Page 358 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Enhanced Oil Recovery Methods 345

and that the MMP increased with temperature. Some workers have shown that
a better correlation is obtained with the molecular weight of the C,+ fraction
of the oil than with the API gravity. In general the recent work shows that the
required pressure must be high enough to achieve a minimum density in the
CO, phase [395,396]. At this minimum density, which varies with the oil com-
position, the CO, becomes a good solvent for the oil, especially the intermediate
hydrocarbons, and the required miscibility can be generated or developed to
provide the efficient displacement normally observed with CO,. Therefore, at
higher temperatures, the higher pressures are needed only to increase the CO,
density to the same value as observed for the MMP at the lower temperature.
Figure 5-175 shows the variation of minimum miscibility pressure with tem-
perature and oil composition [397].
Although the mechanism for CO, flooding appears to be the same as that
for hydrocarbon miscible floods, CO, floods may give better recoveries even if
both systems are above their required miscibility pressures, especially in tertiary
floods. Compared to hydrocarbons, CO, has a much higher solubility in water,
and it has been observed in laboratory experiments to diffuse through the water
phase to swell bypassed oil until the oil is mobile. Thus, not only are the oil
and depth screening criteria easier to meet in CO, flooding, but the ultimate
recovery may be better than with hydrocarbons when above the MMP. It must
be noted, however that this conjecture has not been proved by rigorous and
directly comparable experiments.
I
- CORRELATION OF DATA GATHERED FROM
HOLM and JOSENDAL, M.SILVA AND

5000 -

4000 -

n
3000-
5

2000 -

Figure 5-175. Correlations for CO, minimum miscibility pressure [397].