On the parameterisation of dual-continuum models for reactive transport modelling in fractured media

Abstract

In sparsely fractured rocks, the rock matrix is an important geochemical buffer and provides significant retardation to contaminants advected through the flowing fractures. Accounting for geochemical reactions and mass exchange between these two regions is key to properly capture the overall buffering capacity and the related hydrogeochemical evolution of a fractured medium. Reactive transport modelling in these kinds of fractured media is routinely performed using Equivalent Continuous Porous Media (ECPM) models: i.e. continuum models based on permeability and porosity fields that somehow preserve the underlying fracture properties, which are in turn described by companion Discrete Fracture Network (DFN) models. However, the proper parameterisation of these models, in terms of mass exchange between fractures and the bordering matrix, is still a largely unresolved issue. Here, we leverage the Dual Continuum Disconnected Matrix Model (DCDMM) formulation included in the massively parallel code PFLOTRAN to propose a novel parameterisation approach that honours the local volumetric fracture density (P32). The proposed approach is first benchmarked against a semi-analytical solution with a problem that entails flow and transport along two different and consecutive fractures. Two demonstrative large-scale reactive transport problems are also presented and discussed: the first is related to the generation and migration of radiogenic helium and the second assesses the buffering capacity of a realistic fractured medium against the infiltration of acidic water. The latter simulation, which includes more than three hundred million transport degrees of freedom is one of the largest subsurface reactive transport models ever formulated and solved. This simulation was made possible by the highly efficient implementation of the DCDMM in PFLOTRAN, which makes the solution of the secondary continuum equations embarrassingly parallel.