Nitrogen on the other hand becomes an efficient miscible displacement only for light oils, temperatures greater than 240 ^oF (115 ^oC) and pressures greater than 5,000 psig, where its density is high enough to extract light-hydrocarbons from the oil.  Flue-gas works well at lower pressure and temperature, since it contains around 13% carbon dioxide.

     The following photographs illustrate a CO2 miscible process for a Permian Basin reservoir.   The experiments were conducted in an etched glass model at 105 ^oF (41 ^oC), at pressures of 1,000, 1,500 and 2,500 psig.  The models were viewed with a stereo-microscope so that each millimeter in the original photograph is 50 microns.

Figure 1 – 1000 psi -  Immiscible Carbon Dioxide

      The gas and oil phases are separate phases– Oil spreads between gas and water - Gas can diffuse into pores and form a isolated gas bubble - This helps displace the oil.

Figure 2 – 1,500 psi – Miscibility Between Gas and Oil Begins to Develop

      A clear Middle Phase forms and spreads between the oil and gas.  The interfacial tensions are very low.  Oil recovery will be high and the displacement will appear to be above the Minimum Miscibility Pressure (MMP).
 
Figure 3 – 2,500 psig – CO2 Has Developed Miscibility with the Oil
 

      Well above the MMP - The miscible fluid is the very light fluid flowing in a channel past the darker bypassed oil.   The miscible fluid is Extracting NGL  from the bypassed oil .  First, the fluid gets lighter, then -

Figure 4 – Lower Liquid Begins to Condense
 

      After some time, the light hydrocarbons have been depleted from the bypassed oil and the Lower Liquid (dark spots) begins to condense from the miscible fluid.  The condensed phase appears to coat the wall of the model (wet),

      As more lower liquid condenses from the miscible bank, the lower liquid phase coalesces into a continuous phase, Figure 5.  The dark lower liquid phase (a heavy-oil containing some CO2) is miscible with the bypassed oil, but immiscible with the CO2 rich gas phase.   The dark lower liquid may wet surfaces because it was enriched in the polar components of the oil when the light components were extracted.  Thus, some gas-oil lamellae form at small pore throats (Figure 5) and move along with the gas.  This form of foam is similar to foamy-oil observed in some heavy-oil projects, and may reduce mobility of the gas.

Figure 5 – Final Stage - Gas Immiscible

      The lower liquid coalesces, forms a continuous phase, and the CO2 behaves like an immiscible gas again.

     The behavior presented in the previous pictures is found for oils where the gas is rich in NGL.  The middle liquid phase, Figure 2, is only found in cooler reservoirs, i.e., Permian Basin but not at greater than 200 ^oF in the Williston Basin.  If the reservoir gas is rich in methane true miscibility (Figure 3) may not develop but most of the oil can be recovered anyway because a high concentration of CO2 will dissolve in the oil before a gas phase appears.