A method to generate fractures with rough surfaces was proposed according to the fractal interpolation theory.Considering the particle-particle,particle-wall and particle-fluid interactions,a proppant-fracturing fluid...A method to generate fractures with rough surfaces was proposed according to the fractal interpolation theory.Considering the particle-particle,particle-wall and particle-fluid interactions,a proppant-fracturing fluid two-phase flow model based on computational fluid dynamics(CFD)-discrete element method(DEM)coupling was established.The simulation results were verified with relevant experimental data.It was proved that the model can match transport and accumulation of proppants in rough fractures well.Several cases of numerical simulations were carried out.Compared with proppant transport in smooth flat fractures,bulge on the rough fracture wall affects transport and settlement of proppants significantly in proppant transportation in rough fractures.The higher the roughness of fracture,the faster the settlement of proppant particles near the fracture inlet,the shorter the horizontal transport distance,and the more likely to accumulate near the fracture inlet to form a sand plugging in a short time.Fracture wall roughness could control the migration path of fracturing fluid to a certain degree and change the path of proppant filling in the fracture.On the one hand,the rough wall bulge raises the proppant transport path and the proppants flow out of the fracture,reducing the proppant sweep area.On the other hand,the sand-carrying fluid is prone to change flow direction near the contact point of bulge,thus expanding the proppant sweep area.展开更多
Particle settling in narrow rough fractures is a common but poorly understood phenomenon during hydraulic fracturing.This study first constructs a large slot with two rough surfaces to simulate rock fractures and empl...Particle settling in narrow rough fractures is a common but poorly understood phenomenon during hydraulic fracturing.This study first constructs a large slot with two rough surfaces to simulate rock fractures and employs the particle image velocimetry to measure particle settling.Results show that particle settling in the rough slot is more complex than in the smooth slot.Rough pathways significantly change particle settling characteristics.The rough-walled slot alters the classic settling process by creating preferential pathways,localized trapping,and vortex-driven redistribution.Particles settle along preferential pathways,increasing settling velocity.The wall retardation effect becomes more prominent for large particles,reducing the settling velocity.Particle settling induces vortices throughout the rough surface,affecting particle behavior.Higher particle volume fractions increase settling nonuniformity,leading to unstable fluid flow within fractures,characterized by high vorticity and upward flow.The frequent interplay between particles and particle-walls,and fluid resistance complicates particle trajectories and settling behavior.Fluid viscosity significantly changes settling patterns and promotes particle clusters,forming chain-like and curtain-like clusters in rough fractures.An innovative model is proposed to predict settling velocity in rough fractures.展开更多
Hydraulic fracturing is a key technology for the development of unconventional hydrocarbon resources.The proppant placement morphology determines the fracture conductivity,thus affecting the reservoir stimulation effe...Hydraulic fracturing is a key technology for the development of unconventional hydrocarbon resources.The proppant placement morphology determines the fracture conductivity,thus affecting the reservoir stimulation effect.In this paper,the proppant migration and placement within complex fractures was studied by considering the fracture wall roughness through computational fluid mechanics-discrete element method(CFD-DEM)in numerical simulation,which is a key approach to study the proppant migration and placement.The results show that the proppant placement non-uniformity,proppant migration capacity,and proppant volume filled in the far-end and the secondary branched fracture are enhanced within the rough fracture compared with those within smooth fractures.The proppant migration capacity is increased within the fracture at low inclination angles(<60°)and low approach angles(<90°),and the proppant placement area is larger in the inclined fracture than that in the vertical fracture.The rise of injection rate and fracturing fluid viscosity causes more proppants migrate to far-end or secondary fractures,resulting in a non-proppant area within the near-wellbore fracture.An increase by 1.3 times in the injection rate and 3 times in the fracturing fluid viscosity leads to a decrease by 26.6%and 27%,respectively,in the proppant placement area within the near-wellbore fracture.The staged injection with small size proppants followed by large size proppants increases the proppant placement area in the primary fracture by 13%-26%,and that with large size proppants followed by small size proppants increases the proppant placement area by 19%-25%,which is due to that the latter method facilitates filling of the secondary branched fracture.The injection location mainly affects the proppant filling degree within the near-wellbore fractures.Compared with the upper injection,the middle and lower injection is not beneficial to filling of proppants within the near-wellbore fracture.展开更多
