5:15 PM - 6:45 PM
[SVC28-P02] Simultaneous measurement of permeability and electrical impedance in highly fractured granite samples
Keywords:Resistivity, Permeability, Fluid flow and electrical conduction, Crack network, Rock properties measurement
The resistivity structure of volcanic and geothermal areas is crucial for understanding the distribution of deep-seated thermal water and temperature structures underground. Generally, we estimate the regions where geothermal resources are present based on this resistivity structure in geothermal development fields. At this time, if the permeability of the drilling target area can be estimated, it is expected that the efficiency of resource development will significantly improve. In this study, we conducted simultaneous measurements of resistivity and permeability using Inada granite samples with thermal cracks to develop a method for estimating permeability from resistivity.
In the experiment, artificial brine (0.1 M/L KCl solution) was injected into granite samples with thermal cracks installed under a hydrostatic pressure (PC) of 25 MPa at a constant flow rate (0.1 mL/min). We measured the changes in electrical impedance, pore fluid pressure, and downstream brine flow rate (FR) simultaneously. The downstream fluid pressure (PD) was varied in three stages: 0.5 MPa, 5 MPa, and 10 MPa. Under the pressure condition of 0.5 MPa, with the injection of brine, the upstream pore fluid pressure (PU) rapidly increased, resulting in significant changes in FR. Subsequently, PU and FR became almost constant (steady state). During this pressure increase process, it is considered that brine formed flow paths using the existing crack network. In this study, the process of PU increase was considered as a non-steady state.
Next, PD was gradually increased in steps, and brine was injected at a constant flow rate (0.1 mL/min) to measure PU and FR in both non-steady and steady states. Electrical impedance was also measured at the time of permeability calculation. Then, the pore fluid pressure (Both of PU and PD) was gradually reduced to 8 MPa, 2 MPa, and 1 MPa, and impedance measurements were conducted. During the depressurization process, FR didn’t show obvious changes, so we did not estimate the permeability.
The results of this study showed an inverse relationship between permeability and effective confining pressure (Peff: PC-(PU-PD)/2), in both steady and non-steady states. This result indicates that the formation of a crack network due to high PU from brine injection improves permeability. After brine flow reached a steady state, it was suggested that utilizing the formed flow paths made it less likely for new network formations to occur. Additionally, impedance measurements showed an increasing trend during the flow path formation process. This is considered to be due to brine flowing out of isolated cracks as the network forms, resulting in a decrease in the total amount of charged particles within the impedance measurement area. Subsequently, with the increase in PD, impedance decreased. This result suggests that with the increase in the total amount of brine within the formed flow paths, the total number of charged particles increased.
Finally, resistivity was calculated based on the measured impedance, and the relationship with permeability was examined. In the steady state, resistivity and permeability showed a clear relationship. On the other hand, in the non-steady state (flow path formation process), this relationship was not clear. This result suggests a difference between the flow path of brine and electrical conduction paths in the non-steady state, whereas in the steady state, the flow paths of brine in crack system rocks and the electrical conduction paths are almost the same.
In the experiment, artificial brine (0.1 M/L KCl solution) was injected into granite samples with thermal cracks installed under a hydrostatic pressure (PC) of 25 MPa at a constant flow rate (0.1 mL/min). We measured the changes in electrical impedance, pore fluid pressure, and downstream brine flow rate (FR) simultaneously. The downstream fluid pressure (PD) was varied in three stages: 0.5 MPa, 5 MPa, and 10 MPa. Under the pressure condition of 0.5 MPa, with the injection of brine, the upstream pore fluid pressure (PU) rapidly increased, resulting in significant changes in FR. Subsequently, PU and FR became almost constant (steady state). During this pressure increase process, it is considered that brine formed flow paths using the existing crack network. In this study, the process of PU increase was considered as a non-steady state.
Next, PD was gradually increased in steps, and brine was injected at a constant flow rate (0.1 mL/min) to measure PU and FR in both non-steady and steady states. Electrical impedance was also measured at the time of permeability calculation. Then, the pore fluid pressure (Both of PU and PD) was gradually reduced to 8 MPa, 2 MPa, and 1 MPa, and impedance measurements were conducted. During the depressurization process, FR didn’t show obvious changes, so we did not estimate the permeability.
The results of this study showed an inverse relationship between permeability and effective confining pressure (Peff: PC-(PU-PD)/2), in both steady and non-steady states. This result indicates that the formation of a crack network due to high PU from brine injection improves permeability. After brine flow reached a steady state, it was suggested that utilizing the formed flow paths made it less likely for new network formations to occur. Additionally, impedance measurements showed an increasing trend during the flow path formation process. This is considered to be due to brine flowing out of isolated cracks as the network forms, resulting in a decrease in the total amount of charged particles within the impedance measurement area. Subsequently, with the increase in PD, impedance decreased. This result suggests that with the increase in the total amount of brine within the formed flow paths, the total number of charged particles increased.
Finally, resistivity was calculated based on the measured impedance, and the relationship with permeability was examined. In the steady state, resistivity and permeability showed a clear relationship. On the other hand, in the non-steady state (flow path formation process), this relationship was not clear. This result suggests a difference between the flow path of brine and electrical conduction paths in the non-steady state, whereas in the steady state, the flow paths of brine in crack system rocks and the electrical conduction paths are almost the same.