CO2 huff-n-puff has been identified as a promising method for enhancing oil recovery in unconventional formations. However, the technique has not gained industry-wide adoption partly due to knowledge gaps in several aspects of this method. One such aspect is understanding the influence of critical operating parameters on oil recovery and gas utilization and their optimization. The purpose of this work is to study the impact of five critical operational parameters, namely, injection rate, injection period, soak period, production period and production bottom-hole pressure, on oil recovery and gas utilization. This work leverages compositional reservoir simulation and response surface methodology (RSM) for this purpose. The use of RSM enables the evaluation of the statistical significance of the influencing parameters and, most importantly, their interactions, which has often been neglected in the current literature. We also applied numerical and graphical techniques to find the combination of operating parameters that maximize oil recovery and gas utilization. The Bakken formation is used as a study area. Our findings suggest that the production bottom hole pressure (BHP) and production period are the most influencing parameters on oil recovery. Within our factor design space, the injection rate and period have little influence on oil recovery and may reduce gas utilization when increased. Contrary to several reports in the literature, increasing the soak period negatively influences the oil recovery factor. In addition, we unraveled several significant interactions among these parameters. Finally, numerical optimization indicates that CO2 huff n-puff in the Bakken formation can yield more than 7% incremental recovery with a gas utilization factor of more than 2 bbl/Mscf. We believe the findings in this work can better guide operators to design efficient CO2 huff-n puff strategies in the Bakken formation and other unconventional reservoirs.
