Rock physics & Interpretation

[RP-05] Digital rock physics revealing the relationships between permeability, resistivity and elastic wave velocity of rock fractures

*Kazuki Sawayama¹, Takuya Ishibashi², Fei Jiang³, Tatsunori Ikeda¹, Takeshi Tsuji¹, Osamu Nishizawa¹, Yasuhiro Fujimitsu¹ (1. Kyushu University (Japan), 2. National Institute of Advanced Industrial Science and Technology (Japan), 3. Yamaguchi University (Japan))

Monitoring fracture flow behaviors are of great interest to evaluate the mechanism of earthquakes and fractured reservoirs. During earthquakes and geothermal developments, some geophysical observations have detected the changes in resistivity or seismic velocity. These changes will be triggered by subsurface stress changes, which also affect the fracture flow. However, changes in fracture flow behaviors (e.g., fracture permeability) have not been interpreted from geophysical data due to the absence of a rock physical model for fractured rocks. In this study, we investigated simultaneous changes in permeability, resistivity and elastic wave velocity of rock fractures to establish the empirical model that can correlate them. We take an approach of digital rock physics that simulates hydraulic, electric and elastic properties from digitized rock fracture having natural rough surfaces based on real rock joints (andesite and granite). A series of 3D fracture flow simulations was conducted by using the lattice Boltzmann method and we applied finite element analysis to calculate resistivity and elastic wave velocity after this fluid-flow simulation. The results showed that changes in fracture permeability and resistivity were not affected by the fracture roughness, whereas velocity change shows roughness dependency. This roughness dependence of velocity change can be explained by the heterogeneous distribution of asperities in natural rock fractures. We also correlated the changes in permeability, resistivity and elastic wave velocity. Consequently, the relationship between permeability and resistivity can be simply modeled regardless of the roughness and fracture size. In contrast, the relationship between permeability and elastic wave velocity is depending on the roughness and fracture density, but irrelative to the fracture size. This suggests that laboratory data of the permeability–velocity relationship can be extended to the subsurface fracture. These results indicate that both geophysical properties potentially estimate the change in fracture permeability in a natural setting.