低渗-超低渗透率测量是流体渗流力学和岩石物理学研究领域的热点问题.断层带流体输运的定量化研究对于了解断层的力学性质和预测地下流体的流动具有重要意义.文中介绍了新研制的超低渗透率测量系统和工作原理.这套设备使用稳态法和目前逐渐流行的孔隙压力振荡法进行测量,其孔隙流体为蒸馏水和氮气,最高围压可达200M Pa,孔隙压上限为40MPa.对该测量系统进行了参数标定,并给出了循环加载条件下典型砂岩的渗透率测量结果.对比了2种测量方法得到的结果,分析了产生差异的原因,比较了2种方法的特点和适用条件.探讨了孔隙压力变化对实验结果的影响,按照加载间隔计算了有效压力系数.结果发现有效压力系数依赖于加载路径.使用最小二乘法对所有数据进行拟合,求出了最优有效压力系数并获得渗透率和有效压力函数关系,为分析围压和孔隙压对渗透率的影响提供了依据.%The low-ultralow permeability measurement is one of the most important research topics in fluid mechanics and rock physics. Quantification of fluid transport through fault zones is critical for the understanding of fault mechanics and prediction of subsurface fluid flow. In order to obtain the information about the fluid-flow processes in seismogenic regions,we have designed and constructed a distilled water/gas-medium permeability measurement apparatus capable of achieving ultralow permeability. The new measurement apparatus has used steady-state method and pore pressure oscillation method, which has become very popular recently because of its continuous measurement during processes that might alter the pore space of rocks and can measure samples with different permeability magnitude fast and precisely, thus producing a basic environment in which the confining pressure reaches 200MPa by water, pore pressure reaches 40MPa either by water or gas at room temperature. In this paper,we have introduced some calibrating parameters in pore pressure oscillation method and given the experimental results on typical sandstones under seasonal cycle conditions. Our experimental results have showed that; (1 ) permeability by pore pressure oscillation method decreases with cyclic numbers slightly while permeability by steady state method decreases markedly; (2) we have used empirical power law to fit the experimental values and the accuracy to pore pressure oscillation method is extremely high. We also have compared the results of the two different methods and found that pore pressure oscillation permeability is slightly larger than steady state permeability. Finally,we discussed the reasons that generate the discrepancy due to the method applied. Then we did some preliminary research on the pore pressure, and calculated effective pressure coefficent at intervals during the cycles. Large hysteresis in permeability has been observed, and effective pressure coefficient has been found to be loading path-dependent. At last,we used the least square method on all experimental data to get the optimal effective pressure coefficient to make the best correlation between the permeability and the effective pressure.
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