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This paper presents a novel approach for hydromechanical modelling of fractured rocks by linking a finite-discrete element solid model with a control volume-finite element fluid model based on an immersed-body approach. The adaptive meshing capability permits flow within/near fractures to be accurately captured by locally-refined mesh. The model is validated against analytical solutions for single-phase flow through a smooth/rough fracture and 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., Computers and Geotechnics, 103, ISSN:0266-352X, ISSN:1873-7633

A study about the influence of polyaxial (true-triaxial) stresses on the permeability of a three-dimensional (3D) fractured rock layer is presented. The 3D fracture system is constructed by extruding a two-dimensional (2D) outcrop pattern of a limestone bed that exhibits a ladder structure consisting of a “through-going” joint set abutted by later-stage short fractures. Geomechanical behaviour of the 3D fractured rock in response to in-situ stresses is modelled by the finite-discrete element method, which can capture the deformation of matrix blocks, variation of stress fields, reactivation of pre-existing rough fractures and propagation of new cracks. A series of numerical simulations is designed to load the fractured rock using various polyaxial in-situ stresses and the stress-dependent flow properties are further calculated. The fractured layer tends to exhibit stronger flow localisation and higher equivalent permeability as the far-field stress ratio is increased and the stress field is rotated such that fractures are preferentially oriented for shearing. The shear dilation of pre-existing fractures has dominant effects on flow localisation in the system, while the propagation of new fractures has minor impacts. The role of the overburden stress suggests that the conventional 2D analysis that neglects the effect of the out-of-plane stress (perpendicular to the bedding interface) may provide indicative approximations but not fully capture the polyaxial stress-dependent fracture network behaviour. The results of this study have important implications for understanding the heterogeneous flow of geological fluids (e.g. groundwater, petroleum) in subsurface and upscaling permeability for large-scale assessments., Hydrogeology Journal, 25 (8), ISSN:1431-2174, ISSN:1435-0157

This paper presents a stress-induced variable aperture model to characterise the effect of polyaxial stress conditions on the fluid flow in three-dimensional (3D) persistent fracture networks. Geomechanical modelling of the fractured rock is achieved by the finite-discrete element method (FEMDEM), which can capture deformability of matrix blocks, heterogeneity of stress fields as well as sliding and opening of pre-existing fractures. Propagation of new cracks is not required for this study of persistent fracture systems. The deformed fracture network topologies include details of dilation, opening and closing of fracture apertures, from which the local variations in hydraulic apertures are derived. Stress-controlled distribution of fracture apertures is modelled with both fracture-scale and network-scale effects considered. Under a geomechanical condition with low differential stress ratio, fracture porosity is dominated by the fracture-scale roughness. However, with the increase of stress ratio, some favourably oriented fractures are reactivated for shearing, and matrix blocks are promoted to rotate and generate large openings along their boundaries, which tend to be the key contributors to the aperture field in such persistent systems. The flow behaviour is then considered for these stressed but static solid skeletons and is investigated using a finite element solution to the Laplace problem of single-phase fluid flow. The equivalent permeability tensor of each cube-shaped rock mass is computed based on a series of flow simulations under a macroscopic pressure differential applied at opposite model boundaries with no-flow conditions on the remaining boundaries. Components of the permeability tensor are found to vary more than three orders of magnitude with respect to the change of stress ratio. Large aperture channels formed under a critical stress state accommodate significant localisation features in the flow structure of the network. The results of this study have important implications for upscaling permeability to grid block properties for reservoir flow simulation.