Effectiveness of miscible and immiscible gas flooding in recovering tight oil from Bakken reservoirs in Saskatchewan, Canada

Oil production from unconventional tight Bakken reservoirs in North America has been booming since the early 2000s because of the widespread application of horizontal well drilling and multi-stage hydraulic fracturing technologies. Tight oil recovery is made difficult by the extremely low permeability and porosity of the oil-hosting rocks. Two key characteristics in tight oil production are the steep decline in oil production rate and the low ultimate oil recovery. Therefore, maintaining oil production and improving a field project's economic benefits become urgent issues for the development of tight oil resources. The substantial amounts of residual oil left behind in tight reservoirs demand a viable and effective enhanced oil recovery (EOR) technology after the primary recovery process. This research investigated the effectiveness of miscible or immiscible gas flooding processes using four possible injection gas candidates (i.e., CO2, CO2-enriched flue gas, oilfield-produced gas, and nitrogen) for Bakken tight oil reservoirs in southeast Saskatchewan, Canada. The phase behaviour of the reconstituted live oil-injection gas systems was studied through pressure/volume/temperature (PVT) tests in which important recovery mechanisms of gas flooding, i.e., viscosity reduction and oil swelling effects, were revealed. The minimum miscibility pressures (MMP) for the live oil-injected gas systems were determined using the rising bubble method. The experimental results demonstrated that CO2 had the lowest MMP and largest viscosity reduction and oil swelling among all the tested gases. Therefore, CO2 flooding is considered technically feasible as a miscible process at the current reservoir conditions, while the use of the other three gases would result in an immiscible process. Finally, four coreflood tests using reservoir cores from the Viewfield Bakken field were conducted to evaluate the displacement efficiency. Each coreflood comprised four stages (pressure depletion, initial waterflood, gas injection, and extended waterflood) to represent the actual field production practice. Oil recovery performance confirmed that CO2 miscible flooding was significantly more effective than immiscible flooding with the other injection gases.