Experimental Investigation of Phase Behavior of Condensate Gas and Crude Oil System in Underground Gas Storage

The phase behavior of reservoir fluids is crucial for the smooth operation of an underground gas storage (UGS). In this work, three experiments were conducted to investigate the phase behavior of condensate and crude oil systems within different development stages of gas storage in Bohai Bay Basin of eastern China. In stage 1, constant volume depletion (CVD) test was conducted to simulate the interaction of the original condensate gas-live oil system. In stage 2, the phase behavior of a series of mixtures of live oil with 47.5, 58.3, 69.9, and 77.4 mol% of condensate gas was studied by constant composition expansion (CCE) test. In stage 3, a new experiment was designed to study the mass transfer and phase behavior during cyclic injection-production process of the underground gas storage. The results from the CVD test describe the effect of each pressure stage on produced gas composition and equilibrium oil properties in the condensate crude oil system. The results of the CCE test indicate that condensate gas can expand the oil and improve the properties. The results of cyclic injection-production experiment demonstrate that the mass transfer in condensate gas and oil system is a dual mechanism of condensation and vaporization. With the increase of the cycle, the intermediate components of oil phase are enriched and the heavy components decreases. After four cycles, the oil and gas interphase mass transfer reaches dynamic equilibrium. In addition, test results show that multicycle operation is beneficial to improve the oil properties. This work is of great significance to understand the phase transition mechanism of underground gas storage, which provides a basis for other researchers to further update and improve the EOS model.