11:00 AM - 1:00 PM
[SCG49-P06] Characterization of fracture network of thermally cracked granite rocks and prediction of transport properties
Keywords:permeability, electrical conductivity, persistent homology, fracture network
Spatio-temporal variations in fluid transport properties are critical to understand effects of pore pressure on seismic activity. Structural parameters to characterize fractured media, such as porosity, aperture size, crack width and tortuosity, have been related to transport property, such as permeability and electrical conductivity. Rough correlations between the fracture geometries and the measured transport properties, such as porosity-permeability relationship have been proposed. However, we have not fully clarified the relationship between the structure of fracture network and measured properties. It is mainly due to the complexity of the geometry of fracture network. In this presentation, we will show our challenges to extract characteristics of the fracture network, using 3D volume of microfocus X-ray CT images of thermally cracked granite rock specimen (Inada granite), and will compare between the measured and calculated transport properties: permeability and electrical conductivity.
Cylindrical granite specimens with 25.5 mm in diameter and 30.0 mm in length were thermally shocked to generate cracks; after 1-hour heating at 550 or 650 deg.C, the specimens were cooled rapidly with ice water. The porosity was 0.65~0.69 % in intact specimens, while 2.20~2.31 % in the specimens thermally cracked to 550 deg.C, 4.14~4.36 % in those to 650 deg.C. Permeability and electrical conductivity were measured at Univ. of Toyama. Prior to measure those transport properties, the cylindrical surface of a specimen was coated with a silicone rubber and covered by a heat-shrink tube to avoid fluid flow along the cylindrical surface. By using 0.1 M KCl aqueous solution as a pore fluid, we could measure the permeability and electrical conductivity simultaneously. The permeability (k) increased from 2.6・10-15 m2 to 7.8・10-14 m2, and electrical conductivity (σ) increased, from 8.5・10-3 to 2.1・10-2 S/m, when the porosities of the specimens were increased. The fracture aperture (a) and equivalent fracture width (w) were evaluated based on the dependence of the permeability (k ∝ w・a3) and that of the electrical conductivity (σ ∝ w・a). Estimated a was 2~8 μm, while the estimated w per a unit area, 2.1~3.1 mm/mm2. We also obtained 8-bit grayscale stack images of the thermally shocked specimens, scanned by a microfocus X-ray CT with the resolution of 27.4 μm/voxel (Xradia, Kochi Univ.).
The binarized and skeletonized 3D images of the cracks were obtained by the textured Renyi entropy method (Sahoo and Arora, 2004, Pattern recognition). More complicated structures of the fracture networks, such as intricately branched structures, were observed with larger porosities, namely, higher permeability and higher electrical conductivity. We also tried to estimate permeability from the binarized images based on persistent homology analysis (Suzuki et al., 2021 Scientific reports). In this presentation, we will show the comparison between the measured and estimated permeability.
Acknowledgements: This research is partly supported by JSPS KAKENHI, 19K04047 (MT, MK, SU, and TW) and JST ACT-X JPMJAX190H (AS). We used Xradia thanks to Joint-Use Programs of Center for Advanced Marine Core Research, Kochi Univ., (20A012 and 20B010). We used a python-based software, HomCloud (https://homcloud.dev/index.en.html) to estimate the permeability for 3D crack models.
Cylindrical granite specimens with 25.5 mm in diameter and 30.0 mm in length were thermally shocked to generate cracks; after 1-hour heating at 550 or 650 deg.C, the specimens were cooled rapidly with ice water. The porosity was 0.65~0.69 % in intact specimens, while 2.20~2.31 % in the specimens thermally cracked to 550 deg.C, 4.14~4.36 % in those to 650 deg.C. Permeability and electrical conductivity were measured at Univ. of Toyama. Prior to measure those transport properties, the cylindrical surface of a specimen was coated with a silicone rubber and covered by a heat-shrink tube to avoid fluid flow along the cylindrical surface. By using 0.1 M KCl aqueous solution as a pore fluid, we could measure the permeability and electrical conductivity simultaneously. The permeability (k) increased from 2.6・10-15 m2 to 7.8・10-14 m2, and electrical conductivity (σ) increased, from 8.5・10-3 to 2.1・10-2 S/m, when the porosities of the specimens were increased. The fracture aperture (a) and equivalent fracture width (w) were evaluated based on the dependence of the permeability (k ∝ w・a3) and that of the electrical conductivity (σ ∝ w・a). Estimated a was 2~8 μm, while the estimated w per a unit area, 2.1~3.1 mm/mm2. We also obtained 8-bit grayscale stack images of the thermally shocked specimens, scanned by a microfocus X-ray CT with the resolution of 27.4 μm/voxel (Xradia, Kochi Univ.).
The binarized and skeletonized 3D images of the cracks were obtained by the textured Renyi entropy method (Sahoo and Arora, 2004, Pattern recognition). More complicated structures of the fracture networks, such as intricately branched structures, were observed with larger porosities, namely, higher permeability and higher electrical conductivity. We also tried to estimate permeability from the binarized images based on persistent homology analysis (Suzuki et al., 2021 Scientific reports). In this presentation, we will show the comparison between the measured and estimated permeability.
Acknowledgements: This research is partly supported by JSPS KAKENHI, 19K04047 (MT, MK, SU, and TW) and JST ACT-X JPMJAX190H (AS). We used Xradia thanks to Joint-Use Programs of Center for Advanced Marine Core Research, Kochi Univ., (20A012 and 20B010). We used a python-based software, HomCloud (https://homcloud.dev/index.en.html) to estimate the permeability for 3D crack models.