The effect of stress state on groundwater flow in bedrock: Simulations of in situ experiments

The effect of the stress state on the permeability of bedrock for groundwater was studied by simulating an in situ experiment. Previous studies show that the dependency of permeability on stress can have a significant effect on flow. Several models have been developed, but little has been done in order to develop models suitable for in situ applications, such as the deep underground repositories for spent nuclear fuel. In repositories, stress state evolves during the long time period considered in safety assessment. The effect of the changing flow pattern, due to the evolving stress, has to be estimated for, e.g., radionuclide transport calculations. Previous work done in the field was reviewed, existing relations between stress and permeability were analysed, and suitable relations were selected for the modelling cases. Rock mass permeability and discrete fracture permeability were treated separately. One new empirical model for fracture permeability was presented and three models were further developed to be more suitable for 3-D implementation. Simulations followed in situ experiments conducted in Äspö Hard Rock Laboratory. The modelling geometry was constructed based on the experimental setup and the fracture information from the location. The overall stress state in the area was known and the effect of the measurement tunnel and boreholes was computed. The stress state was used to compute the groundwater flow, and the applicability of the chosen models for in situ modelling was analysed. COMSOL Multiphysics was used as the tool for the simulations. The simulation results followed the measurements reasonably well, but differences were found with one model. The results show that differences between most of the models were relatively small if inflow rates were compared, however, differences between flow patterns were found. Stress dependency could partly explain observed phenomena and qualitative behaviour. Moreover, some of the fracture models were able to identify fractures prone to deformation.