HR: 12:05h
AN: H51H-08    [PDF]
TI: Critical Role of Stress in Flow of Fluids in Fractures
AU: * Sanderson, D J
EM: david.sanderson@imperial.ac.uk
AF: Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London, SW7 2AZ United Kingdom
AB: The role of stress in controlling fluid flow in fractured rock will be examined using numerical (discrete element) models. In any well-connected fracture network there is a critical stress state at which the flow changes from being fairly pervasive through the fracture network to being both enhanced and highly localized in parts of the network. This critical point is produced by the self-organisation of slip and opening on some fractures. The critical stress state has been evaluated for different loading paths and is found to be largely controlled by a simple parameter, the driving stress ratio (R), where R = 2 x (fluid pressure - mean stress) / (differential stress). In numerical models of highly fractured rock, the critical stress state R$_{C}$ can be demonstrated to depend mainly on the frictional properties of the fractures, with other parameters (e.g. fracture geometry, density and rock rheology) controlling the form of the resulting deformation and localization of fluid flow. In most cases the critical driving stress ratio (R$_{C}$) is between -2 and -1.5. A common feature of a critically stressed fracture network is the localized nature of the fluid flow. In situ stress measurements, especially those from deep boreholes and producing oil reservoirs, indicate that driving stress ratios, R, are -2 or higher. In some cases where stress changes locally, either naturally or through human intervention, R remains constant. These observations agree well with numerical models and strongly support the idea that much of the upper crust is maintained at or close to the critical stress state. In such cases, localization of flow should be characteristic.
DE: 1829 Groundwater hydrology
DE: 5104 Fracture and flow
DE: 8010 Fractures and faults
DE: 8164 Stresses--crust and lithosphere
SC: Hydrology [H]
MN: 2003 Fall Meeting