Laboratory experiments were performed to determine the spatial variability of seismic fracture specific stiffness and the dominant flow path within a single fracture subjected to normal stress. Two cylindrical granite specimens were used in the study: an intact sample and a fractured sample with the fracture oriented perpendicular to the load. Compressional and shear waves were measured for the samples in dry conditions and during fluid invasion. The normal fracture specific stiffness increased with increasing stress (0 – 18.9 MPa), while the shear fracture specific stiffness started to approach an asymptote at around 10 MPa. During fluid invasion, the amplitude of both the compressional and shear waves decreased as water filled the fracture while the velocity of the waves increased. By tracking the arrival time changes of the waves, the fluid invasion path was identified. The fluid first invaded portions of the fracture that had a relatively low fracture stiffness and then spread to the regions with higher stiffness. In conclusion, the fluid front can be detected seismically and the spatial variation of fracture stiffness is correlated to the fluid flow path.
A goal of geophysical characterization of rock is to detect fractures and characterize the hydraulic and mechanical properties of the fracture. Experimental data and numerical simulations have shown that fluid flow through a fracture and fracture specific stiffness are implicitly related through fracture geometry, i.e., aperture and contact area distributions [1-9]. This relationship is important because fracture specific stiffness can be determined from seismic wave attenuation and velocity . In this study, we investigated, seismically, the spatial variability in fracture specific stiffness in single fractures subjected to normal stress and whether the variability in fracture specific stiffness is linked to the most conductive flow paths through a fracture.