5:15 PM - 6:45 PM
[SVC28-P01] Simultaneous measurements of hydraulic, electrical, and elastic properties of single fractured samples as elevated normal stress
Keywords:Fractured sample, Permeability, Electrical resistivity, Aperture, 3D printed sample
Subsurface fluid flow plays a significant role in heat and mass transport in geothermal activities. Therefore, it is essential to understand permeability because it directly affects subsurface fluid flow. Previous study has been reported that permeability increases with fracture surface roughness and decreases with normal stress. However, it is not feasible to measure the surface roughness conditions of subsurface fractures. Alternatively, this study aimed to establish a model for evaluating roughness through electrical and elastic properties, which can be obtained from geophysical observations. Previous numerical studies have reported that electrical resistivity is sensitive to fracture roughness; however, the fracture surface deformation due to normal stress has not been accurately modeled. Despite this importance, changes in the hydraulic, electrical, and elastic properties of rough-walled fractures have not been investigated as elevated stress, either numerically or experimentally. Based on these points, this study conducted simultaneous measurements of these properties while actually deforming rough-walled fracture samples via a uniaxial compression test.
In the experiments, we used Aji granite and 3D printed samples (acrylic resin) created from information on the fracture surface of the granite. We prepared samples with different roughnesses controlled based on RMSH (0.2 mm and 0.5 mm), which is the root mean square of the surface height. The size of each sample was the same (60 mm × 30 mm × 30 mm). We conducted uniaxial compression tests of up to ~ 50 MPa, where the normal stress and axial displacement were evaluated using a load cell and an external displacement transducer, respectively. During the tests, a KCl solution (0.1 mol/L) was injected into the sample using a syringe pump. At each stress condition, we evaluated the changes in permeability by measuring the pore pressure difference and flow rate of the KCl solution through the fracture during deformation using a syringe pump and balance. We then measured the electrical impedance and phase during deformation using an LCR meter and calculated the electrical resistivity from these values.
The results of the experiments showed that the electrical resistivity increased with normal stress and decreased with roughness. In addition, the electrical resistivity of the granite was lower than that of the 3D printed samples. This discrepancy may arise from the differences in the sample materials: the matrix of the granite sample is slightly conductive compared to the 3D printed material which is an insulator. Similarly, we found that the permeability decreased with normal stress and increased with roughness. Based on these results, we calculated the slope of the permeability and electrical resistivity in logarithmic coordinates. Consequently, the obtained slopes range from 1.5 ≦ a ≦ 2.2 which is consistent with the values reported in previous studies. This result suggests that the permeability may be high even at relatively high electrical resistivity. The relationships among the permeability, electrical resistivity, elastic property, and fracture roughness estimated in this study are expected to contribute to the estimation of crustal permeability.
In the experiments, we used Aji granite and 3D printed samples (acrylic resin) created from information on the fracture surface of the granite. We prepared samples with different roughnesses controlled based on RMSH (0.2 mm and 0.5 mm), which is the root mean square of the surface height. The size of each sample was the same (60 mm × 30 mm × 30 mm). We conducted uniaxial compression tests of up to ~ 50 MPa, where the normal stress and axial displacement were evaluated using a load cell and an external displacement transducer, respectively. During the tests, a KCl solution (0.1 mol/L) was injected into the sample using a syringe pump. At each stress condition, we evaluated the changes in permeability by measuring the pore pressure difference and flow rate of the KCl solution through the fracture during deformation using a syringe pump and balance. We then measured the electrical impedance and phase during deformation using an LCR meter and calculated the electrical resistivity from these values.
The results of the experiments showed that the electrical resistivity increased with normal stress and decreased with roughness. In addition, the electrical resistivity of the granite was lower than that of the 3D printed samples. This discrepancy may arise from the differences in the sample materials: the matrix of the granite sample is slightly conductive compared to the 3D printed material which is an insulator. Similarly, we found that the permeability decreased with normal stress and increased with roughness. Based on these results, we calculated the slope of the permeability and electrical resistivity in logarithmic coordinates. Consequently, the obtained slopes range from 1.5 ≦ a ≦ 2.2 which is consistent with the values reported in previous studies. This result suggests that the permeability may be high even at relatively high electrical resistivity. The relationships among the permeability, electrical resistivity, elastic property, and fracture roughness estimated in this study are expected to contribute to the estimation of crustal permeability.