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Fracture permeability controls the fluid flow in unconventional geo-energy reservoirs, and achieving an effective and sustainable increase in permeability is a primary objective of hydraulic stimulation treatments. However, the stress dependence of fracture permeability remains insufficiently constrained. In this study, we systematically compiled and re-processed published hydro-mechanical data of tensile, shear, saw-cut, and natural fractures in representative reservoir rocks, including shale, tight sandstone and crystalline rocks, under diverse stress and deformation conditions. We calculated the fracture permeability, synthesized a unified dataset and systematically quantified the permeability variations with Terzaghi effective normal stress using a commonly adopted exponential law characterized by intrinsic permeability ( k 0 ) and normal stress sensitivity ( α ). The synthesized dataset indicates that shale fractures are associated with low k 0 and moderate α , suggesting the potential importance of proppant use for maintaining fracture permeability in shale oil/gas reservoirs; sandstone fractures possess high k 0 and low α , favoring sustainable stimulation effects; and crystalline rock fractures have intermediate k 0 and high α , requiring proppants for maintaining post-stimulation permeability. Slip events are generally accompanied by increases in both k 0 and α , enhancing permeability but making it more sensitive to pressure changes, whereas cyclic fluid pressure changes generally compact fractures and reduce both parameters. Laboratory-scale data suggests that shear stress tends to decrease fracture permeability due to asperity crushing and clogging, while field-scale evidence implies permeability enhancement of critically stressed fractures through brecciation. Overall, this study provides a systematic synthesis of existing data for interpreting stress-dependent fracture permeability, which may inform stimulation strategies designed to enhance and sustain fluid transport in unconventional geo-energy reservoirs.

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.

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.

Geothermal energy is sustainable and gaining momentum as a solution to energy crises and environmental issues. However, challenges like production temperature and thermal breakthrough can impact geothermal project efficiency. One innovative solution to alleviate the thermal breakthrough is to inject polymer‐based materials that are encapsulated in microcapsules into fractures to modify fracture permeability and prevent preferential flow. In our study, we utilized a coupled computational fluid dynamics and discrete element method to simulate the transport of microcapsules under various scenarios controlled by microcapsule size, microcapsule concentration, and fracture roughness. For a smooth fracture, the results indicate that small microcapsules can travel through a smooth fracture regardless of their concentrations. Large microcapsules can transport through a smooth fracture when present in lower concentrations. However, medium and mixed‐size microcapsules tend to cause the sealing of a smooth fracture, irrespective of their concentrations. For a rough fracture, the transport of microcapsules is complicated by their interactions with the rough fracture walls. The presence of two sealing positions in a rough fracture adds further complexity to this transport phenomenon. The size and concentration of microcapsules control one sealing location, while the rough fracture walls determine the other sealing location. The rough walls substantially affect microcapsule transport, rendering the role of microcapsule size and concentration less significant. The simulation results suggest that complex fracture surfaces significantly elevate the occurrence of sealing behavior. To mitigate sealing behavior within more complex fractures, it would be beneficial to use smaller and lower concentrations of microcapsules. Plain Language Summary Geothermal energy is a sustainable solution to energy and environmental challenges, but it faces efficiency issues due to factors like production temperature and thermal breakthrough. A novel solution involves injecting polymer‐based microcapsules into fractures to modify permeability and prevent preferential flow. Our study used numerical simulations to explore how different factors, such as microcapsule size, microcapsule concentration, and rough fracture, affect microcapsule transport. In a smooth fracture, small microcapsules can travel easily, while large microcapsules transport at lower concentrations. However, middle and mixed‐size microcapsules tend to seal a smooth fracture. A rough fracture complicates the transport, with sealing positions influenced by microcapsule size, concentration, and interactions with rough walls. It is favorable to use smaller microcapsules at lower concentrations for complex fractures to mitigate sealing behavior. Key Points Middle and mixed‐size microcapsules have a higher propensity for sealing a smooth fracture compared to small or large microcapsules In the case of a rough fracture, the presence of rough fracture walls substantially increases the likelihood of sealing the fracture In certain rough fracture cases, two sealing locations can be observed

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.

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 investigate both laminar and turbulent flow regimes in a rough-walled rock fracture via numerical CFD simulations. While previous studies were limited to either fully viscous Darcy or inertial Forchheimer laminar flow regimes, we chose to cover the widest possible Reynolds number range of 0.1–106. We introduce CFD simulation of a turbulent flow for rough-walled fractures, implementing RANS approach to turbulence modeling. We focus on 2D fracture geometries and implement changes in both shear displacement and wall roughness, systematically examining their effect on fracture permeability and friction factor in a manner similar to the fundamental studies of the flow in rough-walled pipes. For a curvilinear fracture, laminar flow becomes non-stationary between Re∼102–103, earlier for larger shear displacement and wall roughness. Laminar–turbulent transition starting at Recr∼2300 may lead to a sharp drop in permeability depending on the fracture geometry; this gap vanishes for larger shear displacement and wall roughness. Depending on the fracture geometry, bottlenecks closing with shear become a major negative factor for the overall permeability.HighlightsWe build a realistic 2D rough-walled fracture model from 3D scan data, implementing both shear and roughness variations.We perform CFD simulations of laminar and turbulent flow for wide range of Re = 0.1–106.We analyse flow field data and identify both stationary–non-stationary and laminar–turbulent transitions.We analyse permeability and friction factor data and demonstrate the effects of shear and roughness variations on both parameters, fitting the modified Forchheimer equation for laminar and turbulent flow.