压缩带形成过程中渗透性变化试验研究

 压缩带的形成能够较大地降低岩石渗透性，存在潜在的经济价值。为研究岩石在压缩带形成过程中的渗透性变化特征、卸载围压时渗透性变化、体积应变与渗透性变化的关系，利用风化的高孔隙率岩石进行三轴试验，同时测量渗透性。试验中得到discrete压缩带、高角度剪切带、以及discrete压缩带、diffuse压缩带和高角度剪切带中2种的混合形式。试验中观察到3种不同类型的渗透性变化曲线。渗透性一般随轴向应变的增加而降低，在形成第一个局部化结构过程中渗透性突然大幅度降低。渗透性降低一般在2个数量级以内，大部分在1个量级以内。这比其他2～3个量级渗透性降低的结果小，但与试验中岩石颗粒破碎不严重相符合。在加载的后期，试件体积膨胀，而渗透性降低。因此，可能存在孔隙率增加而渗透性降低的现象。卸载围压过程中渗透性存在不同类型：有的渗透性随围压降低一直增加，而且增加越来越快；有的渗透性随围压降低先降低，而后增加。

EXPERIMENTAL STUDY OF PERMEABILITY CHANGE DURING COMPACTION BANDS FORMATION

The formation of compaction bands could greatly reduce the permeability of rocks，which has potential economic value. In order to research the characteristic of permeability change during the formation of compaction band，permeability change with confining pressure unloading and the relation between volumetric strain and permeability change，weathered high-porosity rocks are used to carry out triaxial compression tests and obtained the permeability during the processes. Discrete compaction bands，high-angle shear bands，and the hybrid modes of two of the three involving discrete compaction bands，diffuse compaction bands and high-angle compaction bands are observed；three patterns of permeability change have been also observed. The permeability reduces with the increase of axial strain，the permeability suddenly reduces when the first localization compaction band formes. The reduction of permeability is blow 2 orders of magnitude，and most is blow 1 order of magnitude. This is lower than other results of 2—3 orders of magnitude；but it is consistent with the result of no severe grain crushing. The volume of the samples dilates but the permeability reduces at the later loading stage. So，there may exist the phenomenon of permeability reduction with the porosity increase. There are different patterns of permeability change during the process of confining pressure unloading. The permeability of some samples consistently increases with the decrease of confining pressure，and the increase speed is also increase. Another pattern is that the permeability decreases first and then increases with the decrease of confining pressure.