3:45 PM - 4:00 PM
[SVC35-07] Rock physics model for fault permeability by simultaneous measurement of electrical resistivity and seismic velocity
Keywords:P-wave velocity tomography, Fault, Asperity, Permeability, Resistivity
Subsurface faults play an important role in controlling the behavior of crustal fluids and are closely related to the formation of hydrothermal systems. In general, conventional geothermal reservoirs develop primarily in fracture systems in impermeable igneous rocks. Therefore, understanding and monitoring permeability in fractured igneous rocks is essential for the sustainable development of geothermal resources. Previous studies have shown that fracture permeability can be several orders of magnitude higher than the host rock and is stress dependent. Field studies have observed changes in electrical resistivity and seismic velocity associated with hydrothermal activity and geothermal development. Since these properties are fracture sensitive, it would be advantageous to correlate them with fracture permeability. However, a quantitative interpretation of subsurface permeability has not been possible due to a lack of experimental data for rough-walled fractures. Therefore, we establish an experimental apparatus to simultaneously measure permeability, resistivity, and seismic velocity under elevated normal stress to evaluate their respective relationships.
We used granite, gabbro, and 3D-printed specimens with different roughness properties (60 mm x 60 mm x 30 mm) for the experiments. Hydromechanical loading up to ~50 MPa was performed by uniaxial compression testing, and permeability was evaluated by injecting KCl solution (0.1 mol/L) into the fracture specimens during the loading process. Impedance and phase during deformation were measured with an LCR meter to determine electrical resistivity. In addition, P- and S-wave velocities were measured for multiple paths as an increased stress. During the tests, the fracture contact state was imaged using pressure sensitive film while the spatio-temporal changes in P-wave velocity with increasing stress were evaluated.
Experimental results at elevated stress showed that the permeability decreased up to ~3 orders of magnitude, the electrical resistivity increased up to ~2 orders of magnitude, and the P-wave velocity increased up to ~0.3 km/s. The contact area reached up to ~60% at this stress level (50 MPa). The spatial distribution of the mapped P-wave velocity was correlated with such a visualized contact area. We also investigated the relationship between permeability and electrical resistivity. This relationship changed with stress at the threshold where the evolution of the seismic velocity is nearly saturated (~15 MPa). These results suggest that simultaneous monitoring of electrical resistivity and seismic velocity can predict changes in permeability during stimulation.
We used granite, gabbro, and 3D-printed specimens with different roughness properties (60 mm x 60 mm x 30 mm) for the experiments. Hydromechanical loading up to ~50 MPa was performed by uniaxial compression testing, and permeability was evaluated by injecting KCl solution (0.1 mol/L) into the fracture specimens during the loading process. Impedance and phase during deformation were measured with an LCR meter to determine electrical resistivity. In addition, P- and S-wave velocities were measured for multiple paths as an increased stress. During the tests, the fracture contact state was imaged using pressure sensitive film while the spatio-temporal changes in P-wave velocity with increasing stress were evaluated.
Experimental results at elevated stress showed that the permeability decreased up to ~3 orders of magnitude, the electrical resistivity increased up to ~2 orders of magnitude, and the P-wave velocity increased up to ~0.3 km/s. The contact area reached up to ~60% at this stress level (50 MPa). The spatial distribution of the mapped P-wave velocity was correlated with such a visualized contact area. We also investigated the relationship between permeability and electrical resistivity. This relationship changed with stress at the threshold where the evolution of the seismic velocity is nearly saturated (~15 MPa). These results suggest that simultaneous monitoring of electrical resistivity and seismic velocity can predict changes in permeability during stimulation.