A three-dimensional reconstruction of rough fracture surfaces of hydraulically fractured rock outcrops is carried out by casting process,a large-scale experimental setup for visualizing rough fractures is built to per...A three-dimensional reconstruction of rough fracture surfaces of hydraulically fractured rock outcrops is carried out by casting process,a large-scale experimental setup for visualizing rough fractures is built to perform proppant transport experiments.The typical characteristics of proppant transport and placement in rough fractures and its intrinsic mechanisms are investigated,and the influences of fracture inclination,fracture width and fracturing fluid viscosity on proppant transport and placement in rough fractures are analyzed.The results show that the rough fractures cause variations in the shape of the flow channel and the fluid flow pattern,resulting in the bridging buildup during proppant transport to form unfilled zone,the emergence of multiple complex flow patterns such as channeling,reverse flow and bypassing of sand-carrying fluid,and the influence on the stability of the sand dune.The proppant has a higher placement rate in inclined rough fractures,with a maximum increase of 22.16 percentage points in the experiments compared to vertical fractures,but exhibits poor stability of the sand dune.Reduced fracture width aggravates the bridging of proppant and induces higher pumping pressure.Increasing the viscosity of the fracturing fluid can weaken the proppant bridging phenomenon caused by the rough fractures.展开更多
The dynamic frictional behaviors of natural discontinuities(joints,fractures,faults)play an important role in geohazards assessment;however,the mechanisms of the dynamic fault weakening/strengthening are still unclear...The dynamic frictional behaviors of natural discontinuities(joints,fractures,faults)play an important role in geohazards assessment;however,the mechanisms of the dynamic fault weakening/strengthening are still unclear.In this paper,a dynamic shear box was used to perform direct shear tests on saw-cut(planar)and natural(rough)granite fractures,with different normal load oscillation amplitudes.Based on the recorded shear forces and normal displacements,the shear forces,apparent friction coefficients and normal displacements are found to change periodically with oscillated normal loads and are characterized by a series of time shifts.The observed changing patterns are similar for the rough and planar fractures.Compared with the test data under constant normal load(CNL),small/large normal load oscillation amplitude enhances/reduces the peak shear strength,with a critical point.The magnitude of critical normal load oscillation for the rough fractures is smaller than the planer fractures.The results imply that dynamic fault weakening/strengthening can be achieved by both normal load oscillation amplitudes and slip surface topography.The rough fractures with larger normal oscillation amplitude can easily cause frictional weakening under stress disturbance.展开更多
The effect of roughness on flow in fractures was investigated using lattice Boltzmann method(LBM).Simulations were conducted for both statistically generated hypothetical fractures and a natural dolomite fracture. The...The effect of roughness on flow in fractures was investigated using lattice Boltzmann method(LBM).Simulations were conducted for both statistically generated hypothetical fractures and a natural dolomite fracture. The effect of increasing roughness on effective hydraulic aperture, Izbash and Forchheimer parameters with increasing Reynolds number(Re) ranging from 0.01 to 500 was examined. The growth of complex flow features, such as eddies arising near the fracture surface, was directly associated with changes in surface roughness. Rapid eddy growth above Re values of 1, followed by less rapid growth at higher Re values, suggested a three-zone nonlinear model for flow in rough fractures. This three-zone model, relating effective hydraulic conductivity to Re, was also found to be appropriate for the simulation of water flow in the natural dolomite fracture. Increasing fracture roughness led to greater eddy volumes and lower effective hydraulic conductivities for the same Re values.展开更多
