Experimental Investigation into Near-Critical Phenomena of Rich Gas Condensate Systems

Abstract The petroleum exploration pushes the frontier to deeper strata with more near critical fluids. However, the documented studies on near critical behavior of complex fluids are absent, especially for unsymmetrical multi-component and real reservoir fluids. Thus the phase behavior of near critical systems has attracted considerable attention. This paper presents some experimental results of near critical phenomena of three rich gas condensate systems. The measurements are performed with mercury-free PVT equipment. The Lab. fluids consist of one six-component hydrocarbon mixture and two field samples. The results presented contain the critical points, bubble- and dew-point lines, optical phenomena and unusual phase transitions in near critical region, etc. The percentage-liquid volumes at several temperatures are recorded in detail for all fluids, and are cross-plotted to determine the critical points in pressure-temperature diagram with no further attempts to experimentally determine the precise critical parameters due to the limitations of the PVT equipment. Instead of the opalescence previously reported for carbon dioxide, the reddish-brown color is observed in near critical regimes for three fluids, this may be attributed to the complexity of the fluids. Comparison of the near-critical phenomena obtained in this study shows a large difference between the synthetic and real systems. Surprisingly, the unusual phase phenomena are observed for the synthetic mixture and one field sample. That is, during the constant composition expansion process the dew-point phase change occurs twice above the critical temperature, and correspondingly dew- and bubble-point phase changes occur sequentially below the critical temperature. In the later case, an interesting finding is that the fluid can not be clearly classified into gas or liquid according to the usual experimental classification of fluid types in PVT cell. In two cases, the first-appeared fluid at the bottom of PVT cell first disappears at the same lower-pressure with pressure depletion, although the temperatures are different. To our best knowledge, the unusual phenomena occurring at high temperature up to 100 Celsius degrees is reported for the first time. These observations present a new challenge on the gas-liquid concepts widely used in engineering fields, and strongly imply limitation of popular two-phase assumption of the PVT calculation in numerical simulation. The results presented herein are of high value from the theoretical and practical viewpoints, and can find wide applications in various fields involving petroleum industry, e.g., formation fluid sampling, classification of fluid types, well testing, the optimum of the profitable development strategies of gasfield, etc..