Dr. Kazuki Sawayama

Biography

Kazuki Sawayama has been an assistant professor at the Institute for Geothermal Sciences, Kyoto University since 2021. He received his Ph.D. from the Department of Earth Resources Engineering, Kyushu University. His main research interests include (1) fracture flow behavior, (2) digital (computational) rock physics, (3) experimental rock physics, (4) rock mechanics in geothermal fields, and (5) subsurface flow characterization using rock physics models. He was awarded the Rocha Medal 2024 by the International Society for Rock Mechanics.

Introduction of the Lecture

Characterizing fracture permeability using electrical and seismic properties: insight from experimental and digital rock physics

Subsurface fractures play a critical role in regulating the behavior of crustal fluids, which is essential for assessing hydraulic integrity and developing underground spaces, such as enhanced geothermal systems (EGS). Recent advances in geophysical techniques have enabled the detection of changes in electrical resistivity and seismic velocity induced by crustal stress variations, which may correlate with changes in subsurface fluid flow. To interpret these geophysical observations, rock physics models provide essential insights; however, the relationships between geophysical and hydraulic properties remain poorly understood due to limited data on rock fractures. Consequently, estimating subsurface permeability using remotely monitored geophysical indicators (e.g., electrical and seismic properties) is highly desirable.

This talk presents rock physics models designed to estimate changes in fracture permeability via geophysical monitoring, utilizing both laboratory experiments and Digital Rock Physics (DRP) simulations. The hydro-mechanical-electrical-seismic properties of digital rock fractures were simulated using a coupled approach combining the Boundary Element Method, Lattice Boltzmann Method, and Finite Element Method. The results demonstrate that changes in fracture permeability and electrical resistivity under elevated normal stress are significantly affected by both surface roughness and shear displacement. Conversely, the relationship between permeability and resistivity shows minimal dependence on roughness, shear displacement, or fracture size. Although seismic properties are influenced by fracture aperture and contacting asperities, these effects can be normalized by applying a power-law scaling. The distinct effects of microstructures on each physical property result in bilinear trends for both the permeability–electrical resistivity and permeability–seismic velocity relationships, which are largely independent of geometric characteristics. The threshold separating these regimes is clearly evident in the resistivity–velocity relationship. These findings suggest that electrical resistivity is highly sensitive to permeability as long as flow paths remain connected, whereas seismic velocities are more sensitive prior to reaching the percolation threshold.