This study aims to propose an empirical prediction model of hydraulic aperture of 2D rough fractures through numerical simulations by considering the influences of fracture length,average mechanical aperture,minimum m...This study aims to propose an empirical prediction model of hydraulic aperture of 2D rough fractures through numerical simulations by considering the influences of fracture length,average mechanical aperture,minimum mechanical aperture,joint roughness coefficient(JRC)and hydraulic gradient.We generate 600 numerical models using successive random additions(SRA)algorithm and for each model,seven hydraulic gradients spanning from 2.5×10^(-7)to 1 are considered to fully cover both linear and nonlinear flow regimes.As a result,a total of 4200 fluid flow cases are simulated,which can provide sufficient data for the prediction of hydraulic aperture.The results show that as the ratio of average mechanical aperture to fracture length increases from 0.01 to 0.2,the hydraulic aperture increases following logarithm functions.As the hydraulic gradient increases from 2.5×10^(-7)to 1,the hydraulic aperture decreases following logarithm functions.When a relatively low hydraulic gradient(i.e.,5×10^(-7))is applied between the inlet and the outlet boundaries,the streamlines are of parallel distribution within the fractures.However,when a relatively large hydraulic gradient(i.e.,0.5)is applied between the inlet and the outlet boundaries,the streamlines are disturbed and a number of eddies are formed.The hydraulic aperture predicted using the proposed empirical functions agree well with the calculated results and is more reliable than those available in the preceding literature.In practice,the hydraulic aperture can be calculated as a first-order estimation using the proposed prediction model when the associated parameters are given.展开更多
The roughness of the fracture surface directly affects the strength,deformation,and permeability of the surrounding rock in deep underground engineering.Understanding the effect of high temperature and thermal cycle o...The roughness of the fracture surface directly affects the strength,deformation,and permeability of the surrounding rock in deep underground engineering.Understanding the effect of high temperature and thermal cycle on the fracture surface roughness plays an important role in estimating the damage degree and stability of deep rock mass.In this paper,the variations of fracture surface roughness of granite after different heating and thermal cycles were investigated using the joint roughness coefficient method(JRC),three-dimensional(3D)roughness parameters,and fractal dimension(D),and the mechanism of damage and deterioration of granite were revealed.The experimental results show an increase in the roughness of the granite fracture surface as temperature and cycle number were incremented.The variations of JRC,height parameter,inclination parameter and area parameter with the temperature conformed to the Boltzmann's functional distribution,while the D decreased linearly as the temperature increased.Besides,the anisotropy index(Ip)of the granite fracture surface increased as the temperature increased,and the larger parameter values of roughness characterization at different temperatures were attained mainly in directions of 20°–40°,60°–100°and 140°–160°.The fracture aperture of granite after fracture followed the Gauss distribution and the average aperture increased with increasing temperature,which increased from 0.665 mm at 25℃to 1.058 mm at 800℃.High temperature caused an uneven thermal expansion,water evaporation,and oxidation of minerals within the granite,which promoted the growth and expansion of microfractures,and reduced interparticle bonding strength.In particular,the damage was exacerbated by the expansion and cracking of the quartz phase transition after T>500℃.Thermal cycles contributed to the accumulation of this damage and further weakened the interparticle bonding forces,resulting in a significant increase in the roughness,anisotropy,and aperture of the fracture surface after five cycles.展开更多
Understanding the complex flow behavior along a rough rock fracture under high-temperature,high-stress,and high-seepage pressure(HTHM)coupling conditions is of great significance for optimizing deep resource extractio...Understanding the complex flow behavior along a rough rock fracture under high-temperature,high-stress,and high-seepage pressure(HTHM)coupling conditions is of great significance for optimizing deep resource extraction.This study investigates the complex flow behavior of a single rock fracture under coupled HTHM conditions using a self-developed multi-field coupling experimental system,considering real-time high temperatures(20–90℃),confining pressures(30–120 MPa),and seepage pressures(5–60 MPa).Experimental results show that as confining pressure increases,two typical nonlinear flow behaviors are observed,which are Forchheimer flow and low-velocity nonlinear flow.The increase in temperature and decrease in roughness significantly promote the fluid flow and enhance the nonlinear relationship between the volumetric flow rate and the hydraulic gradient at lower confining pressures(30 MPa).However,the change in temperature and fracture surface roughness does not affect the nonlinear type of fluid flow.Under a given hydraulic gradient,the influence of temperature and fracture roughness on the volumetric flow rate varies with changes in confining pressure.Additionally,this study considers both the viscous and inertial terms,and a modified Forchheimer equation is proposed using two parameters:the contact area ratio and the thermal expansion coefficient of the rock.The proposed model can effectively predict the nonlinear flow behavior of fluid along rough fractured rocks under varying temperatures and surface roughness.The experimental results and the proposed model provide valuable data and theoretical guidance for deep oil and gas exploration as well as hydraulic fracturing design.展开更多
