Macroscopic and Mesoscopic Characteristics of Radial Flow in Rough Rock Fractures.

For various rock engineering, injection of fluids into rock fractures through boreholes is quite common. It is of great significance to investigate the characteristics of radial flow (RF) in rock fractures for these activities. In this study, macroscopic and mesoscopic characteristics of RF in rough rock fractures were investigated and compared with those of unidirectional flow (UF) by theoretical analysis, tests and simulations. An equation for nonlinear RF was derived for rock fractures according to conservation law of mass and Izbash's law. Four scanned rough rock fracture models were established and used to experimentally investigate the macroscopic flow characteristics in both UF and RF. Numerical simulations were performed to clarify the mesoscopic differences in fluid pressure distributions and the flowlines of RF and UF in rock fractures. The parameters of hydraulic aperture and equivalent width for RF were obtained and correlated to those for UF. A method to calculate fracture roughness coefficient of fractures for RF related to the flow direction was proposed. The characteristic parameters, i.e., critical Reynolds numbers for the flow transition from linear to nonlinear flow, effective hydraulic apertures and non-Darcy coefficients, were obtained for the UF and RF based on the test results. It was indicated that the fracture roughness plays a critical role in the macroscopic and mesoscopic characteristics of both RF and UF. According to the test results, the macroscopic characteristic parameters for RF are related to those for UF, and the nonlinearity of RF was stronger than that of UF at a specified flow rate, which was consistent with the mesoscopic characteristics observed in the simulation that the distribution of water pressure, flow velocity and the streamlines in RF were more non-uniform than that in UF. The study results were useful to describe the RF characteristic in rock fractures with the characteristic parameters for UF, which have been investigated extensively in literature. Highlights: A nonlinear flow equation for radial flow in rock fractures was derived to describe the relationship between the hydraulic head and flow rate. The differences and relations between radial and unidirectional flow were studied from macroscopic and mesoscopic aspects. The parameters of hydraulic aperture and equivalent width for radial flow were obtained and correlated to those for unidirectional flow. The effect of fracture roughness on radial and unidirectional flow was related to the flow direction and was incorporated in the Forchheimer equation. [ABSTRACT FROM AUTHOR]