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Enhanced oil recovery (EOR) processes using CO2 in tight unconventional plays like the Bakken Formation are expected to be very different from the processes which control EOR in conventional reservoirs. During CO2 EOR in conventional reservoirs, CO2 flows through the permeable rock, and the minimum miscibility pressure (MMP) is an important operational parameter for achieving a successful "miscible" flood. In contrast, in tight fractured systems like the Bakken, CO2 flow may be dominated by fracture flow, and not by CO2 flowing through the rock matrix as in a conventional reservoir flood. Since fracture-dominated CO2 flow could essentially eliminate the "flushing" mechanisms responsible for increased recovery in conventional reservoirs, operation at or slightly above MMP may or may not be relevant for the success of an EOR flood in such tight fractured reservoirs. To investigate this concept, capillary-rise vanishing interfacial tension (VIT) was used to measure MMP values for a typical Bakken crude oil (API gravity 41.5) with CO2, methane, and ethane at 110°C (230°F) typical for the reservoir. The effect of these different fluids, as well as the effect of pressures at, above, and well above MMP on recovering crude oil hydrocarbons was determined for small rock core samples from the productive Middle Bakken laminated zone as well as from Upper and Lower Bakken samples. Compared to the MMP value for CO2 of 2520 psi, MMP with methane nearly doubled at 4510 psi, but was nearly cut in 1/2 for ethane at 1360 psi. The recovery of crude oil hydrocarbons from both the Middle Bakken and Lower Bakken shale samples with 24-hour exposures to these fluids at reservoir pressures showed efficiencies that parallel the MMPs determined with each fluid; i.e., ethane yielded faster and more efficient recovery of the crude oil than CO2, but both CO2 and ethane were much more efficient than methane at recovering the crude oil from the 11-mm round rod rock samples. Although hydrocarbon recoveries from the rock samples paralleled each injectant's respective MMP values, extractions with CO2 at the MMP, and at ca. double and triple the MMP pressure showed much more efficient crude oil recoveries at higher pressures from both the Middle Bakken and Lower Bakken shale, demonstrating that EOR pressures much higher than the MMP could substantially increase oil recoveries in tight unconventional systems like the Bakken.

Over 40 rock samples were obtained from six Bakken wells which penetrate through the major oil pay including two shale intervals: Upper and Lower Bakken, and two tight intervals that are the targets for drilling: Middle Bakken and Three Forks. Detailed petrographic and petrophysical analyses were performed on the samples to better correlate the extraction results with the physical and geochemical properties of the rocks. Round rods (11.2-mm diameter X ca. 30–40 mm long) drilled from each of the 40 samples were individually exposed in a "bath" of CO2 for 24 hours at reservoir temperature and pressure of 5000 psi and 230°F (34.5 MPa, 110°C), and the recovered crude oil hydrocarbons were collected periodically and analyzed to determine the rates and efficiencies of oil recovery. For the 26 Middle Bakken and Three Forks rocks, hydrocarbon recovery upon CO2 exposure averaged 86% after 7 hours, and 99% after 24 hours. Recoveries of the crude oil (not including kerogen) from the 15 Upper and Lower shales were surprisingly high with an average of 30% recovered after 7 hours, and 50% recovered after 24 hours. While the Middle Bakken and Three Forks TOC values were ca. 0.3 wt.% (similar to their crude oil content), TOCs for the Upper and Lower Bakken shales were typically 10 to 15 wt.%, with ca. one-tenth of that organic content being crude oil hydrocarbons as opposed to kerogen. The Upper and Lower shales also had significantly smaller pore throat sizes (averaging ca. 3 nm) than the Middle Bakken and Three Forks samples (which averaged ca. 10–26 nm). Additional studies are being performed to determine whether the small pore throat sizes (which approach molecular dimensions) and/or the sorption of crude oil hydrocarbons onto the kerogen in the Upper and Lower shales are responsible for the slower hydrocarbon recovery than that achieved from the Middle Bakken and Three Forks rocks under CO2 exposure. Currently, the main targets for horizontal drilling are Middle Bakken and Three Forks, where thousands of multistage hydraulically fractured wells have been drilled in the past decade. The high oil recovery factor observed in cores from these intervals, especially when compared to the 7% average recovery in the field, indicates the huge potential for oil recovery factor improvement in these units by increasing oil production based upon supercritical CO2 extraction.

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