CT尺度砂岩渗流与应力关系试验研究

EXPERIMENTAL STUDY ON RELATION BETWEEN SEEPAGE AND STRESS OF SANDSTONE IN CT SCALE

岩石渗流与应力关系研究是进行岩石渗流场与应力场耦合分析的关键。运用岩石高压三轴加载装置和渗透压加载装置,对砂岩进行了渗流与应力关系试验,同时借助SOMATOMPLUS螺旋CT扫描机进行实时观测。通过试验结果分析,推出了基于CT数的岩石孔隙率公式,在此基础上,分析了岩石应力–应变过程中孔隙率、渗透速度、渗流速度、微孔隙直径、渗透率等的变化规律。结果发现:岩石渗透参数的变化与岩石受力损伤–破裂过程密切相关。在初期的压密阶段,岩石的孔隙率、渗透速度、渗流速度、微孔隙直径、渗透率等随应力的增大而减小;当岩石的内部出现微裂纹后,岩石的孔隙率、渗透速度、渗流速度、微孔隙直径、渗透率等随应力的增大而增大,从宏观应力–应变关系看,从微裂纹出现到宏观破坏出现前,岩石还处于弹性变形阶段;当岩石宏观破坏时,岩石的孔隙率、渗透速度、渗流速度、微孔隙直径、渗透率等达到最大值。同时还发现:在渗透水压力作用下,受压砂岩的微裂纹起裂应力占岩石峰值强度的45%,而同样干岩样中微裂纹起裂应力占岩石峰值强度的55%以上,也就是说,渗透水压力使砂岩样的强度损失10%。

The relation between seepage and stress is an important problem in numerical analysis of coupled seepage and stress fields of rock mass. The triaxial compressive and seepage experiment of sandstone sample is conducted in the laboratory using triaxial loading apparatus, seepage pressure loading apparatus and medical SOMTOM PLUS CT scanner. By analyzing the experimental results, the porosity equation based on CT numbers is proposed in CT scale. Using the equation, the porosities, real flow rates in porous rock, specific discharge, permeability and the micro-pore diameters during the processes of stress and strain of a sandstone sample are calculated. Results show that the changes of the seepage parameters have closely relations to the processes of damaging and cracking in the sandstone sample. At the first stage of compression test, with the increase of axial compression, the porosity, real flow rate in porous rock, specific discharge, permeability and the micro-pore diameters in a sandstone sample decrease： and the porosity in the sample is closed under compression. After the micro-fracturing initiation in the sample, the porosity, the real flow rate in porous rock, the specific discharge, permeability and the micro-pore diameters increase with the increase of the axial compression. According to the relation between stress and strain of the sandstone sample in macro-scale, the deformation of the sandstone sample is in the elastic deformation stage before the macro-cracking occurrence. At the point of macro-failure, the porosity, the real flow rate in porous rock, the specific discharge, the permeability and the micro-pore diameters reach the maximum values. Meanwhile, it is discovered that the micro-fracture occurs at the elastic deformation stage of rock and the stress of micro-fracturing initiation occupies 45% of the peak strength of the sandstone sample under seepage pressure, but 55% without seepage pressure. This indicates that the strength of the sandstone sample reduces 10% due to seepage pressure.