After the technology advances in horizontal drilling and hydraulic fracturing, unconventional plays came to the foreground of the oil and gas industry. Although there are vast amounts of original oil in place in unconventional plays, the recovery factor remains very low. It is imperative to find the solution to unlock the billions of barrels left behind after the primary depletion phase. In unconventional plays, all the previous studies showed that, due to the unfavorable reservoir conditions, secondary recovery methods, such as water-flooding, are not feasible. After the natural depletion stage, only tertiary oil recovery techniques can be considered. In this study we investigated the potential of the CO2 enhanced oil recovery technique in one of the largest tight oil-bearing formations in North America. We used four samples to test CO2 H-n-P under typical reservoir temperature of 105°C and different injection pressures. The experimental results showed that increasing the soaking time from 3 hours up to 17 hours increased the recovery factor from 4% and 9% up to 42% and 70 % for middle Bakken and Three Forks plugs, respectively. Allowing the sample to soak more than 24 hours resulted in additional oil recovery of 15% and 9% of the original oil in place for middle Bakken and Three Forks, respectively. Moreover, after increasing the injection pressure 500 Psi above the MMP, the recovery factor increased from 25% and 45 % to 62% and 84% for middle Bakken and Three Forks samples, respectively.
The production from tight reservoirs became possible and economically efficient after the development of hydraulic fracturing and horizontal drilling. The Bakken is one of the largest oil-bearing tight formations in North America that covers parts of the United States in Montana and North Dakota and parts of Saskatchewan and Manitoba in Canada (Fig. 1). Oil was initially discovered in the Bakken in 1951, but with a very limited production capacity before a tremendous oil production increase took place in 2006 (Fig. 2).
Different studies yielded different values of original oil in place in the Bakken petroleum system and the estimated recovery factor is between 5 and 10%. Clark (2009) estimated the recovery factor in Mountrail County to be 7%. With such low recovery factor, Bakken is a potential target for enhanced oil recovery techniques.