Study On Discontinuum Numerical Models For Flow In Fractured Rock And Its Engineering Applications

The hydraulic properties of rock masses are likely to be highly heterogeneous even within a single lithological unit if the rock is fractured. The main difficulty in modeling fluid flow in fractured rock is to describe this heterogeneity. According to the representation of the heterogeneity, mathematical models for predicting flow in fractured geological media fall into one of three broad classes: equivalent continuum models, discrete fracture network simulation models and hybrid techniques. The equivalent continuum models, which may be deterministic or stochastic, and may incorporate dual porosity, have been used extensively in the past. But there are two greatest limitations of this method: the scale at which the continuum approximation is justified can be difficult to quantify and the process of spatial averaging restricts model predictions to scales greater than or equal to that of the representative elemental volume(REV). Hence, in the resent years, the last two models, which can both be ranked in discontinuum models, have been developed and have been increasingly powerful. The advantage of these models is that volume-averaging approximations are avoided at the scale of the fracture network. In cases where an equivalent continuum cannot be defined, discontinuum network simulation is a viable alternative. Howerer, these models are very demanding in terms of computer resources, which restraints their application to engineering practice.The main objective of this dissertation was to develop methodologies with capability of engineering application for simulating flow in fractured rock using discontinuum models. The study work includes three parts. In the first part, numerical methodology of three-dimensional discrete fracture network(DNF) model casing on hybrid BEM-channel was presented, and the technique for improve the model's computing efficiency was also studied. Firstly in this part, computer simulation methodology based on the Baechermodel for generating network of discrete fractures was presented, which includes the follow details: probability distributions of fracture density,orientation, trace length, size, and aperture and estimation of their statistical parameters; stochastic models of fracture network; Monte-Carlo's simulation method; numerical simulation procedure and technicality. Then, boundary element method was used to calculate flow through the generated fractured network. Assuming single fracture as a two-dimension inexpressible isotropic porous media, boundary element method equations for flow in single fracture and then in fracture network were derived using the weighted residual method. These equations were incorporated into a numerical procedure to discretize and solve the boundary value problem for hydraulic head and fluxes within fractured network. Some numerical technicalities, such as choice of fundamental solution to governing equation of flow problem, treatment process to corner point problems, automatic meshing, treatment process for fractures partly beyond the flow domain boundaries, were also discussed. Thirdly, a hybrid BEM-channel model was presented, and the numerical procedure for which was described. This model can combine the advantage of accuracy of the BEM model and the advantage of numerically efficient of the channel model. Finally, some simplification methods to the model were studied and a modified block elimination contraposing the features of the model's linear system was developed, which can greatly decrease the model's requirement to computer resources.In the second part, numerical methodology of hybrid techniques was developed, which uses DFN models developed in the fist part to build continuum approximations of fractured rock. The possibility of building porous media equivalents for networks of discrete fractures was analyzed. Tensor form of permeability of fractured rock was studied. The estimation process of continuum properties of fractured rock, including REV size and equivalent permeability tensor, were described.In the third part, a prima...