冻融作用下砂岩孔隙结构损伤特征研究

为研究冻融作用下砂岩孔隙结构损伤特征,选取5个岩样进行100次冻融循环试验,并采用核磁共振技术对砂岩孔隙结构进行测试,得到了冻融作用下砂岩的核磁共振弛豫时间T2谱分布、孔隙度等细观结构特征。根据孔隙的孔径分布特征,按孔径尺寸划分为小孔隙、中孔隙、大孔隙3类,并采用扩散双电层理论对不同尺寸孔隙水分布规律进行了分析。结果表明:随着冻融循环次数增加,核磁共振T2分布右移,砂岩的孔隙度增大;同时,水-岩作用导致部分矿物质溶解在孔隙水中,使得孔隙水中离子浓度升高,导致岩石内部产生大量的次生孔隙。随着孔径尺寸的增加,孔隙中的束缚水含量逐渐减少,且小孔隙的束缚水含量远远大于大孔隙;在低温冻结时,自由水先于束缚水结冰,小孔隙束缚水离子浓度的增长幅度小于大孔隙,进而产生了离子浓度差,使得小孔隙中的水分子向大孔隙迁移,造成小孔隙的损伤速率远小于大孔隙。因此,小孔隙在水-岩作用与冻胀压力的作用下不断发育,大孔隙则在冻胀压力下快速扩张、发育,直至岩样发生宏观破坏。

Damage characteristics of sandstone pore structure under freeze-thaw cycles

To study the damage characteristics of sandstone pore structure under freeze-thaw cycles, five rock specimens were selected to conduct 100 freeze-thaw cycles, and the pore structure of sandstone was measured by nuclear magnetic resonance technology (NMR). The mesostructure characteristics such as T2 spectrum distribution and sandstone porosity were obtained under the freeze-thaw effect. According to the distribution of pores, the pore size was divided into three categories: mini-pores, meso-pores, and macro-pores. Meanwhile, the diffusion electric double layer theory was used to analyze the distribution of pore water with different pore sizes. The results show that as the number of freeze-thaw cycles increases, the T2 distribution of NMR shifts to the right and the porosity of the sandstone increases. At the same time, some minerals are dissolved in the pore water under the water-rock effect, which causes the ion concentration to increase in the pore water and results in a large number of secondary pores within the rock. With the increase of pore size, the bound water content in the pores gradually decreases, and the bound water content of small pores is much larger than that of macro-pores. At a low temperature, the free water freezes before the bound water, and the increase of ion concentration of bound water in small pore is lower than that of macro-pores, which leads to a difference in ion concentration. As a result, the water molecules in the small pores migrate to the macro-pores and the damage rate of small pores is much smaller than that of macro-pores. Therefore, the mini-pores continuously deteriorate under the effect of water-rock interaction and the frost heaving pressure; the macro-pores are rapidly developed and expanded under the frost heaving until the macroscopic damage of the rock samples.