Through high-precision engraving,self-affine sandstone joint surfaces with various joint roughness coefficients(JRC=3.21e12.16)were replicated and the shear sliding tests under unloading normal stress were conducted r...Through high-precision engraving,self-affine sandstone joint surfaces with various joint roughness coefficients(JRC=3.21e12.16)were replicated and the shear sliding tests under unloading normal stress were conducted regarding various initial normal stresses(1e7 MPa)and numbers of shearing cycles(1 e5).The peak shear stress of fractures decreased with shear cycles due to progressively smooth surface morphologies,while increased with both JRC and initial normal stress and could be verified using the nonlinear Barton-Bandis failure criterion.The joint friction angle of fractures exponentially increased by 62.22%e64.87%with JRC while decreased by 22.1%e24.85%with shearing cycles.After unloading normal stress,the sliding initiation time of fractures increased with both JRC and initial normal stress due to more tortuous fracture morphologies and enhanced shearing resistance capacity.The surface resistance index(SRI)of fractures decreased by 4.35%e32.02%with increasing shearing cycles due to a more significant reduction of sliding initiation shear stress than that for sliding initiation normal stress,but increased by a factor of 0.41e1.64 with JRC.After sliding initiation,the shear displacement of fractures showed an increase in power function.By defining a sliding rate threshold of 5105 m/s,transition from“quasi-static”to“dynamic”sliding of fractures was identified,and the increase of sliding acceleration steepened with JRC while slowed down with shearing cycles.The normal displacement experienced a slight increase before shear sliding due to deformation recovery as the unloading stress was unloaded,and then enhanced shear dilation after sliding initiation due to climbing effects of surface asperities.Dilation was positively related to the shear sliding velocity of fractures.Wear characteristics of the fracture surfaces after shearing failure were evaluated using binary calculation,indicating an increasing shear area ratio by 45.24%e91.02%with normal stress.展开更多
Subsurface fluid injections can disturb the effective stress regime by elevating pore pressure and potentially reactivate faults and fractures.Laboratory studies indicate that fracture rheology and permeability in suc...Subsurface fluid injections can disturb the effective stress regime by elevating pore pressure and potentially reactivate faults and fractures.Laboratory studies indicate that fracture rheology and permeability in such reactivation events are linked to the roughness of the fracture surfaces.In this study,we construct numerical models using discrete element method(DEM)to explore the influence of fracture surface roughness on the shear strength,slip stability,and permeability evolution during such slip events.For each simulation,a pair of analog rock coupons(three-dimensional bonded quartz particle analogs)representing a mated fracture is sheared under a velocity-stepping scheme.The roughness of the fracture is defined in terms of asperity height and asperity wavelength.Results show that(1)Samples with larger asperity heights(rougher),when sheared,exhibit a higher peak strength which quickly devolves to a residual strength after reaching a threshold shear displacement;(2)These rougher samples also exhibit greater slip stability due to a high degree of asperity wear and resultant production of wear products;(3)Long-term suppression of permeability is observed with rougher fractures,possibly due to the removal of asperities and redistribution of wear products,which locally reduces porosity in the dilating fracture;and(4)Increasing shear-parallel asperity wavelength reduces magnitudes of stress drops after peak strength and enhances fracture permeability,while increasing shear-perpendicular asperity wavelength results in sequential stress drops and a delay in permeability enhancement.This study provides insights into understanding of the mechanisms of frictional and rheological evolution of rough fractures anticipated during reactivation events.展开更多
