关键词: 富凝析气, 近临界, 相态, 临界点, 异常相变, 实验

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 scarce,especially the unsymmetrical multi-component and real reservoir fluids.Thus the phase behavior of near critical systems has attracted considerable attention in recent years.This paper presents some experimental results of near critical phenomena of three rich gas condensate systems.The measurements are made with mercury-free PVT equipment. The fluids used consist of one six-component hydrocarbon mixture and two actual field samples.The presented results include the critical points,bubble-&dew-point locus,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 crossplotted to determine the critical points in pressure-temperature phase diagram with no further efforts to determine experimentally the precise critical parameters due to the limitations of the PVT equipment.Instead of the opalescence previously reported on carbon dioxide,the reddish-brown color is observed in near critical regimes for three fluids,this may be attributed to the complexities of the fluids.Comparison of the synthetic and real systems used in this study shows a large difference in the near-critical phenomena. 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-&bubble-point phase changes occur sequentially below the critical temperature.In the later case,of special interest 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 firstappeared fluid at the bottom of PVT cell first disappears at the same lower-pressure with pressure decline,although the temperatures are different.To the best of our knowledge,it is for the first time reported that the unusual phenomena simultaneously occurring at high temperature of 100×℃.These observations throw new light on the near critical phenomena of a gas-liquid system. The results presented herein are of high value from the theoretical and practical viewpoints,and may find wide applications in various fields such as reservoir fluid sampling,classification of fluid types,well testing, and the optimum of the profitable development strategies of gas field,etc.

Key words: rich gas condensate, near-critical, phase behavior, critical point, unusual phase change, experimental method