Site modelling using DFN.lab:
flow and transport
Development of a DFN-Based Numerical Model for Tracer Test Simulations in Fractured Crystalline Rock
Context
The Forsmark site in Sweden, known for its sparsely fractured crystalline bedrock, has been selected as the location for a deep geological repository for nuclear waste. Highly detailed, DFN (Discrete Fracture Network)-based site models are essential for post-closure safety assessments and construction planning. DFN-based models provide the most accurate representation of the fractured rock system and geological context.
The project aims to develop and parametrize a DFN-based numerical model to simulate a generic doublet (injection/pumping in one borehole pair) tracer test under conditions relevant to the Forsmark site. Key tasks include assigning realistic transmissivities to the fractures, with in-plane distributions of open and sealed portions, and setting fracture properties proportional to fracture sizes. Additionally, the model applies specific hydraulic boundary conditions, such as a regional head gradient and controlled injection and pumping flow at the boreholes.
Project results
We developed a comprehensive numerical model that includes DFN generation, assignment of fracture transmissivities and apertures, calculation of connectivity between hydraulically active boundaries, and computation of steady-state head and velocity fields. Additionally, the model simulates tracer tests using a particle tracking algorithm. The developed scripts include post-processing functions for generating visual outputs, such as model images, breakthrough curves, and flow logs.
Initial tests demonstrated the model’s ability to successfully simulate in-situ steady-state doublet tracer tests under sparsely fractured crystalline rock conditions. The model is fully parametrized, allowing users to easily control the resolution of the DFN, including the lower cut-off size of fractures and the size of the study domain, ensuring flexibility for future applications.
