トランジェントパルス法を用いた低透水性岩石の水理定数の高精度評価

Abstract

The transient pulse method is suitable for measuring hydraulic constants of low-permeability rocks in the laboratory. This method is established and reliable; however, there is room for improvement. In this study, we examined analytical solutions of the transient pulse method and introduced a versatile technique of precise test data analysis with sensitivity analysis and experimental error estimation that is based on the nonlinear least-squares method and is applicable to other permeability tests. We also carried out highly precise transient pulse permeability tests in the laboratory and obtained highly precise experimental data. Based on the results, we validated our proposed data analysis technique based on nonlinear least squares. We conducted transient pulse permeability tests using Inada granite, which is a low-permeability rock, and evaluated its hydraulic constants (hydraulic conductivity and specific storage) using the exact solution of the transient pulse method. We estimated the experimental error between the analytical solution and the experimental data simultaneously. When the approximate solution is used, we can increase precision by considering the compressible storage of the upstream and downstream reservoirs and tubing, which is estimated by a calibration test, instead of considering only water compression. Sensitivity analysis revealed that the sensitivity of the hydraulic head difference between the upstream and downstream reservoirs in terms of hydraulic conductivity is an order of magnitude greater than that in terms of specific storage. This means that the relative error of hydraulic conductivity obtained in a laboratory test is an order of magnitude less than that of specific storage. In precise experiments, control of the room temperature is important because the fluids used in pore and confining lines have thermal expansion characteristics that are sensitive to temperature variation, and the differential pressure transducer is also directly affected by temperature change. The pressure pulse should be as small as possible because if a large change in pressure occurs, the effective stress state of the specimen changes and the permeability changes with it.