Understanding reservoir rock characteristics and the forces that mobilize oil in unconventional reservoirs are critical in designing oil recovery schemes. Thus, we conducted laboratory experiments in three preserved Middle Bakken cores using centrifuge and nuclear magnetic resonance (NMR) instrument to understand oil recovery mechanisms in the Bakken. Specifically, we measured capillary pressure, pore size distribution, and oil and brine distributions.

A series of oil and brine replacement experiments (drainage and imbibition) were conducted in the preserved cores using a high-speed centrifuge. T2 time distribution and one-dimensional saturation profile measurements were obtained using a 2-MHz nuclear magnetic resonance instrument before and after centrifuge experiments. Moreover, pore size distribution was determined from Mercury Intrusion Capillary Pressure (MICP) and Nitrogen Gas Adsorption experiments. We conducted scanning electron microscope (SEM) imaging on polished core slabs to determine pore shapes and mineralogy of pore walls using a field emission-scanning electron microscope (FE-SEM).

Our measurements show that, contrary to the common notion, the preserved Middle Bakken cores are not oil-wet—but show mixed-wet characteristics. Water resides in smaller pores and oil resides in larger pores in all experiments. Using a low-salinity synthetic brine of 50,000-ppm to surround Bakken cores of much higher salinity, we produced up to 6.33 % (of pore volume) oil from two higher porosity (~8%) cores, and 10.72% (of pore volume) oil from one lower porosity (~2%) core in spontaneous imbibition experiment. Moreover, up to 6.62 % (pore volume) oil from the two higher porosity cores and 11.23% (of pore volume) oil from the lower porosity core was produced in forced imbibition experiment. These experiments indicate that chemical osmosis overrides the wettability effects in tight Middle Bakken cores. The new observations regarding osmosis have altered our classical notion of capillary imbibition in shale reservoirs.

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