页岩气纳米级孔隙渗流动态特征

页岩储层的孔隙结构比较复杂，孔隙直径较小，纳米级孔隙普遍发育，大量的页岩气是以吸附态储存于页岩中的。页岩气开采时，纳米级的孔隙结构和吸附气解吸会引起孔隙结构改变，从而使页岩渗透率产生动态变化。为此，基于毛细管模型，引用固体变形理论，研究了气体分子在纳米级孔隙中渗流动态特征。结果表明：孔隙直径小于10 nm时，受扩散与解吸作用的影响，渗透率随储层压力下降呈现出先增加后减小的趋势；孔隙直径越大，渗透率拐点压力值越低，渗透率下降速度越快；孔隙直径大于20 nm，气体分子间的扩散作用对渗流影响较小；压力较低（小于10 MPa）时，气体渗流受分子扩散效应作用明显。

Dynamic characteristics of shale gas flow in nanoscale pores

The pore structure of a shale gas reservoir is complex and the pores are in rather small diameter,especially the nanoscale pores are the most widely distributed,and a great volume of gas is adsorbed in the shales.During the shale gas development,the desorption of the absorbed gas in the shales will result in the change in the pore structure,thus causing dynamic change in the permeability of shales.In view of this,based on the capillary model,the flow dynamic characteristics of gas molecules in nanoscale pores are studied with the solid deformation theory being introduced.The following results are achieved in this study.(1) When the pore diameter is less than 10 nm,the decrease of permeability with formation pressure takes on a fast-slow trend due to the influence of diffusion and desorption of gas;the larger the pore diameter,the lower the pressure of the permeability knee point and the faster the decrease of permeability.(2) When the pore diameter is larger than 20 nm,diffusion among the gas molecules shows minor influence on seepage;but when the pressure is low(less than 10 MPa),diffusion shows major influence on gas seepage.