Stress Influence on Fracture Aperture and Permeability of Fragmented Rocks

The influence of far-field stresses on fracture apertures in a fragmented rock layer is investigated using finite element analysis of a three-dimensional mechanical model. The model implements realistic boundary conditions, interactions between the fragmented layer and neighboring plastic rock layers, and frictional interfaces between the rock blocks. Stress-strain analysis is conducted to obtain stress variations within the fragmented rock layer and the block displacements and rotations. The fracture apertures are calculated using the local stress states instead of the far-field stresses simply being projected on the fractures. It is observed that fracture apertures can vary for the fracture segments over the individual blocks. Ensemble permeability is calculated by running a single-phase flow analysis considering the obtained fracture apertures for fracture segments. The influence of the rock block displacements, rotations and deformations, difference between the mechanical properties of the rock layers, and the orientation of the horizontal stresses is investigated on the ensemble permeability. It is demonstrated that the compressibility of the neighboring layers and block rotations and deformations have significant influence on the permeability of the fragmented rock layer. These effects, which may be ignored in simpler aperture calculation models, can result in considerable inaccuracies in the estimation of fracture apertures and ensemble permeability. Hence, such methods may only be used as indicative tools.