This study investigates the influence of aperture heterogeneity induced by the fracture surface roughness on the hydraulic properties of 3D discrete fracture network (DFN) models. Numerical simulations of fluid flow were performed on a series of 3D DFN models with different fracture densities. Four different aperture distribution functions with increasing mean values were used to generate the model with anisotropic apertures. The corresponding parallel fracture model was also generated to compare the flow behavior with the anisotropic fracture model. The results show that there exists obvious channeling flow effect in the anisotropic fracture model. The fluid flows through the channels with large aperture in each fracture plane, and at the same time, it selects the most transmissive fractures at the fracture network scale. The ratio between the permeability of models with anisotropic and uniform apertures is smaller than 1, indicating that fracture roughness tends to decrease the permeability of 3D DFN model. The difference of flow patterns between parallel fracture model and anisotropic fracture model decreases as the mean aperture and fracture density increase. In practice, to realistically simulate the fluid flow in rock fractures, the 3D DFN model with heterogeneous apertures should be utilized and the channeling flow should be considered.
Accurate description and quantitative estimation of permeability of fractured rock masses have attracted substantial attentions in the past few decades in various perspectives such as radioactive waste storage, geothermal energy extractions, and oil production (Neuman 2005). In engineering practices, owing to the computational limitations and the insufficient 3D geological information, analysis of hydraulic properties of rock fractures is often reduced to a 2D problem (Li et al. 2016; Liu et al 2016). In recent years, a great amount of efforts has been exerted to study flow characteristic of 2D fracture networks, and significant progress has been made towards the theoretical modeling, laboratory experiments and numerical methods. However, the 2D fracture model, which is a cut plane of the 3D fracture network, cannot capture the geometry of real features. Whereas 3D fracture networks have the outstanding advances of describing the orientation, connectivity, and permeability tensor of the real rock masses (Kirkby et al. 2017).
In analyzing fluid flow through fracture networks, discrete fracture network (DFN) approach is a widely used modeling technique (Maillot et al. 2016). In contrast to continuum methodologies which are mainly applied for flow analyses over a lager domain, DFN approach is better treated in explicit representation of fracture geometries (fracture length, density, location, orientation and aperture) together with specific description of their intersections. In the DFN approach, rock fractures are described by smooth parallel plates. Whereas, natural rock fracture usually displays a strong hydraulic complexity coming from the heterogeneous aperture distributions induced by the topography of fracture (Luo et al. 2016; Huang et al. 2018). The assumption of parallel plate would lead great deviations from hydraulic responses of natural fractured rock masses. Many previous studies have been focused on the hydro-mechanical properties of single fracture considering the fracture aperture heterogeneity, where hydraulic property of 3D fracture network with heterogeneous aperture distribution remains poorly understood.
This paper provides a brief summary of a continuous research programme by the authors since 2004, and highlights the research approach, achievements and outstanding issues for conceptual understanding, laboratory testing and mathematical modeling of the coupled stress-shear-fluid flow-solute transport processes of rock fractures. The focuses are put on stress and shear induced fluid flow anisotropy, transport pass channeling, and impact of considering different retardation mechanisms in single fractures of crystalline rocks, typically granites, due to its importance for the performance and safety assessments of geological radioactive waste disposal projects.
Rocks are natural geological materials containing fractures of different origins, sizes, mineral fillings, weathering degrees, orientations, termination patterns, thickness and shapes, and especially surface roughness features. In addition, rocks in-situ are under stress, caused by dynamicmovements in the upper crust of the Earth, such as tectonic plate movements, earthquakes, land uplifting/subsidence, glaciation cycles and tides, in addition to gravity. A rock mass is also a fractured porous medium containing fluids in either liquid or gas phases (e.g. water, oil, natural gases and air), under complex in-situ conditions of stresses, heating or cooling, freezing or thawing, fluid pressures, and complicated geochemical reactions, with connected fractures most often serve as the major energy and mass transport pathways and most active areas of geochemical reactions, especially for fractured hard crystalline rocks. This is the reason why the coupled thermal (T), hydraulic (H), mechanical (M) and chemical (C) processes is an issue of importance in the field of rock mechanics.
The terms “discontinuity” and “fracture” are used interchangeably in the rock mechanics literature. The term “fracture” is adopted throughout this paper as a collective term for all types of natural or artificial discontinuities such as faults, joints, dykes, fracture zones and other types of weakness surfaces or interfaces, unless specifically stated otherwise. The rock fractures are usually not just open voids with fresh and smooth surfaces. Their surfaces (or walls) are often rough, weathered and fully or partially filled with precipitated minerals, and their relative positions are often modified by geological history and loading conditions, such as opening, closing, faulting or shearing, with large or small relative displacements. The complexity in the surface topography makes understanding and quantitative representation of the physical-chemical behavior and rock fracture properties difficult issues.