Numerical and experimental analysis of fluid flow in fracture replicas with variable aperture

Sammanfattning: Flow in fractured rocks has been intensively studied in the last decades, in part due to plans in manycountries to site repositories for high level nuclear waster in deep geologic formations. All investigated crystalline rocks have been found to be fractured and the majority of water flows through these fractures andfracture zones. Therefore, it is of interest to be able to understand and model flow rates, flow pathwaysand discharge through realistic rough-surfaced fractures. Conventional experiments using rock samples havedifficulty controlling and observing fracture properties, in particular the highly variable fracture aperturevoid space. Therefore, taking advantage of 3D printing technologies, two samples were created representinga constant and varying aperture fracture. This allows the properties of the fracture to be controlled andidentical geometries for both experiment and simulation to be prepared. Development of a laboratory experiment allowed for flow tests over different hydraulic gradients to be conducted through the printed samples.Discharge was numerically simulated through 9 single fracture cases, 5 of which have a constant aperture,assuming the Navier Stokes equations and using a computational fluid dynamics solver (OpenFOAM). Theresult of the simulation based on the Navier Stokes equations was within 5% of the experimental result forhydraulic gradients of 0.01 and 0.05, this suggests that the results from the experiment and simulation arereliable. In order to observe the transition from Darcy to non Darcy flow through a single fracture the hydraulic gradient was systematically increased. The results from both the experiment and the simulation werecompared to the simplified cubic law assumption using various estimates of aperture. Inertial forces influencethe discharge and have more importance on varying aperture geometry, therefore the transition to non Darcyflow in smooth apertures occurs at a higher hydraulic gradient. Calculation of the cubic law using the meanaperture value including zeros can reduce the difference between simulated discharge. A two-fracture systemwith one intersection was also simulated to investigate the influence of intersection geometry on discharge,which seems to be dependent on flow velocities, with low and high flows producing less variable discharge.Uncertainties between numerical simulation and laboratory experiments can be reduced by using 3D printedfracture networks, hence they can be a beneficial for understanding complex interactions that can happenwithin networks.