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Title: Numerical modelling of hydraulic fracturing in naturally fractured rock
Author: Obeysekara, Asiri
ISNI:       0000 0004 7658 6423
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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This thesis presents a novel approach for hydro-geomechanical modelling of fractured rocks by linking a solid mechanics model with a multi-fluids and multi-phase fluid flow model using the immersed-body approach for coupling fluids and solids. The method uses conservative mesh-to-mesh projection to pass variables between an mesh adapting multi-phase fluid-flow solver and a deformation and fracturing solid geomechanics solver. The adaptive mesh refining and coarsening of the fluid model further permits the flow within and near to fractures (e.g. localised flow, leak off) to be represented using a locally refining mesh. The mass conservation between the coupled fluid and solid fields is ensured through a globally conservative Galerkin projection-based mesh interpolation. To simulate the nonlinear deformation of natural fractures in rock, the geomechanics model is integrated with a joint constitutive model that can capture the aperture variation of rough fractures under normal and/or shear loading. The solid mechanics is modelled using a Lagrangian specification in a Finite-Discrete Element Method (FEMDEM) framework called Solidity Project. Multi-phase (incompressible and compressible) fluid flow in porous media is considered through a Control-Volume Finite-Element Method (CV-FEM) based Darcy Flow solver and is called the Imperial College Finite-Element Reservoir Simulator (IC-FERST). The one-way (solid to fluid) coupled model is validated against analytical solutions for single-phase flow through a smooth/rough fracture and other reported numerical solutions for multi-phase flow through intersecting fractures. Examples of modelling single- and multi-phase flows through fracture networks under in-situ stresses are further presented, illustrating the important geomechanical effects on the hydrological behaviour of fractured porous media. The two-way coupled model for fluid-driven fracturing is validated using available laboratory hydraulic fracturing experiments in un-stressed and stressed regimes. The coupling framework is extended to three-dimensional, coupled modelling of flow modelling and fluid-driven fracturing through the creation of locally enriched shell-mesh elements at pre-existing and new fracture surfaces. The coupled framework can be extended to other perturbed states than fluid pressure driven fracturing, such as during underground excavation and dewatering. Application of the coupled framework to assess the stability of tunnel walls during tunnel excavation in saturated highly-fractured porous rock has been investigated in the context of deep-geological disposal of nuclear waste.
Supervisor: Latham, John-Paul ; Xiang, Jiansheng Sponsor: NERC Centre for Doctoral Training in Oil and Gas
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral