Numerical study of fluid flow of deforming fractured rocks using dual permeability model

Abstract

To investigate the transport behaviour of fractured rocks (tightly compacted granular material) under loading/unloading, a dual permeability model was applied by means of numerical simulation. The numerical samples had a size of 80 × 80 mm2 and consisted of granular material with average grains of 1.6 × 1.6 mm2 in size. The grain boundaries were strongly bonded and the samples behaved like intact rock blocks mechanically, but the grain boundaries were permeable and provided the primary permeability for the matrix. Fractures might initiate and grow along the grain boundaries where the level of stress exceeded their strength. The fractures newly created and the pre-existing ones that were embedded within the samples might be much more permeable than the passive grain boundaries, and they would dominate the transport behaviour of the samples if a connected fracture network formed. By increasing fluid pressure (constant differential stress) and increasing differential stress (constant fluid pressure), two types of loading schemes were applied to the samples. As a result, connected fracture networks developed at a certain stress state. The evolution of fractures and fracture networks greatly altered the flow patterns; main flows were being concentrated within those continuously open fracture networks. On the other hand, the change in hydraulic conduits has led to a strong modification of the fluid pressure distribution. Under low effective mean stress (high fluid pressure), secondary percolation phenomena occurred in the loading plane because of the presence of a connected network of open fractures. In this case, the permeability in the loading plane and in the direction perpendicular to the plane increased greatly. Under high effective mean stress (highly differential stress and constant fluid pressure); however, such a secondary percolation threshold was unlikely to exist, although a connected network developed at a critical stress state. This was because most of those fractures created did not open under high effective mean stress, and the deformation was characterized by grain boundary sliding and isolated, dilational jogs at the intersections of grain boundaries. In this case, the permeability in the loading plane had a slight reduction caused by lack of continuously open fractures, but the permeability in the direction perpendicular to the loading plane increased sharply and became highly localized. The deformation and permeability of the samples were examined when subjected to cycling loads. The fluid pressure alternated between the hydrostatic fluid pressure and a higher fluid pressure (supra-hydrostatic) at which the samples became unstable. During the cycling loading the fractures opened and closed periodically, and the permeability increased and decreased correspondingly. When the fluid pressure decreased from a high level to a low level (loading) in a cycling load, the permeability and deformation of the samples did not totally return to the previous state, indicating a clear plastic deformation. After a number of cycling loads, the plastic deformation and permeability increased progressively, which suggests strong path-dependent behaviour. The overall permeability of the samples increased with increasing extensional strain. For the same extensional strain, the permeability was greater under a low effective mean stress than under a high effective mean stress, and greater during continuous loading than cyclic loading.