[S-CG52] Dynamics in mobile belts

[SCG52-P12] Fluid flow behaviors and rock physical properties of anisotropic fractures: Insight into the scale dependency simulated by digital rock physics

*Kazuki Sawayama¹, Takuya Ishibashi², Fei Jiang³, Takeshi Tsuji⁴ (1.Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, 2.Fukushima Renewable Energy Institute, National Institute of Advanced Industrial Science and Technology, 3.Department of Mechanical Engineering, Faculty of Engineering, Yamaguchi University, 4.Department of Earth Resources Engineering, Faculty of engineering, Kyushu University)

Keywords:digital rock physics, permeability, resistivity, elastic wave velocity, sheared fracture

Geothermal and seismogenic zones are dominated by fractures. Fluid flow in fractures controls subsurface heat and mass transport, and thereby is important for understanding these active subsurface systems. The heterogeneity in subsurface permeability may also affect volcanic eruptions, slow and fast earthquakes. However, in-situ fluid flow is difficult to be estimated, so the detailed mechanism of the fluid-induced eruptions or earthquakes has not been clarified yet. Recently changes in electrical resistivity and seismic velocity due to stress changes have been observed during geothermal activities and earthquakes. Crustal stress must change the fracture aperture which is related to the fluid flow behavior. Therefore, the subsurface fluid flow would be predictable from observed data if the correlation between the changes in geophysical properties and permeability is clarified. We have investigated the relationship between geophysical properties (resistivity, elastic wave velocity, and stiffness) and permeability change in fractures, and found that their respective relationship is not scale-dependent in mated fractures. On the other hand, shear deformation would have a significant effect on these properties in actual faults. In this study, we newly clarified the relationship between these properties for fractures subjected to shear displacement so that the result can be compared with the observed properties in the field. Numerical simulations of digitized rock fractures (digital rock physics) enable us to investigate the scale-dependency on rock properties, which is not feasible in the laboratory.
We first prepared the different sizes of digital rock fractures (24 mm, 48 mm, 96 mm and 144 mm square) based on the fractal nature of natural fractures (Aji granite and Inada granite) and fractional Brownian motion. These synthetic fractures are then subjected to shear displacement and normal stress simulated by a half-space dry contact model so as to simulate the deformation of the contacting asperities. We then calculated the changes in resistivity, elastic wave velocity, and permeability on the same digital fracture model by using the lattice Boltzmann method and the finite element method to discuss the effects of shear displacement, normal stress, and fracture size on each property.
As a result, the geophysical properties of the sheared fracture showed a significant scale dependency with the increase of normal stress. The microscopic analysis of the fracture structure further suggests that the scale dependency of the permeability-resistivity relationship can be explained by the tortuosity and that of the permeability-elastic wave velocity relationship is related to the difference in asperity size. Based on these dimensionless parameters, we discuss the scaling law for the correlation between permeability and geophysical properties.