Investigating Geophysical Indicators of Permeability Change During Laboratory Hydraulic Shearing of Granitic Fractures with Surface Roughness

• Published in: Rock mechanics and rock engineering 2024-08, Vol.57 (8), p.5431-5445
• Main Authors: Ishibashi, Takuya, Asanuma, Hiroshi
• Format: Article
• Language: English
• Subjects: Acoustic emission; Amplitude; Amplitudes; Civil Engineering; Earth and Environmental Science; Earth Sciences; Flow paths; Fluid flow; Fracture permeability; Fractures; Geophysics; Geophysics/Geodesy; Granite; Hydraulics; Indicators; Laboratory experimentation; Mechanical properties; Original Paper; Permeability; Porosity; Preferential flow; Pressurized fluids; Seismic response; Shear; Shearing; Surface roughness
• ISSN: 0723-2632; 1434-453X
• DOI: 10.1007/s00603-023-03590-y

In order to investigate geophysical indicators of permeability changes in subsurface fractures, we conducted an experimental study on the hydraulic–mechanical–seismic coupled behaviors of granite fractures with surface roughness under stress conditions during hydraulic shearing. Our laboratory experiment yielded the following insights: (1) The “self-propping shear slip concept” unequivocally serves as the primary mechanism for maintaining the increase in fracture permeability of granite, even under stress conditions exceeding approximately 50 MPa. (2) The Gutenberg–Richter b -value gradually decreases during shear dilation and accompanying increase in fracture permeability. Thus, it could serve as an indicator for assessing changes in fracture permeability. (3) The evolution amplitude in acoustic emissions (AEs), as well as the classification of tensile/shear modes and the timing of our maximum amplitude of AE occurrence, do not seem to provide useful information for estimating fracture permeability changes during hydraulic shear slip. The reduction in b -value can be attributed to the spontaneous formation of preferential flow paths during the injection of pressurized fluid into the rock fracture and the subsequent detachment of the small contacting asperities due to localized shear slips, which naturally lead to the creation of porosity and irreversible increase in fracture permeability. Highlights Hydraulic-mechanical-seismic coupled behaviors of granitic fracture are investigated during hydraulic shearing experiments in the laboratory. b -value gradually decreases during shear dilation and associated permeability increase of granitic fracture with rough surfaces. There is no clear correlation between fracture permeability change and amplitude/occurrence timing of AE max amp . during hydraulic shearing.