Selection and Calibration of a Hydromechanical Model for Discrete Fracture Networks

Abstract

As part of a preliminary evaluation of a deep geological repository for spent nuclear fuel, the hydraulic properties of a discrete fracture network were calibrated using published permeability data from comparable sparsely fractured crystalline rock. These permeability data exhibit two primary trends: a relatively gradual reduction with depth below c. 300 m and a marked increase above c. 300 m depth. Respectively, these are hypothesized to be caused by progressive fracture closure with increasing confining stress and periodic shear dilation of fractures proximal to stress criticality due to long-term natural fluctuations in and redistribution of stress. The links between bedrock stress, fracture geometry, and fracture hydraulic properties are widely recognized, and many numerical models describe these relationships. Several combinations of models were considered: four models for the relationship between fracture stress and hydraulic aperture, two models to identify critically stressed fractures, and two models to calculate critically stressed apertures. Appropriate calibration permitted these models to correlate well with both the major trends in the data and additional subtleties, indicating they can capture the interactions of the modelled natural processes. In contrast to empirical fitting of observed data, this suggests the models can produce predictive results.