Assessment of Fracture Aperture and Tortuosity through Electrical Impedance

Electrical techniques are increasingly being used to evaluate changes in fractures during geothermal operations. The interpretation of fracture properties from electrical measurements relies heavily on theoretical models that link the electrical response of a fracture to its geometric characteristics, such as aperture, porosity, and tortuosity. Here, laboratory experiments were performed to investigate the factors affecting the interpretation of aperture and flow path tortuosity from impedance measurements on electrolytic-filled fractures. Complex impedance measurements were conducted on 3D printed cylindrical specimens with idealized fractures, including a smooth fracture and a sawtooth-patterned, tortuous fracture. A direct 3D reconstruction of the fracture geometry was achieved using 3D X-ray microscopy. The results of this study are consistent with the equivalent channel model, which introduces current path tortuosity in the relationship between the electrical response of saturated rocks and their pore geometry. For fractures with a geometric tortuosity between 1 and 1.25, the electrical aperture estimated using geometric tortuosity was found to be within 10% of the aperture obtained using X-ray imaging. However, when electrical tortuosity was used in the estimation of aperture instead of the geometric tortuosity, the interpreted aperture was found to be within 1% of the imaged aperture.