Much special attention has been devoted in this paper to the near critical phenomena of gas condensate systems, including the critical points, bubble and dew point locus, dynamically optical phenomena, and especially the unusual phase transitions in near critical region.

The experimental measurements are carried out during constant composition expansions in high-pressure PVT cell. The percentage-liquid volumes in near critical region are crossplotted to determine the critical points in pressure-temperature diagram. The reddish-brown color is observed in near critical regimes for the fluids due to the complexities of components.

What most fascinated and surprised us is that unusual phase phenomena are unexpectedly observed in the near-critical region. That is to say, during the constant composition expansion process the retrograde dewpoint phase change occurs twice above the critical temperature, and correspondingly retrograde-dew and normal-bubble point phase changes occur sequentially below critical temperature. In two cases, the first-appeared liquid at the bottom of PVT cell first disappears at the same lower-pressure during different temperature CCE processes. These unusual phenomena occur simultaneously for the same fluid at temperature high up to about 100°C. As far as we know, this is the first report of such an observation in the literature.

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