Miscibility Development and Miscibility Pressure

Miscibility development in CO2-oil systems is of multiple contact (MCM) type in that a

few contacts are required for miscibility to be developed. First contact miscible (FCM) CO2 flooding is not operationally achievable as the required pressure would be very high.

A CO2 swelling experiment at 212°F with the above fluid description showed that

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Figure 2.7 (left) shows the development of miscibility in a slimtube simulation at 2500psi and 212°F. The slimtube model parameters have been depicted in Table 2.5. This 1D slimtube model will be used for the majority of the 1D simulations conducted in this Chapter. The slimtube model permeability is high enough (4000mD) to minimise pressure variations across the model. CO2 injection velocity was also adjusted in

accordance with the recommendation of Yellig and Metcalfe (Yellig & Metcalfe 1980).

Note that upon miscibility development, oil and gas densities approach each other, though they never converge completely (Figure 2.7-left). This is because of dispersion. Multiple contact miscibility development is sensitive to the level of dispersion which always exists in any system. The dispersion in this slimtube simulation is only due to numerical gridding. Figure 2.7 (right) shows the corresponding evolution of k-values as they approach unity upon miscibility development; implying existence of only one phase.

Table 2.5: Slimtube model parameters

Grid 500×1×1

Grid Dimensions 0.1ft × 0.1ft × 0.1ft Average horizontal permeability 4000mD

Porosity 0.25

Temperature 212°F

MMP 2400psi

Well locations Injector on the left, producer on the right

Fluid model Table 2.1

Relative permeability model Table 2.3

Figure 2.7: Left: oil and gas densities after 0.01, 0.05 and 0.1 HCPV at 2500psi and 212°F. Right: equilibrium K-values at 0.5PV CO2 injection; individual colours represent each of the

seven components k-values. Model properties have been depicted in Table 2.5.

Figure 2.8 (next page) shows the predicted miscibility pressure after injecting 1.2PV of CO2 in the above slimtube models at two different temperatures of 113°F and 212°F. The

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estimated minimum miscibility pressures are around 1200psi and 2400psi (the point where two tangents intersect) for the 113°F and 212°F temperatures, respectively.

While these figures show that at high flooding pressures recovery factor approaches 100% in both models, in reality the ultimate slim-tube recoveries never attain 100% (Stalkup 1983). Some factors are responsible for this, including the wall effects, dead end pores and dispersion which were not taken into account for these slimtube simulations.

A significant difference for miscibility developments between the two representative temperatures is the onset development of miscibility at each respective temperature. The rapid onset development of miscibility at 113°F compared to 212°F is due to the impact of a smaller transition zone between CO2 and oil at lower temperatures. This behaviour

has also been observed experimentally by Yellig and Metcalfe (Yellig & Metcalfe 1980) which indicates that at lower temperatures, miscibility development could be more sensitive to pressure variations. In other words, the miscibility development in a CO2-

EOR process in the Permian Basin is likely more sensitive to pressure variations than in the North Sea. Transition between miscibility and immiscibility affects the balance between microscopic and macroscopic sweep efficiencies and injectivities as will be shown later.

Figure 2.8: Minimum miscibility pressure estimated at 113°F and 212°F by slimtube simulations

Figure 2.9 shows oil viscosity and saturation profiles after injecting 0.4HCPV CO2 in the

above slimtube model at 2400psi and 212°F; oil viscosity ahead of the CO2 gas front has

been significantly reduced from slightly below 1 to 0.2. This phenomenon has also been observed by other researchers (Gardner et al. 1981, Mungan 1982). The stepwise change

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of oil viscosity behind the CO2 front is due to the chromatographic evaporation of oil

components as further CO2 is injected.

Figure 2.9: Oil viscosity and saturation after 0.4HCPV CO2 injection (212°F) in the slimtube

described above (Table 2.5).

Holm & Josendal (1974) reported that the formation of a methane bank ahead of the CO2

front could be an indication of immiscible displacement. Figure 2.10 shows the effluent methane and CO2 concentrations for two slimtube simulations conducted below and

above minimum miscibility pressure at 212°F (MMP=2400psi); a bank of methane can be identified prior to CO2 breakthrough at lower than miscibility pressure.

Figure 2.10: Outlet methane and CO2 mole fractions in two slimtube simulations below (left)

and above (right) minimum miscibility pressure.

The presence of impurities such as methane can significantly increase the CO2 minimum

miscibility pressure. The effect is, however, different at different temperatures. This can affect the need for CO2 separation and recycling should the methane presence

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In this regard, Figure 2.11 compares the results of MMP measurements with mixtures of CO2 and methane at two different temperatures (113ºF and 212ºF), representative of the

Permian Basin and North Sea provinces. The MMPs are measured by Winprop (CMG- WinProp 2014.10) with the method developed by Ahmadi et al. (2011). It can be seen that the difference in the MMPs become progressively smaller as the presence of methane (or impurities) in the CO2 stream increases. This result indicates two things; first, the

measured MMPs are less sensitive to temperature at high concentrations of impurities. Second, the impact of the presence of methane is less considerable at elevated temperatures; note that the difference between pure CO2 and pure methane MMPs is

around 3462psi at 113ºF, while at 212ºF it is only 2037psi.

Figure 2.11: Impact of methane on the measured CO2 MMP at different temperatures,

numbers show MMPs at the corresponding conditions of temperature and methane mole fraction.