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The transport properties of complex fracture networks are of great significance to predicting the subsurface flow. Tree-like fracture networks were generated according to the fractal theory, and the roughness of fracture surfaces was created by using the Weierstrass–Mandelbrot function. The forward and reverse flows were numerically investigated via the lattice Boltzmann method by employing positive and negative pressure differences respectively. The results indicate that the bifurcation angle θ, fractal dimension D and flow direction have significant influences on the transport properties of fracture networks. The maximum velocity increases quaveringly from the entrance or the last bifurcation point but suddenly decreases at the next bifurcation point, which is mainly due to the variation of local roughness. Both the energy loss ratio and the local pressure loss coefficient increase with the bifurcation angle. The cavity vortex can form within the concave of rough surface for fractal dimensions larger than 1.2 which result in the reduction of the hydraulic aperture and the alteration of the streamline from being parallel with the surface to a favorable flow path. The ratio of fracture hydraulic width to physical width decreases with the fractal dimension and fracture level. The flow rate of reverse flow is higher than that of forward flow under the same driven pressure because the flow convergence has lower local resistance than the flow diversion at the bifurcations. The theoretical solution of the flow rate through the rough-walled tree-like fracture networks was derived and verified by the modelling results.

Fluid flow in a rock fracture bounded by two rock surfaces with surface asperities is a complex phenomenon to study. However, precise knowledge of the flow characteristics through a real rock fracture is essential in order to design and estimate the efficiency and production of unconventional oil and gas exploration and geothermal energy extraction projects. The aim of this numerical study, is therefore to incorporate a rock fracture in the modelling platform using a pre-processing procedure and to couple it with the flow parameters. 3-D scanning technology was used to obtain the rock fracture surfaces and to generate the fracture profile in a grid matrix form. In addition, the generated fracture profile was imported in to the COMSOL Multiphysics software package to simulate the flow characteristics of the rock fracture. The COMSOL model was validated using experimental permeability results conducted under triaxial conditions. According to the results, the COMSOL numerical model can simulate the flow characteristics through the rock fracture with more than 90% accuracy compared to the experimental data. The numerical results also reveal that the pressure gradient through a rock fracture is nonlinear and depends on the fracture profile. Furthermore, the nonlinearity of pressure gradient varies on different sections of the fracture, confirming the heterogeneity nature of the fracture. In addition, the results illustrate that the entire fracture width does not contribute to the final flowrate and that it is essential to consider the effective fracture width in flow calculations.

Underground coal mining usually develops vertical stresses and reduces the horizontal stress in front of the coal face, which may trigger the occurrence of coal ruptures and gas outburst hazards. According to the stress change characteristics of the working face using different mining methods, different loading paths (constant confining pressure path and unloading confining pressure path) were selected in this study. Methane permeability tests of coal samples under different loading and unloading paths were conducted. Based on these tests, the effects of the unloading rate on the mechanical behaviour and permeability evolution (including energy evolution and fractal dimension) of the coal was investigated quantitatively. The tests revealed that the higher confining pressure unloading rates yielded a lower compressive strength and ductile strain. The increase in the unloading rate reduced the elastic energy and dissipation energy of the samples. Moreover, the higher unloading rate corresponded to more complex failure structures of coal and a greater fractal dimension, which resulted in greater permeability after the rupture of the coal samples. Furthermore, the higher unloading rates accelerated the rock damage rate and the permeability increase rate, which had a notable effect on the emergence of gas outburst hazards.

Since the secure storage of CO2 in any geological sink is largely dependent on its caprock and reservoir rock flow properties, it is necessary to check the permeability of both in assessing geological reservoirs for CO2 storage. Siltstone is a common rock type in both. On the other hand, the highly complicated thermodynamic properties of CO2 cause its flow behaviour through any rock mass to become highly complex. This intention of this study is to understand this complexity and to propose an accurate technique to evaluate the apparent permeability of CO2 through siltstone under laboratory conditions, considering the possible phase transitions of CO2 inside the rock mass. A series of tri-axial drained experiments on siltstones at room temperature was therefore performed. According to the results, the proposed method accurately predicts permeability through siltstone in tri-axial drained tests, because it more precisely considers the influence of CO2 phase transition on its flow performance, and the permeability in the sample is separately evaluated for the liquid and gas CO2 regions. The new approach shows that the actual CO2 pressure distribution along the sample has a curvilinear shape. Consideration of the possible phase transition between the sample inlet and the outlet is particularly important for liquid CO2 injection in tri-axial drained tests, due to the certainty of the occurrence of phase transition under this liquid inlet and gas outlet condition. According to the test results, the apparent permeability calculated for liquid CO2 injection using the proposed method is around 50% lower than that evaluated using the traditional method. This suggests the importance of the use of more accurate approaches such as that proposed under such situations. In addition, CO2 permeability in siltstone is found to be significantly increased with increasing injection pressure, and increasing the injection pressure from 3 to 6 MPa (in the gas CO2 region) caused the CO2 permeability in the tested siltstone to be increased from 0.00102 to 0.00228 mD at 15 MPa confining pressure, which is related to the related pore structure modification in siltstone.

Brittleness is commonly used to characterize the possible failure features in rocks, quantified by the brittleness index (BI(26)BIs, the applicability of each BI should be well understood to enable the selection of the most suitable for each application. This study reports on a detailed review of existing BI definitions in the rock mechanics field, the transition from brittle to ductile and the application of BIs to shale fracturing. The success of shale gas recovery using hydraulic fracking is greatly dependent on the shale's brittleness, since brittle shales have many pre-existing fractures and are easy to fracture in tensile and shear modes. A combination of laboratory and geophysical approaches are recommended for shale brittleness quantification. Precise quantification of brittleness is important, both in the laboratory and the field. Brittleness indices based on the elastic moduli (Young's modulus and Poisson's ratio) and mineral composition are common in field applications, and can be derived from both laboratory tests and field log data.