页岩气藏微裂缝表观渗透率动态模型研究

为研究页岩气藏开发过程中介质变形和滑脱效应对微裂缝表观渗透率动态变化的影响规律，分析有效应力和多孔介质结构参数等对气体渗流影响机制，采用光滑平板模型，结合分形及气体微观渗流理论，建立了介质变形和滑脱效应耦合作用下的微裂缝表观渗透率动态模型，并对模型进行可靠性验证和参数敏感性分析。研究表明，页岩气藏降压开采过程中受介质变形和滑脱效应“一负一正”耦合影响，微裂缝表观渗透率呈先减小后增大趋势，且临界压力值约为5 MPa；不同有效应力状态下，由于介质变形和滑脱效应耦合机制的差异性，导致表观渗透率变化规律不同，从微观作用机制角度对实验中不同加载条件下页岩应力敏感性的差异做出了理论解释；微裂缝最大开度越小，表观渗透率曲线“凹槽”越深，同时微裂缝孔隙度及开度分形维数越高、迂曲度分形维数越低，表观渗透率值越大。

A dynamic model of apparent permeability for micro fractures in shale gas reservoirs

The object of this paper is to study the coupled impacts of medium deformation and slippage effect on variation of apparent permeability for micro fractures, and to investigate the mechanism of effective stress condition and pore structure parameters on gas flow in shale gas reservoirs. A dynamic apparent permeability model for micro fractures is presented, which accounts for the coupled effects of medium deformation and gas slippage simultaneously based on the smooth plate model and combined with the fractal theory, as well as microscopic gas flow mechanism. Then the reliability verification and sensitivity analysis of the established model are performed. The results show that: The apparent permeability decreases firstly and then increases slowly as pore pressure reduces, and the critical pressure is about 5 MPa. The huge difference between the two coupled mechanism (medium deformation and slippage effect), resulting in the variation of apparent permeability curve with different effective stress states, which also explained the variation of stress sensitivity for shale with different loading modes in testing from microscopic perspective. The deeper "groove" of the apparent permeability curve when the maximum aperture is smaller. In addition, the higher the porosity and fractal dimension of the aperture, and the lower the tortuosity fractal dimension, the greater the apparent permeability is.