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The permeability of a coal seam is an important index for coal mine gas control and coalbed methane development, and its magnitude determines the degree of difficulty of gas drainage. To obtain the permeability value, a dimensionless mathematical model for dual-porosity borehole gas-coupled flow in a coal seam was established and adopted using a simulator developed by our group. A new method of inversion was developed to determine the fracture permeability coefficient λ f and the matrix micro-channel diffusion coefficient K m by fitting the simulated results with onsite measured data. A range of simulations quantified the effects of different dimensionless parameters on gas migration. The results verified the feasibility of the inversion method based on the high matching degree of the fitted results, and the dimensionless mathematical model was accurate. The desorption and release of adsorbed gas from the center to the surface in coal matrices were heterogeneous, and unsteady states and gas migration times in coal matrices cannot be neglected. The new method can be introduced to analyze the problem of gas migration in different coal reservoirs, simplify the corresponding calculation and computational processes, and provide guidance in determining the permeability of coal seams.

We present a series of controlled fluid injection experiments in the laboratory on a pre‐stressed natural rough fracture with a high initial permeability (∼10−13 m2) in granite using different fluid pressurization rates. Our results show that fluid injection on a fracture with a slight velocity‐strengthening frictional behavior exhibits dilatant slow slip in association with a permeability increase up to ∼41 times attained at the maximum slip velocity of 0.085 mm/s for the highest‐rate injection case. Under these conditions, the slip velocity‐dependent change in hydraulic aperture is a dominant process to explain the transient evolution of fracture permeability, which is modulated by fluid pressurization rate and fracture surface asperities. This leads to the conclusion that permeability evolution can be engineered for subsurface geoenergy applications by controlling the fluid pressurization rate on slowly slipping fractures. Plain Language Summary Understanding the evolution of fracture permeability during hydraulic stimulation of subsurface reservoirs is the key to characterizing fluid transport and formulating strategies to limit induced seismicity. Accordingly, there is a significant interest in deciphering how the fluid pressurization rate, a constitutive operational parameter during injection, influences the transient permeability change during fracture slip. We conducted a series of experiments in the laboratory using different fluid pressurization rates on a natural rough fracture in granite under a pre‐stressed state. The fracture had a high initial permeability. Our findings show that when fluid is injected into a fracture with a slight velocity‐strengthening frictional behavior, it causes slow slipping with significant permeability enhancement. The change in hydraulic aperture caused by slip velocity is the main reason for the temporary change in permeability, and this effect is modulated by fluid pressurization rate and fracture surface irregularities. Our results suggest that we can modulate the permeability of subsurface geoenergy reservoirs by controlling the fluid pressurization rate on slowly slipping fractures. Key Points We conducted fluid injection experiments on a pre‐stressed natural rough fracture in granite at different pressurization rates The velocity‐strengthening fracture exhibits slow slip accompanied by a significant increase in permeability during fluid injection Transient fracture permeability is controlled by injection‐induced slip velocity, modulated by pressurization rate and surface asperities

We perform a set of reactive transport simulations in three‐dimensional fracture networks to characterize the impact of geochemical reactions on flow channelization. Flow channelization, a frequently observed phenomenon in porous and fractured subsurface rock formations, results from the spatially variable hydraulic resistance offered by a geological structure. In addition to geo‐structural features such as network connectivity, geometry, and hydraulic resistance, geochemical reactions, for example, dissolution and precipitation, can dynamically inhibit or enhance flow channelization. These geochemical processes can change the fracture permeability leading to increased flow channelization, which are localized connected regions of high volumetric flow rates that are seemingly ubiquitous in the subsurface. In our simulations, fractures partially filled with quartz are gradually dissolved until quasi‐steady state conditions are obtained. We compare the flow field's initial unreacted and final dissolved states in terms of flow and transport observations. We observe that the dissolved fracture networks provide less resistance to flow and exhibit increased flow channelization when compared to their unreacted counterparts. However, there is substantial variability in the magnitude of these changes which implies that the channelization strongly depends on the network structure. In turn, we identify the interplay between the particular network structure and the impact of geochemical dissolution on flow channelization. The presented results indicate that geological systems that have been weathering or reactive for longer times in older landscapes are likely to have increased flow channelization compared to their equivalent but younger counterparts, which implies a time dependence on flow channelization in fractured media. Plain Language Summary Fractures are the primary pathways for fluid flow and solute transport in Earth's subsurface. In many of these systems, fluids passing through the fractures are out of equilibrium with the resident minerals, and various geochemical reactions occur. These geochemical processes can change the resistance to flow offered by the fractures, leading to increased flow channelization, which are localized connected regions of high flow rates. However, quantification of these impacts has been limited to the computational burden of performing requisite simulations. Through a series of reactive transport simulations in fractured media, we characterize the influence of dissolution on flow channelization using observations of flow and transport properties. We compare flow and transport in the initial unreacted state and in the final dissolved state to better understand how geochemical reactions influences the flow properties of the network medium. We observe that the unreacted fracture networks provide lower resistance to flow and exhibit increased flow channelization. The results suggest that older geological systems ought to have increased flow channelization when compared to their younger counterparts. Key Points We characterize the impact of geochemical dissolution on flow channelization in fractured media using reactive transport simulations We observe that the dissolved fracture networks provide less resistance to flow and exhibit increased flow channelization There is a dissolution feedback loop between primary sub‐networks and geochemical reactions on flow channelization

