Simulating flow, transport and hydrogeochemical processes within
fracture networks
Abstract
Modelling physical processes such as flow and transport within a
fracture network can be challenging, both conceptually and numerically.
A typical approach is to upscale the properties of the network onto a
regular grid of elements, which is then used to simulate the required
physical processes. However, this method can be inaccurate if not
carefully applied. For example, when most of the flow passes through a
few choke points in the network, upscaling may introduce extra
connectivity that is not present in the original network. The
ConnectFlow software package has an alternative method for representing
fracture networks, whereby the fractures are explicitly modelled as a
network of intersecting two dimensional planes. The algorithms within
ConnectFlow are very efficient, allowing millions of separate fractures
to be simulated, each discretised using hundreds of finite elements.
Here we present recent updates to this functionality: 1) to allow the
advection-diffusion equation to be solved for multiple solute species
(which is fully coupled to the pressure solution via the buoyancy term
in Darcy’s equation); 2) to model the diffusion of solutes into the pore
space of the surrounding rock (i.e. matrix diffusion); and 3) to carry
out chemical reactions between solutes and minerals (which coat the
fractures and/or the rock pores) using an interface to the IPhreeqc
library. The ConnectFlow algorithms have also been parallelised to
improve the tractability of this new functionality. This implementation
represents a significant step forward in capability that allows
groundwater flow, transport and hydrogeochemical reactions to be
properly represented in the context of structurally constrained
fractured bedrock. This has a wide range of potential applications,
particularly for future safety assessments of nuclear waste facilities.
Example calculations are presented for the Onkalo disposal facility in
Olkiluoto, Finland, and the proposed Swedish repository for long-lived
waste, SFL.