The Analysis of Compositional Effects on Global Flow Regimes in CO₂ Near-Miscible Displacements in Heterogeneous Systems

This study investigates how compositional effects interact with the flow behavior during near miscible (and immiscible) CO₂-oil displacements in heterogeneous systems. A series of numerical simulations modeling 1D slim-tube and 2D areal systems were performed using a fully compositional simulator. With negligible numerical dispersion, the fine-scale (Δx=0.005m) slim-tube simulations were performed to provide the "truth case" in terms of the compositional effects and oil/component recovery. A number of grid resolutions were tested to examine cell-size effects on the simulation accuracy. It was found that coarse cell size not only leads to spreading of the displacing front, but also lowers the displacement efficiency by reducing the component stripping effects, as noted by Orr (2007). The corresponding 2D cases are based on a small heterogeneous sector model of dimensions 50m × 10m, in order that the finest scale displacement physics can be modelled accurately. We investigated various flow regimes ranging from viscous fingering to channeling displacements within heterogeneous random correlated fields. CO₂ dissolves in oil at near-miscible conditions and improves the mobility of the oil, but leads to earlier breakthrough of CO₂ in both fingering and channeling flow. It was also found that the instability of fingering flow could introduce considerable variation in the composition paths during oil displacement by CO₂, compared with the slim-tube simulations, particularly in the later stages of the flood. For this reason, heavier component recovery is more likely to be affected and reduced by viscous instability. In the case of channeling flow, compositional effects were less important since the permeability channel dominated the displacement. Both the ultimate oil recovery and component recovery are significantly and about equally reduced, when the underlying heterogeneity is of dominant influence.

To summarize, compositional effects can have a very significant impact on the prediction of near-miscible CO₂ EOR projects. Issues such as front stability, local displacement efficiency and formation of fingering/channeling during CO₂ near-miscible displacement can lead to behavior that is significantly different from immiscible flooding in these systems. The process of mass transfer between CO₂ and oil can be hampered to a certain degree by unstable flow depending on the level of heterogeneity. This leads to a further reduction in component recovery, particularly of the heavier components. Lastly, the appropriate upscaling methods considering mass transfer still require further investigation for CO₂ near-miscible displacement in field-scale applications. The complete dataset and results of this study are available online as a model case example for testing out potential upscaling techniques for compositional flows in heterogeneous systems (Wang et al. 2019).