Effective Continuum Approximations for Permeability in Brown-Coal and Other Large-Scale Fractured Media

Abstract

The stability of open-pit brown-coal mines is affected by the manner in which water is transmitted or retained within their slopes. This in turn is a function of the in-situ fracture network at those mines. Fracture networks in real mines exhibit significant degrees of heterogeneity; encompassing a wide range of apertures, inter-fracture separations, and orientations. While each of these factors plays a role in determining fluid movement, over the scale of a mine it is often impractical to precisely measure, let alone simulate, the behaviour of each fracture. Accordingly, effective continuum models capable of representing the bulk effects of the fracture network are needed to understand the movement of fluid within these slopes. This article presents an analysis of the fracture distribution within the slopes of a brown coal mine and outlines a model to capture the effects on the bulk permeability. A stress-dependent effective-fracture-permeability model is introduced that captures the effects of the fracture apertures, spacing, and orientation. We discuss how this model captures the fracture heterogeneity and the effects of changing stress conditions on fluid flow. The fracture network data and the results from the effective permeability model demonstrate that in many cases slope permeability is dominated by highly permeable but low-probability fractures. These results highlight the need for models capable of capturing the effects of heterogeneity and uncertainty on the slope behaviour.