Modelling the Effect of Heterogeneity on the Hydromechanical Response of Large-Scale Fracture-Networks in Brown Coal Slopes

Abstract:

Rehabilitation of brown-coal mine-sites requires numerical models that are capable of predicting their behavior over the long term. In particular, understanding the long-term stability of brown coal slopes requires accurate means of assessing the coupling between the hydrodynamic behavior of the slope and its mechanical response. However, brown coal often contains many fractures that affect the flow of fluid and influence the stability of the formation. These fractures are heterogeneous in nature, with widely distributed characteristics. While discrete fracture models may be used to determine the mechanical response of individual fractures, such models are often not suited to large-scale applications such as open-cut coal-mines. Instead, here we describe how continuum permeability models can be used to capture fracture heterogeneity and its effect on the physical deformation of the fracture-network by coupling the effects of fluid flow and the fracture strength.

In this presentation, we will discuss the influence of heterogeneity on the fluid flow and how these variations affect the fracture strength. To capture these effects, a numerical model that couples the effects of water pressure and solid mechanics was implemented using the Multiphysics Object Orientated Simulation Environment (MOOSE). The numerical model was informed by real-world data from a brown coal formation and uses a stress dependent fracture aperture that allows the fracture to expand or contract depending on the pressures acting on it. In addition, the presence of fractures decreases the strength of the formation along particular directions. This is represented in the numerical model as planes of weakness which are added to the plasticity law. These, like the aperture distributions in the hydrodynamic model, are chosen to best conform to the existing fracture distribution. The results of the simulation show that the heterogeneous nature of the fracture network intensifies fluid flow and the deformation of the formation and must be accounted for when assessing its stability.