Summary

Tight oil formations are known to hold hundreds of billions of barrels of oil in place; however, the primary recovery factor for these plays is typically less than 10%. Tight oil formations, therefore, may be attractive candidates for enhance oil recovery (EOR) using CO2. Multiphase fluid behavior and flow in fluid-rich shales can vary substantially depending on the size of pore throats, and properties such as fluid viscosity and density are much different in nanoscale pores than in macroscale pores. To design effective CO2 injection and EOR schemes, it is necessary to better understand fluid permeation and transport within tight reservoirs. Thus it is critical to understand the nature and distribution of nano-, micro-, and macroscale pores and fracture networks. A combination of advanced computer tomography (CT) imaging and scanning electron microscopy (SEM) techniques, including whole-core and micro x-ray CT imaging, field emission (FE)–SEM, and focused ion beam (FIB)–SEM, have been applied to characterize 26 samples of Bakken reservoir rock and shales. X-ray CT imaging yielded detailed information on the occurrence of fractures, bedding planes, fossils, and bioturbation in core. The CT images were calibrated to produce bulk density and photoelectric factor logs, which were used to interpret porosity, organic content, and mineralogy. FE-SEM is capable of 1,000,000× magnification with a spatial resolution of 1.2 nm and was used for characterization of nano- and microscale features, including determination of clay occurrence and grain geometries, nanoscale pore visualization, micropore and pore throat mineralogy, and nano- and microfracture imaging and analysis. FIB-SEM was used to develop 3-D volumes of selected areas of interest that yielded visualization of fracture networks, porosity and pore-size distribution, connected versus isolated porosity, and distribution of organics and mineral phases. Results provide previously unavailable insight on nanoscale fracture apertures, intensity and orientation; pore throat mineralogy and connectivity; rock matrix characteristics, mineralogy, and organic content; and calculated absolute permeability in the vertical and horizontal direction. These results are being integrated into laboratory and modeling research activities to determine the fundamental mechanisms controlling fluid transport in the Bakken, which will support EOR scheme design and estimation of CO2 storage potential in tight oil formations.

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