Numerous intersected rock fractures constitute the fracture network in enhanced geothermal systems.The complicated convective heat transfer behavior in intersected fractures is critical to the heat recovery in fractur...Numerous intersected rock fractures constitute the fracture network in enhanced geothermal systems.The complicated convective heat transfer behavior in intersected fractures is critical to the heat recovery in fractured geothermal reservoirs.A series of three-dimensional intersected fracture models is constructed to perform the flow-through heat transfer simulations.The geometry effects of dead-end fractures(DEFs)on the heat transfer are evaluated in terms of intersected angles,apertures,lengths,and the connectivity.The results indicate that annular streamlines appear in the rough DEF and cause an ellipse distribution of the cold front.Compared to plate DEFs,the fluid flow in the rough DEF enhances the heat transfer.Both the increment of outlet water temperatureΔToutand the ratio of heat production Qrpresent the largest at the intersected angle of 90°while decline with the decrease of the intersected angle between the main flow fracture(MFF)and the DEFs.The extension of the length of intersected DEFs is beneficial to heat production while enhancing its aperture is not needed.Solely increasing the number of intersected DEFs induces a little increase of heat extraction,and more significant heat production can be obtained through connecting these DEFs with the MFF forming the flow network.展开更多
In fractured reservoirs, the fractures not but also form the main flow channels which connect productivity of reservoirs. However, because of the only provide the storage space for hydrocarbons, the pores of the matri...In fractured reservoirs, the fractures not but also form the main flow channels which connect productivity of reservoirs. However, because of the only provide the storage space for hydrocarbons, the pores of the matrix, so fractures dominate the heterogeneity and randomness of the distribution of fractures, exploration and evaluation of fractured reservoirs is still one of the most difficult problems in the oil industry. In recent years, seismic anisotropy has been applied to the assessment of fractured formations, whereas electrical anisotropy which is more intense in fractured formations than seismic anisotropy has not been studied or used so extensively. In this study, fractured reservoir models which considered multiple sets of fractures with smooth and partly closed, rough surfaces were established based on the fractures and pore network, and the vertical and horizontal electrical resistivities were derived as a function of the matrix and fracture porosities according to Ohm＇s law. By using the anisotropic resistivity equations, variations of the electrical anisotropy of three types of fractured models under the conditions of free pressure and confining pressure were analyzed through the variations of the exerted pressure, matrix porosity, fracture aperture and formation water resistivity. The differences of the vertical and horizontal resistivities and the anisotropy between the connected and non-connected fractures were also analyzed. It is known from the simulated results that an increase of the confining pressure causes a decrease of electrical anisotropy because of the elasticity of the closed fractures and the decrease of the fracture aperture. For a fixed fracture porosity, the higher the matrix porosity, the weaker the electrical anisotropy in the rock formation.展开更多
In this study, the lattice Boltzmann method （LBM） was used to simulate the solute transport in a single rough fracture. The self-affine rough fracture wall was generated with the successive random addition method. T...In this study, the lattice Boltzmann method （LBM） was used to simulate the solute transport in a single rough fracture. The self-affine rough fracture wall was generated with the successive random addition method. The ability of the developed LBM to simulate the solute transport was validated by Taylor dispersion. The effect of fluid velocity on the solute transport in a single rough fracture was investigated using the LBM. The breakthrough curves （BTCs） for continuous injection sources in rough fractures were analyzed and discussed with different Reynolds numbers （Re）. The results show that the rough frac~＇e wall leads to a large fluid velocity gradient across the aperture. Consequently, there is a broad distribution of the immobile region along the rough fracture wall. This distribution of the immobile region is very sensitive to the Re and fracture geometry, and the immobile region is enlarged with the increase of Re and roughness. The concentration of the solute front in the mobile region increases with the Re. Furthermore, the Re and roughness have significant effects on BTCs, and the slow solute molecule exchange between the mobile and immobile regions results in a long breakthrough tail for the rough fracture. This study also demonstrates that the developed LBM can be effective in studying the solute transport in a rough fracture.展开更多
