Evaluation of CO2 enhanced oil recovery in unconventional reservoirs: Experimental parametric study in the Bakken

Previous CO2 enhanced oil recovery (EOR) pilot tests performed in the Bakken Formation indicated that cyclic CO2 injection might be a promising technique for enhanced oil recovery; however, no clear consensus has been reached, and reported results have revealed that CO2 EOR mechanisms in unconventional reservoirs are still poorly understood. This research investigates and analyzes the effects of different parameters on CO2 performance using samples from the Middle Bakken member and Three Forks Formation. The factors studied include sample size, the CO2 Huff-n-Puff (HnP) injection schedule, water presence within the fractures, and the volume of CO2 in contact with the rock matrix during the HnP experiments. Results indicated that reducing the sample size could overestimate the oil recovery factor and water presence could impede the CO2 potential to permeate the rock. Lowering the CO2 volume in contact with the tested sample resulted in lower oil recovery for the same cores. We also observed that cyclic injection outperforms CO2 flooding in fractured Middle Bakken and Three Forks samples. The obtained results provide a better understanding of CO2 EOR performance in tight formations and enlighten future pilot tests and experimental CO2 injection studies in unconventional reservoirs.