Multiple-Contact Miscibility

Key Takeaways

• The minimum miscibility pressure (MMP) is the critical design parameter that determines whether a gas injection project achieves miscible displacement and its associated high recovery — below the MMP, injected gas and reservoir oil remain as separate phases separated by interfacial tension, and displacement is immiscible, limited by viscous fingering and capillary trapping of residual oil; above the MMP, the multi-contact enrichment process drives the displacement front to miscibility, effectively eliminating the interfacial tension between oil and solvent and reducing residual oil saturation to near zero in the swept volume; MMP determination is typically done in the laboratory using slim-tube tests (where the gas is injected through a coiled tube packed with reservoir sand, and recovery versus pressure is plotted to identify the inflection point where additional pressure no longer increases recovery significantly, indicating that the MMP has been reached) or rising bubble apparatus tests; accurate MMP determination is critical for CO2 and gas injection project design because insufficient injection pressure (below MMP) delivers immiscible displacement with much lower recovery, while excess pressure above MMP wastes compression energy without improving recovery further.
• The vaporizing drive MCM mechanism dominates in CO2 injection into light and intermediate crude oils — in vaporizing drive MCM, the injected CO2 (which is initially too lean in heavy components to be miscible with the oil) contacts fresh reservoir oil at the displacement front and vaporizes the C2-C6 intermediate components from the oil phase into the CO2-rich phase; this progressively enriches the CO2 front in heavier components, moving the gas composition toward the critical point where oil and gas become indistinguishable; simultaneously, the oil left behind by the stripping process loses its intermediate components and becomes heavier and less compatible with the advancing enriched CO2 front; the key variable is the C2-C6 content of the reservoir crude oil — oil with higher intermediate component content achieves MCM at lower pressure (lower MMP), while very heavy or asphaltic crudes with low C2-C6 content may require extremely high pressure to achieve MCM with CO2 or may not achieve MCM at reservoir conditions at all.
• Condensing drive MCM is the dominant mechanism for enriched gas injection with C2+ solvents — in condensing drive MCM, the injected gas (which is enriched in intermediate-to-heavy components such as propane, butane, and C5-C7 hydrocarbons) contacts the reservoir oil and condensable components from the gas transfer into the oil phase, enriching the oil and moving its composition toward the critical point from the oil side; the gas phase correspondingly loses its heavy components, becoming leaner and less miscible, but the enriched oil at the displacement front can eventually become fully miscible with the injected gas; enriched gas injection (adding C2-C4 to lean gas) is used when the reservoir pressure is insufficient to achieve miscibility with lean gas alone, because the addition of heavier components to the injection gas lowers the MMP to within the achievable injection pressure range; this approach is more expensive than lean gas injection but enables miscible displacement in reservoirs where it would otherwise be impossible.
• Gravity override and viscous fingering are the primary mechanisms that reduce MCM sweep efficiency despite the elimination of residual oil at the pore scale — MCM eliminates capillary trapping of residual oil at the pore scale, potentially enabling very high microscopic displacement efficiency; but MCM injection projects achieve overall recovery well below the theoretical maximum because the gas (which is much less dense and less viscous than reservoir oil) tends to override the oil column under gravity (gravity override) and to finger through the oil in high-permeability streaks (viscous fingering due to the unfavorable mobility ratio where gas mobility far exceeds oil mobility); these macroscopic sweep efficiency problems limit actual MCM project recoveries to typically 30-60% of original oil in place despite the near-perfect pore-scale displacement; water-alternating-gas (WAG) injection mitigates these effects by alternating water slugs with gas slugs, with water improving sweep efficiency and reducing gas mobility while the gas slugs maintain miscibility conditions at the displacement front; WAG design optimization is a major focus of MCM EOR project engineering.
• CO2 storage in depleted oil reservoirs combines MCM EOR with carbon sequestration objectives — at reservoir conditions above the MMP, injected CO2 dissolves extensively into the reservoir crude oil (reducing oil viscosity and swelling the oil to improve recovery — additional mechanisms that contribute to CO2 EOR beyond miscibility alone), contacts the reservoir brine (dissolving to form carbonic acid), and is adsorbed on clay and organic surfaces in the reservoir rock; when CO2 injection continues beyond the economic EOR phase, the residual CO2 in the reservoir pore space, the CO2 dissolved in the formation brine, and the CO2 retained through mineral trapping (slow geochemical reactions forming stable carbonate minerals) represent a permanent geological CO2 storage mechanism; the Permian Basin CO2 EOR industry, which has injected CO2 into West Texas oil fields since the 1970s using natural CO2 sourced from fields in New Mexico and Colorado, currently stores approximately 30-40 million tonnes of CO2 per year in producing oil reservoirs — demonstrating at scale the concept of combined CO2 EOR and geological storage that is central to many carbon capture and storage project designs.