Development of Flow and Transport Model for Three-dimensional Discrete Fracture Networks

• Main Authors: I-hsien Lee, 李奕賢
• Other Authors: Chuen-fa Ni
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/27267943795444104159

博士 === 國立中央大學 === 應用地質研究所 === 104 === Development of flow and contaminant transport models for three-dimensional (3D) discrete fracture networks (DFNs) is critical to characterize flow and transport in fractured rocks. The fractures in a rock are relatively permeable as compared with the rock matrix. The difficulty in resolving complex fracture and matrix interactions in 3D domains has motivated investigators to focus mainly on fracture networks for characterizing flow and transport in fractured rocks. Because of the complex fracture geometry and connectivity, generations of fracture unstructured mesh and simulations of flow and transport in 3D DNF become challenging tasks. The objectives of this study are to develop, test, and implement numerical models for generation of 3D DFNs, generation of DFN meshes, and simulations of DFN flow and advection-dispersion transport. The developed DFN generator enables the Poisson and uniform distributions to be implemented for fracture locations and other fracture properties such as sizes, trend, and plunge. Other distributions of fracture properties can be employed in the developed DFN generation model. The DFN mesh generation model employed the Delaunay triangulation algorithm and applied a boundary recovery technique to resolve detailed fracture intersections and fracture concurrent points. This mesh generation model can automatically export mesh formats for other public domain models such as TOUGH series models and HYDROGEOCHEM model. Unlike previous investigations that focused on the particle tracking algorithm for DFN transport simulations, this study proposed a back rotation process (BRP) for fracture orientations and directly solved Eularian-based advection-dispersion equation for 3D fracture networks. The developed flow and transport model were validated with the TOUGH2 and HYDROGEOCHEM models using single porous fracture plate. The implementations of the developed models were simply divided into two different parts. The first part is implementations of DFN and DFN mesh generations for the TOUGH2 model and the associated equation of state (EOS) modules, including ECO2N and EOS7R. The second part focuses on implementations of the developed DFN flow and advection-dispersion models for issues of equivalent hydraulic conductivity in DFN upscaling and for analysis of transport uncertainty based on different monitoring strategies. The results of model validations showed that our DFN flow and transport models can reproduce identically the solutions of flow and concentration obtained from TOUGH2 and HYDROGEOCHEM models. The estimations of equivalent hydraulic conductivity based on multiple DFN realizations showed that different fracture intensities can lead to variations of equivalent hydraulic conductivity values in 2 to 3 orders of magnitude lower than the value of the fracture hydraulic conductivity. The comparisons of 2D fracture intensity (P21) and 3D fracture intensity (P32) indicate that the variations of P21 values are relatively high as compared with a specified range of P32. High variations of concentration breakthrough curves were obtained when fronts of plumes passing the observed locations. The maximal mean concentration breakthrough curves for different averaging strategies might not reach the maximal concentration value released at continuous sources. The standard deviations of concentration at different times can be higher than half of the source concentration at different monitoring locations. Additionally, the values of concentration standard deviations are similar for different monitoring points and monitoring (averaging) strategies.