挥发油非平衡相变对脱气影响的定量评价模型

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

为了定量评价非平衡相变对油气系统脱气的影响和准确确定不同采油速度下油气系统的泡点压力,以亨利模型为基础,考虑挥发油非平衡相变,建立了考虑压降影响的气体溶解度计算模型。利用变压器油和二氧化碳组成的油气系统在不同压降速度下进行脱气实验,分别测量非平衡相变和平衡相变状态下的脱气速度,经计算、拟合得到表征油气系统非平衡性的重量函数,最后计算出不同压降速度时的脱气速度和泡点压力,并定量评价非平衡相变对脱气的影响程度及泡点压力偏离程度。考虑油气系统非平衡相变模型的计算精度高于未考虑非平衡相变的亨利模型,计算结果表明,挥发油藏开发过程中,油气非平衡相变降低了油藏泡点压力,延缓了挥发油脱气进程,压力降落速度和压力降落幅度决定非平衡相变对脱气的影响程度:压力降落幅度越大、压力降落速度越大,非平衡相变对脱气影响越明显,泡点压力越低、脱气速度越小。

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.