Using an innovative experimental set-up (Punch-Through Shear test), we initiated a shear zone (microfault) in Flechtingen sandstone and Odenwald granite under in situ reservoir conditions while monitoring permeability and fracture dilation evolution. The shear zone, which has a cylindrical geometry, is produced by a self-designed piston assembly that punches down the inner part of the sample. Permeability and fracture dilation were measured for the entire duration of the experiment. After the shear zone generation, the imposed shear displacement was increased to 1.2 mm and pore pressure changes of ±5 or ±10 MPa were applied cyclically to simulate injection and production scenarios. Thin sections and image analysis tools were used to identify microstructural features of the shear zone. The geometry of the shear zone is shown to follow a self-affine scaling invariance, similar to the fracture surface roughness. The permeability evolution related to the onset of the fracture zone is different for both rocks: almost no enhancement for the Flechtingen sandstone and an increase of more than 2 orders of magnitude for the Odenwald granite. Further shear displacement resulted in a slight increase in permeability. A fault compaction is observed after shear relaxation which is associated to a permeability decrease by a factor more than 3. Permeability changes during pressure cycling are reversible when varying the effective pressure. The difference in permeability enhancement between the sandstone and the granite is related to the larger width of the shear zones.

Freeze-thaw induced fracturing coal by liquid nitrogen (LN 2 ) injection exerts a significant positive effect on the fracture permeability enhancement of the coal reservoir. To evaluate the different freeze-thaw variables which modify the mechanical properties of treated coals, the effects of freezing time, number of freeze-thaw cycles, and the moisture content of coal were studied using combined uniaxial compression and acoustic emission testing systems. Freezing the samples with LN 2 for increasing amounts of time degraded the strength of coal within a certain limit. Comparison to freezing time, freeze-thaw cycling caused much more damage to the coal strength. The third variable studied, freeze-thaw damage resulting from high moisture content, was restricted by the coal’s moisture saturation limit. Based on the experimental results, equations describing the amount of damage caused by each of the different freeze-thaw variables were empirically regressed. Additionally, by using the ultrasonic wave detection method and fractal dimension analyses, how freeze-thaw induced fractures in the coal was quantitatively analyzed. The results also showed that the velocity of ultrasonic waves had a negative correlation with coal permeability, and the freeze-thaw cycles significantly augment the permeability of frozen-thawed coal masses.

The permeability of shale reservoirs is an important parameter in evaluating the feasibility of shale gas commercial exploitation. The influence of structural anisotropy, bedding planes, and effective stress is of great significance to the permeability of shale reservoirs; thus, a further study on the permeability of shale rock containing bedding planes and fractures is necessary. In this paper, to investigate the gas conductivity of shale rock in different bedding directions and fracture surfaces, permeability tests were conducted on the intact specimens with different bedding inclinations and specimens containing fractures of Longmaxi shale. The pulse decay method is adopted in the determination of the intact shale specimen, and the steady-state method is adopted to measure the permeability of fractured shale specimen. Combined with experiments and theoretical analysis, the anisotropic characteristics of the permeability of intact shale specimens and the permeability of fractured specimens under the applying of effective stress are studied. The permeability of the two kinds of shale specimens decreased exponentially with the increase in effective stress. The main controlling factors on permeability were studied, besides, the flow characteristic of fluid inside the rock with bedding plane or sandwich structures were described. Furthermore, a new model was proposed to describe the anisotropy of rock permeability characteristics. As for shale specimen contains fracture, the function between its equivalent permeability and fracture permeability was derived, as well as the function between equivalent permeability and effective stress, and the function between fracture permeability and effective stress.

