Quantification of CO2 Retention During CO2 EOR: A Laboratory and Numerical Modeling Approach

Abstract In response to the need to reduce greenhouse gas (GHG) emissions, it has been identified that CO2 can be simultaneously sequestered while used to enhance oil recovery (EOR). The objective of this study is to quantify CO2 retention during CO2-EOR. We investigate different parameters that influence CO2 retention and oil recovery to provide recommendations for optimal field implementation. The study also investigates the optimal quantity of natural gas that can be combined with CO2 to achieve optimal oil recovery in CO2-EOR. This study combines laboratory experiments and numerical modeling to achieve the objectives. The laboratory experiments were conducted using dolomite-rich carbonate cores. The experiment involved three stages: fully saturating the core with water, oil drainage to irreducible water saturation, and Water-Alternating-Gas (WAG) flooding. CT scan was used to monitor CO2 retention and fluid saturation. Subsequently, a numerical model was developed to validate the experimental results and ensure credibility. A sensitivity analysis was conducted on key parameters, including the CH4-CO2 mix, injection rate, and injection temperature, to assess their impact on CO2 retention and oil recovery. The numerical model successfully reproduced laboratory trends and provided information about free-gas banking, oil recovery, and CO2 retention. For gas composition, mixtures with ≥40% CO2 yield similar recoveries, but the gas composition with 60% CO2 achieved the highest oil recovery at the expense of the lowest CO2 retention due to a large mobile gas bank. A practical balance between oil recovery and CO2 retention was obtained with an 80% CO2/20% CH4 gas blend. For temperature, a clear trade-off emerges: 48.8 °C maximizes recovery but minimizes CO2 retention, 30 °C maximizes retention but has poor recovery, and 60 °C appeared to be the most favorable condition for a balance between oil recovery and CO2 retention. For rate, with a two-hour injection window, higher throughput dominates. Doubling the rate from the base case yields the highest recovery and the largest absolute storage, while reducing the base case rate by half yields the highest CO2 retention efficiency but low oil recovery. Overall, the study demonstrates that operational factors such as injected gas mixture ratio, temperature, and injection rate, govern the balance between oil recovery and retention. The use of a model informed by laboratory studies provides a pathway for providing actionable guidance for designing field programs that co-optimize incremental oil and CO2 storage, thus advancing practical decarbonization within CO2-EOR projects.