Numerical simulation on CO2-EOR and CO2 sequestration in shale oil reservoir

• CO₂-EOR showed higher recovery than waterflooding or nitrogen in the Bakken. • CO₂ methods boosted oil recovery by ∼18,9% over natural depletion in the Bakken. • Fractured wells saw a 27.5% recovery, showing strong synergy with CO₂ injection. • Fracture conductivity and injection pressure are the dominant control factors. Shale oil formations across North America, notably the Bakken, hold vast hydrocarbon resources estimated between 100 and 900 billion barrels, yet their primary recovery rarely exceeds 10%, underscoring the need for more effective recovery techniques. This study presents a high-resolution compositional reservoir model designed to assess both enhanced oil recovery (EOR) and CO₂ storage potential in the Bakken shale of the Williston Basin. The model integrates realistic geological parameters, including depths of 2,743–3,200 m, porosity of 5–10%, and matrix permeability of 0.01-0.1 mD. Five recovery strategies were simulated : natural depletion, water flooding, nitrogen injection, cyclic CO₂ (huff-n-puff), and continuous CO₂ flooding under both fractured and unfractured well configurations. CO₂-based methods demonstrated superior efficiency, with cyclic and continuous injection yielding 16.8% and 19.2% increases in recovery, respectively, compared to natural depletion. Fractured wells achieved markedly better outcomes, averaging 27.5% of original oil in place (OOIP) for huff-n-puff compared to 18.9% for unfractured wells, while also improving CO₂ storage efficiency (76.5% vs. 61.2%). Trapping mechanisms were partitioned among residual (14.3%), adsorption (11.7%), and solubility (21.4%) processes. Sensitivity analysis highlighted fracture conductivity and injection pressure as dominant factors influencing storage performance. Monte Carlo simulations confirmed the model ' s robustness, producing a mean incremental recovery of 16.8% (95% CI: 15.9–17.7%) and an average storage ratio of 74.2%. Overall, this study demonstrates how to optimize recovery in unconventional reservoirs while promoting partial CO₂ retention and a potentially net-negative carbon outcome, depending on surface-system efficiency and recycling losses.