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The effectiveness of CO2 injection as a Huff-n-Puff process in tight oil reservoirs with complex fractures needs to be investigated due to the fast decline of primary production and low recovery factor. Although numerous experimental and numerical studies have proven the potential of CO2 Huff-n-Puff, relatively few numerical compositional models exist to comprehensively and efficiently simulate and evaluate CO2 Huff-n-Puff considering CO2 molecular diffusion, nanopore confinement, and complex fractures based on an actual tight-oil well. The objective of this study is to introduce a numerical compositional model with an embedded discrete fracture model (EDFM) method to simulate CO2 Huff-n-Puff in an actual Eagle Ford tight oil well. Through non-neighboring connections, the EDFM method can properly and efficiently handle any complex fracture geometries without the need of local grid refinement (LGR) nearby fractures. Based on the actual Eagle Ford well, we build a 3D reservoir model including one horizontal well and multiple hydraulic and natural fractures. Six fluid pseudocomponents were considered. We performed history matching with measured flow rates and bottomhole pressure using the EDFM and LGR methods. The comparison results show that a good history match was obtained and a great agreement between EDFM and LGR was achieved. However, the EDFM method performs faster than the LGR method. After history matching, we evaluated the CO2 Huff-n-Puff effectiveness considering CO2 molecular diffusion and nanopore confinement. The traditional phase equilibrium calculation was modified to calculate the critical fluid properties with nanopore confinement. The simulation results show that the CO2 Huff-n-Puff with smaller CO2 diffusion coefficients underperforms the primary production without CO2 injection; nevertheless, the CO2 Huff-n-Puff with larger CO2 diffusion coefficients performs better than the primary production. In addition, both CO2 molecular diffusion and nanopore confinement are favorable for the CO2 Huff-n-Puff effectiveness. The relative increase of cumulative oil production after 7300 days with CO2 diffusion coefficient of 0.01 cm2/s and nanopore size of 10 nm is about 12% for this actual Eagle Ford well. Furthermore, when considering complex natural fractures, the relative increase of cumulative oil production is about 8%. This study provides critical insights into a better understanding of the impacts of CO2 molecular diffusion, nanopore confinement, and complex natural fractures on well performance during CO2 Huff-n-Puff process in the Eagle Ford tight oil reservoirs.

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Gas injection is one of the most effective enhanced oil recovery methods for the unconventional reservoirs. Recently, CH4 has been widely used; however, few studies exist to accurately evaluate the cyclic CH4 injection considering molecular diffusion and nanopore effects. Additionally, the effects of operation parameters are still not systematically understood. Therefore, the objective of this work is to build an efficient numerical model to investigate the impacts of molecular diffusion, capillary pressure, and operation parameters. The confined phase behavior was incorporated in the model considering the critical property shifts and capillary pressure. Subsequently, we built a field-scale simulation model of the Eagle Ford shale reservoir. The fluid properties under different pore sizes were evaluated. Finally, a series of studies were conducted to examine the contributions of each key parameter on the well production. Results of sensitivity analysis indicate that the effect of confinement and molecular diffusion significantly influence CH4 injection effectiveness, followed by matrix permeability, injection rate, injection time, and number of cycles. Primary depletion period and soaking time are less noticeable for the well performance in the selected case. Considering the effect of confinement and molecular diffusion leads to the increase in the well performance during the CH4 injection process. This work, for the first time, evaluates the nanopore effects and molecular diffusion on the CH4 injection. It provides an efficient numerical method to predict the well production in the EOR process. Additionally, it presents useful insights into the prediction of cyclic CH4 injection effectiveness and helps operators to optimize the EOR process in the shale reservoirs.