Numerical Analysis of CO2 Storage Associated with CO2-EOR Utilization in Unconventional Reservoirs

Carbon dioxide (CO2) emissions resulting from natural gas flaring are significant contributors to atmospheric greenhouse gases, posing a substantial risk to the Earth’s climate by exacerbating global warming. As a response, both the oil industry and government authorities are actively exploring cost-effective strategies to address this issue through carbon capture, utilization, and storage (CCUS), as well as reducing natural gas flaring and CO2 leaks in the oil fields to mitigate the adverse consequences of greenhouse gas emissions. This study presents a numerical investigation of CO2 utilization for enhanced oil recovery (EOR) and associated CO2 retention in unconventional reservoirs, using the Bakken Formation as a representative case. A compositional reservoir model is developed to simulate CO2 Huff-n-Puff (HnP) processes in a fractured horizontal well. The model incorporates dual-porosity and dual-permeability formulations, fluid–rock interactions, and an equation-of-state-based compositional framework to capture multiphase flow behavior. Key operational parameters, including reservoir pressure, injection rate, injection duration, and CO2 molecular diffusion, are systematically evaluated to assess their impact on oil recovery and CO2 retention. The results show that lower bottom-hole pressures enhance oil recovery through increased drawdown, while operating pressures near the minimum miscibility pressure (MMP) improve CO2 solubility and overall retention. Extended injection durations and higher diffusion coefficients increase CO2 dissolution in the oil phase but exhibit diminishing marginal benefits beyond an optimal injection time. The study quantifies residual and solubility trapping mechanisms during the operational timeframe of CO2-EOR and provides mechanistic insights into optimizing CO2-HnP performance in tight formations. The proposed framework establishes a technical basis for integrating CO2-EOR with emission mitigation strategies in unconventional reservoirs.