The correlation of flow structure, displacement efficiency and relative permeability for two-phase flow in variable-aperture fractures is critical for many fractured rock applications. A systematic visualization method for two-phase flow experiments was developed and four transparent variable-aperture fracture samples were duplicated by epoxy casting method. A total of 112 times two-phase flow experiments of nitrogen displacing water under various gas flow rates were performed and the corresponding results of images and pressure data during flow process were recorded and analyzed based on optical principles. On the basis of the competition between capillary resistance and inertia force of gas, the flow structure can be generally divided into four states including capillary resistance dominant (CR) state with few flow paths in two lateral sides without fingering flow, competitive (CP) state with the occurrence of clear fingering flow phenomenon, inertia force of gas dominant (IF) state showing more chaotic gas distributions, and steady (SD) state that has few changes in flow paths. The growth of displacement efficiency is greatly affected by the four transition states, in which the increment of displacement efficiency in Frac-1 that has gone through the three states of flow structure approaches 30%, and that of Frac-2 and Frac-3 are about 15%, while Frac-4 with little change of flow structure is less than 10%. As a consequence, the relationship between saturation and relative permeability of gas becomes steeper from Frac-1 to Frac-4 with greater deviation from X, V–C and Corey models due to the smaller variation range of gas saturation, and an improved -type model is obtained where the exponent shows a positive correlation to fracture mean aperture.

中文翻译：

---

变孔径裂缝两相流可视化实验：流动结构、驱替效率和相对渗透率之间的相关性

可变孔径裂缝中两相流的流动结构、驱替效率和相对渗透率的相关性对于许多裂缝岩石应用至关重要。开发了一种系统的两相流实验可视化方法，并通过环氧树脂浇注方法复制了四个透明变孔径断裂样品。共进行了112次不同气体流量下氮气置换水的两相流实验，并基于光学原理记录和分析了流动过程中的图像和压力数据的相应结果。根据毛细管阻力与气体惯性力的竞争，流动结构一般可分为毛细管阻力主导状态（CR）、两侧流道较少、无指进流、竞争状态（CP）四种状态随着明显的指状流动现象的发生，气体惯性力主导（IF）状态表现出更加混乱的气体分布，而稳定（SD）状态则流路变化很少。驱油效率的增长受四种过渡状态的影响较大，其中经过三种流动结构状态的Frac-1驱油效率增幅接近30%，Frac-2和Frac-3驱油效率增幅接近30%。约15%，而流动结构变化不大的Frac-4不到10%。因此，由于天然气饱和度变化范围较小，从Frac-1到Frac-4，天然气饱和度与相对渗透率的关系变得更陡，与X、V-C和Corey模型的偏差更大，并且改进型获得的模型中指数与裂缝平均孔径呈正相关。