Proppant transport within fractures is one of the most critical tasks in oil,gas and geothermal reservoir stimulation,as it largely determines the ultimate performance of the operating well.Proppant transport in rough...Proppant transport within fractures is one of the most critical tasks in oil,gas and geothermal reservoir stimulation,as it largely determines the ultimate performance of the operating well.Proppant transport in rough fracture networks is still a relatively new area of research and the associated transport mechanisms are still unclear.In this study,representative parameters of rough fracture surfaces formed by supercritical CO_(2) fracturing were used to generate a rough fracture network model based on a spectral synthesis method.Computational fluid dynamics(CFD)coupled with the discrete element method(DEM)was used to study proppant transport in this rough fracture network.To reveal the turning transport mechanism of proppants into branching fractures at the intersections of rough fracture networks,a comparison was made with the behavior within smooth fracture networks,and the effect of key pumping parameters on the proppant placement in a secondary fracture was analyzed.The results show that the transport behavior of proppant in rough fracture networks is very different from that of the one in the smooth fracture networks.The turning transport mechanisms of proppant into secondary fractures in rough fracture networks are gravity-driven sliding,high velocity fluid suspension,and fracture structure induction.Under the same injection conditions,supercritical CO_(2)with high flow Reynolds number still has a weaker ability to transport proppant into secondary fractures than water.Thickening of the supercritical CO_(2)needs to be increased beyond a certain value to have a significant effect on proppant carrying,and under the temperature and pressure conditions of this paper,it needs to be increased more than 20 times(about 0.94 m Pa s).Increasing the injection velocity and decreasing the proppant concentration facilitates the entry of proppant into the branching fractures,which in turn results in a larger stimulated reservoir volume.The results help to understand the proppant transport and placement process in rough fracture networks formed by reservoir stimulation,and provide a theoretical reference for the optimization of proppant pumping parameters in hydraulic fracturing.展开更多
In order to understand the mechanical properties and the fracture surface roughness characteristics of thermally damaged granite under dynamic splitting,dynamic Brazilian splitting tests were conducted on granite samp...In order to understand the mechanical properties and the fracture surface roughness characteristics of thermally damaged granite under dynamic splitting,dynamic Brazilian splitting tests were conducted on granite samples after thermal treatment at 25,200,400,and 600℃.Results show that the dynamic peak splitting strength of thermally damaged granite samples increases with increasing strain rate,showing obvious strain‐rate sensitivity.With increasing temperature,thermally induced cracks in granite transform from intergranular cracks to intragranular cracks,and to a transgranular crack network.Thermally induced damages reduce the dynamic peak splitting strength and the maximum absorbed energy while increasing the peak radial strain.The fracture mode of the thermally damaged granite under dynamic loads is mode Ⅱ splitting failure.By using the axial roughness index Z2 a,the distribution ranges of the wedge‐shaped failure zones and the tensile failure zones in the fracture surfaces under dynamic Brazilian splitting can be effectively identified.The radial roughness index Z_(2)^(r)is sensitive to the strain rate and temperature.It shows a linear correlation with the peak splitting strength and the maximum absorbed energy and a linear negative correlation with the peak radial strain.Z_(2)^(r)can be used to quantitatively estimate the dynamic parameters based on the models proposed.展开更多
Fracture/fault instability induced by fluid injection in deep geothermal reservoirs could not only vary the reservoir permeability but also trigger hazardous seismicity.To address this,we conducted triaxial shear expe...Fracture/fault instability induced by fluid injection in deep geothermal reservoirs could not only vary the reservoir permeability but also trigger hazardous seismicity.To address this,we conducted triaxial shear experiments on granite fractures with different architected roughnesses reactivated under fluid injection,to investigate the controls on permeability evolution linked to reactivation.Our results indicate that the fracture roughness and injection strategies are two main factors affecting permeability evolution.For fractures with dif-ferent roughnesses,a rougher fracture leads to a lower peak reactivated permeability(k_(max)),and varying the fluid injection strategy(in-cluding the confining pressure and injection rate)has a less impact on kmax,indicating that the evolution of permeability during fluid pressurization is likely to be determined by the fracture roughness along the shear direction.Both the fracture roughness and injection strategies affect the average rates of permeability change and this parameter also reflects the long-term reservoir recovery.Our results have important implications for understanding the permeability evolution and the injection-induced fracture/fault slips in granite reser-voirs during the deep geothermal energy extraction.展开更多
