data that can be used for in situ
fracture characterization. Production logs provide insights into
fluid distribution and flow regimes within the reservoir. Advanced
magnetic resonance logs yield data on matrix porosity and permeability, and the nature of fluids. Capture spectroscopy/pulsed
neutron logs generate information on mineralogy, lithology, and
water/oil/gas fluid saturation.
Microseismic data can provide a means of assessing the induced fracture network distribution and stimulated reservoir volume. Pressure monitoring, stimulation fluid communication tests,
stimulation pressure tests, and pulse tests can help determine fluid communication pathways between offset wells. Using tracers
is another reliable and cost-effective means of evaluating movement of fluids within the reservoir, and also offers insight into
scheme conformance.
Well completions will be critical to the success of any EOR
test. Designing the injection well completion needs to be considered in the context of a test's goals. Specifically, the completion
should be compatible with the injection test and monitoring operations. This will support interpreting results, particularly with
respect to understanding where fluids are going.
A detailed understanding of the completions and production
history of offset wells also is necessary for valid test result in-

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American Oil and Gas Reporter - February 2016

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