Observations from pilot wells along with laboratory experiments have revealed the significant potential of CO2 as an EOR agent in unconventional liquid reservoirs (ULR). This study focuses on unveiling the mechanisms of gas injection EOR through a combination of experimental results, ternary diagram analysis, and core-scale simulation. In addition, laboratory results were upscaled to the field-scale to evaluate the effectiveness of the CO2 injection in production enhancement from ULR.

Gas injection experiments were performed at different pressures, and the laboratory results were upscaled to evaluate the production enhancement through gas injection EOR in ULR. A CT-generated core-scale model was utilized to investigate the mechanisms of gas injection EOR. Mechanisms such as diffusion and multi-contact miscibility were determined from core-scale simulation through history-matching experimental results, then upscaled to the field-scale model. Ternary diagrams reveal that EOR by gas injection is only effective at pressures greater than the Minimum Miscibility Pressure (MMP). Alteration of the injected gas and composition of crude oil clearly has an implication on changing the ternary diagram.

The primary production mechanisms of CO2 EOR are multi-contact miscibility, vaporizing/condensing gas drive, oil swelling, and diffusion. Gas injection experiments recovered up to 45% of the Original Oil In Place (OOIP) at 3,500 psi, but the recovery factor was less than 5% when operating below the MMP. Diffusion has a minor effect in enhancing oil recovery in ULR based on the core-scale history-matching results. The multi-contact miscibility is found to be the primary driving mechanism for oil extraction during gas injection. Ternary diagrams analysis clearly demonstrates that MMP plays a significant role in gas injection and that miscible conditions need to be achieved for EOR projects in ULR. CT-scan technology is utilized to demonstrate the movement of the fluids inside the cores throughout the experiments. Thus, we can determine the high flow path regions of the core plugs. Additionally, the impact of injection pressure and the start time of the gas injection process were analyzed using the field-scale model. The simulation results indicate that gas injection has significant potential of enhancing oil production in ULR. This study not only reveals the mechanisms of gas injection in ULR, but also provides a method for designing and optimizing gas injection for Huff-n-Puff EOR.

This study challenges the paradigm that diffusion is the dominating parameter of CO2 injection EOR in ULR. The novelty comes from the establishment of gas injection EOR mechanism in ULR through a thorough analysis of laboratory experiments, core-scale simulation, and ternary diagram analysis. In addition, a new modeling workflow for the design of gas injection strategies is proposed to unveil the real potential of gas injection.

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