A Multi-continuum Method for Studying the Effect of Inactive Fractures on Solute Transport in 2-D Discrete Fracture Network

摘要

Fractures in a discrete fracture network can be divided into two parts: Active fractures, which form a connected fracture network and dominate fluid flow and solute transport; and inactive fractures, which are dead-end parts of the fractures (isolated fractures will be incorporated into rock matrix) and do not contribute significantly to the fluid flow, but maybe important for the solute transport, especially for rock matrix diffusion. We present a multi-continuum method (including active fracture continuum, inactive fracture continuum and matrix continuum), which is based on the "multiple interacting continua" method, to describe fluid flow and solute transport in fractured media, including interactions of (1) active fractures with inactive fractures, (2) active fractures with matrix and (3) inactive fractures with matrix. A 2-D discrete fracture network is transformed into a coarse-scale grid-based equivalent continuum model, and each coarse-scale block is discretized into overlying sub-blocks including active fracture continuum, inactive fracture continuum and nested matrix continua with equivalent properties based on local fracture geometry information. The permeability tensor for the sub-block associated with active fracture continuum is determined from local flow simulations using the underlying discrete fracture network. The permeability for inactive fracture continuum and matrix continuum is assigned with very small value as they do not significantly contribute to the fluid flow. With this upscaling method, we established a heterogeneous, anisotropic permeability tensor field in the study domain. The above methodology was applied to a 2D BMT (benchmark test) of the international cooperative project DECOVALEX 2011. This benchmark test consists of a 20×20 m model domain including a 2-D fracture-network of 7797 individual fractures with apertures of each fracture correlated to their length. The simulation results show that the inactive fractures will enhance rock matrix diffusion, which is consistent with observations at field experiments as reported in the literatures, and thus play an important role in solute transport in fractured media.