In Enhanced Geothermal Systems (EGS), rock fractures in the reservoir are often subjected to cyclic changes in effective stress at elevated temperatures, causing permeability variations. In this study, the effect of cyclic normal stress on permeability evolution of rough fractures in Gonghe granite at elevated temperature was investigated through flow-through experiments. The results show that the fracture permeability decreases with increasing normal load and partially recovers as the normal load releases. Under a constant normal stress, the fractures gradually close, exhibiting viscoelastic-plastic behavior that can be characterized using the Nishihara model. Based on these characteristics, a fracture-creep deformation model has been developed considering the stress history using a combination of the Hopkins fracture closure model and the Nishihara model. This study investigated the effects of cyclic stress on the internal geometric features of fractures at elevated temperature, calculated the fracture deformation and the corresponding permeability during the loading and unloading process, and validated the accuracy of the proposed model. The study reveals the primary mechanisms responsible for fracture permeability evolution under cyclic effective stress at elevated temperatures, providing valuable insights for the sustainable development of EGS.HighlightsThe viscoelastic–plastic behavior of rough fractures in Gonghe granite subjected to cyclic normal stress has been demonstrated in the flow-through experiments.A fracture-creep deformation model for a single rough fracture is developed considering the stress history using a combination of the Hopkins fracture closure model and the Nishihara model.The fracture permeability is calculated according to the geometric characteristics of the fracture, which has been validated by laboratory permeability measurements.

We re-investigate the laboratory acoustic emission (AE) source location from a hydraulic fracturing test. This test produces temporally well-separated AE releases exhibiting first the reverse Omori–Utsu law (ROU-AEs), and following later the normal Omori–Utsu law (NOU-AEs) behaviors. One fault was cut in the hydraulic fracturing specimen. The spatial permeability changes along the direction of minimum in-situ stress were previously measured on two series of sub-cores. However, the previously published AE source location, which is produced from a commercial software, suffers from several issues. Through data re-analysis, we discover: (1) areas of high AE source concentration in the period of ROU-AEs can support the reverse permeability-distance relationship which is discovered by the commercial software, as well as can be closely correlated with the actual hydraulic fracturing path. (2) The AE point cloud in the period of NOU-AEs after pressure breakdown has re-oriented towards the pre-existing fault, highlighting the influence of nearby structure on hydraulic fracture growth. (3) The peak of ROU-AEs is well synchronized with the borehole pressure breakdown, while the NOU-AEs are released under significantly decreased borehole pressure. (4) The evidence suggests the ROU-period is primarily associated with hydraulic fracture creation, while the NOU-AEs may be correlated with the re-activation of the hydraulic fracture under the influence of the nearby fault.

The permeability evolution of fractures in geologic media plays a pivotal role in various geo-mechanical activities, ranging from energy resources recovery to the deep geologic disposal of hazardous materials. This paper presents both laboratory and computational investigations into the evolution of fracture permeability under the influence of varying confining stress levels in a crystalline granitic rock from the Canadian Shield. Steady state permeability experiments were conducted to assess variations in fracture permeability in areas where stress conditions may significantly change due to large-scale underground excavations. A high accuracy laser scanner was used to capture the geometric features of the fracture surface, enabling the numerical simulation analysis of stress-induced hydromechanical behavior. The fracture permeability evolution was systematically investigated under both isotropic stress and deviatoric stress conditions, including four loading–unloading sequences and a series of sequentially increased deviatoric stresses. Simulations of fracture closure under increasing confining pressure and corresponding fluid flow process were performed incorporating a model constructed from fracture elevation data obtained through scanning. The results demonstrate a reduction in permeability exceeding 50% as the triaxial confining stress increases from 5 to 40 MPa. Additionally, permeability hysteresis during stress relief was observed. These findings are relevant to deep geological construction activities where excavation leads to the development of an excavation damage zone and stress alterations, affecting overall fluid flow process.HighlightsThe fracture permeability evolution in Lac du Bonnet granite under four loading–unloading sequences has been systematically investigated.Stress-induced fracture closure process and its corresponding fluid flow process were simulated based on a model constructed from fracture elevation data obtained through scanning techniques.Fracture permeability showed a clear dependence on stress level and permeability hysteresis was observed during the stress relief.

Faulted and fractured systems form a critical component of fluid flow, especially within low-permeable reservoirs. Therefore, developing suitable methodologies for acquiring structural data and simulating flow through fractured media is vital to improve efficiency and reduce uncertainties in modelling the subsurface. Outcrop analogues provide excellent areas for the analysis and characterization of fractures within the reservoir rocks where subsurface data are limited. Variation in fracture arrangement, distribution and connectivity can be obtained from 2D fractured cliff sections and pavements. These sections can then be used for efficient discretization and homogenization techniques to obtain reliable predictions on permeability distributions in the geothermal reservoirs. Fracture network anisotropy in the Malm reservoir unit is assessed using detailed structural analysis and numerical homogenization of outcrop analogues from an open pit quarry within the Franconian Basin, Germany. Several events are recorded in the fracture networks from the Late Jurassic the Alpine Orogeny and are observed to be influenced by the Kulmbach Fault nearby with a reverse throw of 800 m. The fractured outcrops are digitized for fluid flow simulations and homogenization to determine the permeability tensors of the networks. The tensors show differences in fluid transport direction where fracture permeability is controlled by orientation compared to a constant value. As a result, it is observed that the orientation of the tensor is influenced by the Kulmbach Fault, and therefore faults within the reservoirs at depth should be considered as important controls on the fracture flow of the geothermal system.