軟砂岩微觀組構與異向性滲透性質探討

Abstract

軟弱岩石的透水係數不但與試體飽和度、顆粒粒徑分佈、孔隙大小、泥質含量及膠結物種類等因素有關，且深受滲流方向之影響。如欲以傳統之定水頭或變水頭滲透實驗方法決定材料不同方向之滲透性常有其困難。為求正確決定材料滲透性，本研究嘗試研發一試體準備方法程序來製作恰當試體，藉而能夠求得異向性岩石的滲透性質。由苗栗天花湖及台中大坑分別取回灰色粉砂岩及黃色砂岩之岩樣，進行一系列室內定流量三軸透水試驗。所求得的滲透係數整理如下：苗栗天花湖灰色粉砂岩平行層面之取樣介於2.82×10-6~9.85×10-6cm/s，垂直層面之取樣介於9.71×10-7~1.65×10-6cm/s；台中大坑黃色砂岩方向P取樣介於7.12×10-5~9.14×10-5cm/s，方向V取樣介於4.56×10-5~6.27×10-5cm/s。 此外，本研究對軟弱砂岩施以滲流弱化模擬，再以微觀方式觀察不同方向取樣試體之組構變化，分析岩石顆粒與膠結物的改變情形，探討滲流及不同方向取樣對微組構的影響。本研究利用三軸透水儀器模擬滲流作用，再觀察其滲流前後微組構的變化，發現岩石的平面孔隙率在滲流後比滲流前增大了1~3％。推測主要是細顆粒與填充物之間會有孔隙局部增大的情形，可能是滲流水經過時先溶解掉具有膠結性的黏土礦物，使細顆粒間的凝聚力降低，造成崩解分離。而不同方向取樣的試體，其平面孔隙率約相差2~4％。

The permeability of soft rock depends on several factors, including saturation, grain characteristics, pores, and cementation, of the material. In this study, a method of specimen preparation was designed to investigate the anisotropic permeability of soft rock. A series of constant-flow permeability tests on samples of soft sandstone from Tian-Hua Lake, Miaoli (denoted THL) and from Ta-kang, Taichung (denoted TK) were carried out. For THL sandstone, the permeability along bedding direction lies within 2.82 ×10-6~9.85 ×10-6cm/s, while the permeability perpendicular to bedding direction lies within 9.71 ×10-7~1.65 ×10-6 cm/s. For TK sandstone, the permeability along two mutually perpendicular directions (denoted directions P and V, respectively) were determined and compared: the permeability along the P direction lies within 7.12 ×10-5~9.14 ×10-5cm/s, while the permeability along the V direction lies within 4.56 ×10-5~6.27 ×10-5cm/s. This study also attempts to investigate the microstructure change of soft sandstone after the degradation due to seepage. By comparing the microstructures of samples before and after seepage, it was observed that the void in sandstone increases significantly after degradation. On the basis of microscopic observation, the possible cause is likely due to the void increase near the grain-matrix interface in the material and/or the dissolution of the cementation/matrix. The void ratio estimated from sections prepared along two mutually perpendicular directions may differ about 2~4%.