The phase behavior of two Middle East reservoir fluids mixed with CO2 has been studied. The experimental work includes quantification of VLLE/LLE behavior studied in a visual high pressure PVT cell.

To investigate the effect of dispersion on slim tube recoveries, experiments were performed using two different tube sizes. One of the reservoir fluids was found to be miscible with CO2 at the reservoir temperature while a liquid-liquid split persisting at high pressure was observed for the other fluid. As the two liquid phases had approximately the same mobility, a high slimtube recovery was seen for the latter fluid despite the liquid-liquid split. EOS models have been developed matching the swelling and slimtube data and confirming the observed VLLE and LLE phase behavior.

The work has led to an improved understanding of how swelling and MMP data for CO2-crude oil systems is to be interpreted. An important observation is that a miscible drive is not always attainable for a reservoir fluid undergoing CO2 gas injection.

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