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Title: Modelling the flow and transport properties of two-dimensional fracture networks, including the effect of stress
Author: Leung, Colin Tsee-On
ISNI:       0000 0004 2735 4204
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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This thesis focuses on the effective hydraulic transmissivity of two-dimensional fracture networks in rocks. The main simulation tool used in this work is the discrete fracture network code NAPSAC. There are four main topics in this thesis: (1) estimating permeability from network properties, (2) comparing discrete fracture network with effective continuum models, (3) using DFN for hydro-mechanical coupled modelling, and (4) solute transport simulations. For fracture networks with uniform aperture, the permeability can be estimated using segment density, fracture density, and fracture lengths of the fracture network. For fracture networks with apertures directly proportional to their lengths, the individual conductance of each of the fracture segments was used to calculate an effective conductance for the whole network. The arithmetic mean of the segment conductance gives a good approximation for the effective conductance of the whole network. A series of effective continuum models of a fracture network were created using different element sizes, and their flow behaviours were compared against results obtained from discrete fracture network model. The permeability tensors of each of the elements in the effective continuum meshes were calculated using discrete fracture network methods. It was found that the flow through effective continuum model with any element size gave good agreement with the discrete fracture network results. Hydro-mechanical coupled simulations were carried out using NAPSAC, where the applied far field stresses are applied to each fractures independently. Simulations were then carried out using the distinct element code UDEC to justify the simplified physics used in NAPSAC. It was shown that for random 2D fracture networks under a range of loadings, NAPSAC and UDEC seem to predict similar overall flows. Different ways for modelling the effects of rock matrix diffusion were explored. The significance of rock matrix diffusion, as well as the diffusion distance, was linked to the magnitude of the pressure gradient across the fracture network. A semi-analytical method for estimating the diffusion distance was proposed: using the perimeter and the area of each of the matrix blocks, it is possible to estimate the diffusion distance using the 'shape factor' concept.
Supervisor: Zimmerman, Robert Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral