A quantitative model for evaluating the impact of volatile oil non-equilibrium phase transition on degassing

To quantitatively evaluate the impact of non-equilibrium phase transition on degassing in an oil-gas system and accurately determine the bubble point pressure of the system with different oil rates, a model, which takes the influence of pressure drop and non-equilibrium phase transition into consideration, is established based on the Henry model to calculate the gas solubility. To measure the degassing speeds with non-equilibrium phase transition and equilibrium phase transition respectively, degassing experiments are done at different pressure drop speeds with the oil-gas system composed of transformer oil and carbon dioxide. A function characterizing the non-equilibrium nature of the oil and gas system is derived after calculation and matching, based on which, degassing speeds and bubble point pressures at different pressure drop speeds are calculated. The impact of non-equilibrium phase transition on degassing and the deviation degree of the bubble point pressure are evaluated quantitatively. The model considering the non-equilibrium phase transition has higher computational accuracy than the Henry model not considering the non-equilibrium phase transition. The computation results indicate that the non-equilibrium phase transition reduces the bubble point pressure in the reservoir and slows down the degassing process of the volatile oil during the development of volatile oil reservoirs. The extent to which the non-equilibrium phase transition impacts degassing depends on the speed of pressure drop and the value of pressure drop: the bigger the pressure drop value and the faster the pressure drop speed, the more significant impact the non-equilibrium phase transition exerts on degassing, the lower the bubble point pressure and the slower the degassing speed.