砂岩渗透性演化特性的孔隙率分布细观模拟分析

通过不同围压下的三轴渗透试验,获得砂岩全应力-应变过程中渗透率变化数据。基于试验结果,建立砂岩试件离散元颗粒流数值模型,模拟得到砂岩压缩变形过程中体应变、孔隙率分布和黏结断裂特征。考虑围压效应,从细观机制上解释了动态孔隙率分布对渗透性演化特性的影响。研究结果表明,低围压下砂岩出现贯通的剪切带,裂隙易张开,对渗流具有显著的促进作用。围压越高,砂岩试样的黏结断裂数量和范围越大,裂隙数量增多,但裂隙和孔洞也容易被压密,从而对渗流起抑制作用。贯通的张开裂隙会大幅增加渗透性,而曲折间断的张开裂隙只会小幅增加渗透性,周围被闭合裂隙和压密的孔洞包裹的完全孤立的张开裂隙则对渗透性贡献很小。

THE POROSITY DISTRIBUTION MESOSCOPIC SIMULATION ANALYSIS OF THE PERMEABILITY EVOLUTION PROPERTY OF SANDSTONE

In this paper, triaxial tests were carried out to study the changes of the permeability of sandstones under different confining pressures. The DEM numerical model of sandstone was built based on the laboratory testing results. The volumetric strain, porosity distribution and contact fracture characteristics of sandstone under compression was obtained by DEM simulation. The results reveal the influence of the dynamic porosity distribution on the permeability from mesoscopic mechanism. The research shows that under low confining pressure, the connected shear bands of sandstone and fissures develop easily, which has significant effect on the seepage. A higher the confining pressure would bring greater numbers and scope of fracture contacts, as well as more fissures. The seepage is inhibited because the fissures and pores are more compressible. The permeability of sandstones increases quickly with the increasing number of connected opening fissures, but slowly with the tortuous and intermittent opening fissures. The completely isolated opening fissures wrapped around closed fissures and compacted pores contribute little to the permeability.