For hard rock cracking induced by laser irradiation,the failure modes and fracture characteristics among rocks of different types and sizes are still unclear.Therefore,the experiments on laser-induced fracturing of li...For hard rock cracking induced by laser irradiation,the failure modes and fracture characteristics among rocks of different types and sizes are still unclear.Therefore,the experiments on laser-induced fracturing of limestone,sandstone,and various-sized granite specimens were conducted.Real-time acoustic emission monitoring and laser scanning were employed to capture acoustic emission signals inside rocks during laser irradiation and to reconstruct the fracture surfaces after laser irradiation.Results indicate that abundant melts in sandstone and granite dissipated laser energy,leading to lower acoustic emission peak energy compared to limestone.Larger-sized specimen delayed the occurrence of peak energy.Crystal thermal expansion and changes in pore pressure induced tensile-shear composite failure in limestone,whereas thermal expansion of minerals in sandstone and granite promoted tensile failure.Fracture surface morphology was influenced by sampling interval,anisotropy,and size effects.The joint roughness coefficient and fractal dimension of sandstone exceed granite and limestone.Asperity heights and slope angles ranged from 1–14 mm and 0–40°,respectively,with the average aspect angles exceeding 110°.Granite exhibited the highest proportion of macropores after laser irradiation,approximately 4.8%.These findings provide valuable insights for the application of laser-assisted fracturing in hard rock excavation.展开更多
Fractures in rock strata serve as flow pathways for gas flow.The undulation of fracture channels can influence the guidance of gas flow.In this context,four-point bending experiments on prefabricated fractured rocks a...Fractures in rock strata serve as flow pathways for gas flow.The undulation of fracture channels can influence the guidance of gas flow.In this context,four-point bending experiments on prefabricated fractured rocks at different angles under stable stepped loading stress.The experiment results clarified the evolutionary law that the undulation degree of the rock tensile fracture surface is separated by an initial fracture angle of 45°.The high undulation intervals were less than 45°,whereas the low undulation intervals were more than 45°.Furthermore,the relative undulation degree,undulation frequency,and matching degree of the fracture surface were quantified.The relationship between the change in fracture surface undulation and gas flow guidance was established.Based on this,the stability,tortuosity,and uniformity of the gas flow in the fracture channel were quantitatively characterized.Subsequently,numerical models of the fracture channels were constructed to validate the indices proposed in this study.The results of the study clarified the influence of different initial fracture angles on the undulation changes of fracture surfaces,and established the relationship between these changes and gas flow,which is conducive to understanding the role of internal fracture channels in rocks in guiding the gas flow process.展开更多
Large shear deformation problems are frequently encountered in geotechnical engineering.To expose the shear failure mechanism of rock tunnels,compression-shear tests for rock models with circular tunnel were carried o...Large shear deformation problems are frequently encountered in geotechnical engineering.To expose the shear failure mechanism of rock tunnels,compression-shear tests for rock models with circular tunnel were carried out,including single tunnel and adjacent double tunnels.The failure process is recorded by the external video and miniature cameras around the tunnel,accompanied by real-time acoustic emission monitoring.The experiments indicate that the shearing processes of rock tunnel can be divided into four steps:(i)cracks appeared around tunnels,(ii)shear cracks and spalling ejection developed,(iii)floor warping occurred,and(iv)shear cracks ran through the tunnel model.Besides,the roughness of the sheared fracture surface decreased with the increase in normal stress.Corresponding numerical simulation indicates that there are tensile stress concentrations and compressive stress concentrations around the tunnel during the shearing process,while the compressive stress concentration areas are under high risk of failure and the existence of adjacent tunnels will increase the degree of stress concentration.展开更多
基金Supported by National Natural Science Foundation of China(52274020,U21B2069,52288101)General Program of the Shandong Natural Science Foundation(ZR2020ME095)National Key Research and Development Program(2021YFC2800803).
