变开度岩体裂隙多相渗流实验与有效渗透率模型

岩体裂隙的有效渗透率是描述岩体非饱和或多相渗流的关键参数,而裂隙开度是影响有效渗透率的重要因素.通过自主研发的粗糙裂隙多相渗流可视化实验平台,针对天然岩体裂隙复制而成的裂隙模型开展变开度条件下的多相渗流可视化实验,研究开度变化对多相渗流流动结构以及有效渗透率的影响.研究表明:非湿润相流体运动通道,在低流量比条件下呈现出气泡流流动结构,而在高流量比条件下呈现较为稳定的通道流流动结构.随着开度的增加,非湿润相流动通道的分支变少、等效宽度增加,两相流体的有效渗透率均增大,流动结构趋于稳定.可视化结果还阐明了柱塞流流动结构下,两相流体交替占据裂隙空间的竞争机制:当非湿润相流体通道由连续转变为不连续时,裂隙进出口压差显著增加;反之,当该通道由不连续转变为连续时,压差显著减小.最后,基于分形理论以及渗透率统计建模方法,建立了考虑开度效应的岩体裂隙多相渗流有效渗透率理论模型,并通过实验测定的有效渗透率数据验证了该模型的正确性与有效性.

EXPERIMENTS AND EFFECTIVE PERMEABILITY MODEL FOR MULTIPHASE FLOW IN ROCK FRACTURES WITH VARIABLE APERTURES

The effective permeability of rock fractures is a fundamental parameter for describing unsaturated flow and multi-phase flow in fractured media, and the fracture aperture is an important factor affecting this parameter. In this paper, to investigate the effect of aperture on the flow structures of water-oil multiphase flow and on the effective permeability, we develope a visualization experimental system, and perform multiphase flow experiments in fracture models replicated from real rock fractures with three different apertures. Visualization experimental results show that the flow of non-wetting phase in the fracture can be categorized as unstable bubble flow at the low flow-ratio conditions and stable channel flow for high flow-ratios. As fracture aperture increases, the flow channel of non-wetting phase becomes less branching and wider, and the effective permeabilities of the two phases both increase, during which the flow structures become stable. The visualization results also reveal the competing mechanism of fluid-fluid alternately occupying the fracture space in the slug flow structure. When the non-wetting phase fluid channel changes from continuous to discontinuous, the pressure difference between the inlet and the outlet of the fracture increases significantly; conversely, when the channel changes from discontinuous to continuous, the pressure difference decreases significantly. Finally, based on the fractal theory and the statistical model for permeability, the effective permeability model proposed for multiphase flow in rock fractures with variable apertures, and the correctness and reliability of the model is evaluated by the measured effective permeability data.