A miscibility study was conducted to investigate the effects of the C5+ content in injection gases on Minimum Miscibility Pressure (MMP). Currently available common MMP correlations are based on data from injection gases containing no C5+ fraction. However, laboratory analyses suggest that having a small (and yet relatively significant) C5+ fraction could have a large effect on reducing the MMP to considerably below the predicted values from existing correlations. This paper aims at investigating possible benefits for miscibility and CO2 EOR flooding with impurities, particularly with C5+ fraction.

Previously, a PVT and miscibility study was conducted on a Cooper Basin (located in central Australia) reservoir fluid, including solubility-swelling and viscosity studies. Slim tube tests were performed using two injection gases – pure CO2 and a CO2-rich synthetic gas (80mol% CO2), which contained a small amount of C5+ fraction (0.3mol%). The MMP was expected to increase substantially due to the presence of a large amount of methane (15mol%). Correlations from literature predicted the MMP to be above 3433psia, but the MMP was measured to be 2880psia1 . As most currently available correlations are based on injection gases with no C5+ fraction, a major contributing source to error with these correlations may have been the C5+ content of the gas.

In order to investigate the effects of the C5+ fraction in the injection gas, a series of Rising Bubble Apparatus (RBA) experiments were conducted on the reservoir fluid with three injection gases at three different temperatures to investigate the effect of C5+ in the injection gas stream.

Results indicate that the C5+ fraction does have a significant effect on the MMP that needs to be factored into current correlations when the injection gas has a C5+ content.

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