Simulation of petroleum phase behavior in injection and production process of underground gas storage in a gas reservoir

On the basis of analyzing the fluid phase behavior during the transformation from gas reservoir to gas storage, a mathematical model and an experimental simulation method are established to describe the oil-gas phase behavior during the whole injection- production process of gas storage. The underground gas storage in the Liaohe Shuang 6 gas reservoir with oil rim is taken as a typical example to verify the reliability and accuracy of the mathematical model and reveal characteristics and mechanisms of fluid phase behavior. In the gas injection stage of the gas storage, the phase behavior is characterized by mainly evaporation and extraction and secondarily dissolution and diffusion of gas in the cap to oil in the oil rim of the reservoir; the gas in gas cap increases in light component content, decreases in contents of intermediate and heavy components, and increases in density and viscosity. The oil of the ring decreases in content of heavy components, increases in contents of light and intermediate components, decreases in density and viscosity, and increases in volume factor and solution gas oil ratio. In the stable operation stage of periodic injection-production of gas storage, the phase behavior shows that the evaporation and extraction capacity of injection gas in the cap to oil rim is weakened step by step, the phase behavior gradually changes into dissolution and diffusion. The gas in gas cap decreases in content of intermediate components, increases in content of light components slowly, and becomes lighter; but changes hardly in density and viscosity. The oil in the oil rim increases in content of heavy components, decreases in content of intermediate components, rises in density and viscosity, and drops in volume factor and solution gas oil ratio.