文摘A method to generate fractures with rough surfaces was proposed according to the fractal interpolation theory.Considering the particle-particle,particle-wall and particle-fluid interactions,a proppant-fracturing fluid two-phase flow model based on computational fluid dynamics(CFD)-discrete element method(DEM)coupling was established.The simulation results were verified with relevant experimental data.It was proved that the model can match transport and accumulation of proppants in rough fractures well.Several cases of numerical simulations were carried out.Compared with proppant transport in smooth flat fractures,bulge on the rough fracture wall affects transport and settlement of proppants significantly in proppant transportation in rough fractures.The higher the roughness of fracture,the faster the settlement of proppant particles near the fracture inlet,the shorter the horizontal transport distance,and the more likely to accumulate near the fracture inlet to form a sand plugging in a short time.Fracture wall roughness could control the migration path of fracturing fluid to a certain degree and change the path of proppant filling in the fracture.On the one hand,the rough wall bulge raises the proppant transport path and the proppants flow out of the fracture,reducing the proppant sweep area.On the other hand,the sand-carrying fluid is prone to change flow direction near the contact point of bulge,thus expanding the proppant sweep area.
基金supported by the National Natural Science Foundation of China(Grant No.52274035)。
文摘Particle settling in narrow rough fractures is a common but poorly understood phenomenon during hydraulic fracturing.This study first constructs a large slot with two rough surfaces to simulate rock fractures and employs the particle image velocimetry to measure particle settling.Results show that particle settling in the rough slot is more complex than in the smooth slot.Rough pathways significantly change particle settling characteristics.The rough-walled slot alters the classic settling process by creating preferential pathways,localized trapping,and vortex-driven redistribution.Particles settle along preferential pathways,increasing settling velocity.The wall retardation effect becomes more prominent for large particles,reducing the settling velocity.Particle settling induces vortices throughout the rough surface,affecting particle behavior.Higher particle volume fractions increase settling nonuniformity,leading to unstable fluid flow within fractures,characterized by high vorticity and upward flow.The frequent interplay between particles and particle-walls,and fluid resistance complicates particle trajectories and settling behavior.Fluid viscosity significantly changes settling patterns and promotes particle clusters,forming chain-like and curtain-like clusters in rough fractures.An innovative model is proposed to predict settling velocity in rough fractures.
基金financial support of the National Natural Science Foundation of China(Grant No.52074332)express their gratitude to project ZR2020YQ36 supported by Shandong Provincial Science Fund for Excellent Young Scholars。
文摘Hydraulic fracturing is a key technology for the development of unconventional hydrocarbon resources.The proppant placement morphology determines the fracture conductivity,thus affecting the reservoir stimulation effect.In this paper,the proppant migration and placement within complex fractures was studied by considering the fracture wall roughness through computational fluid mechanics-discrete element method(CFD-DEM)in numerical simulation,which is a key approach to study the proppant migration and placement.The results show that the proppant placement non-uniformity,proppant migration capacity,and proppant volume filled in the far-end and the secondary branched fracture are enhanced within the rough fracture compared with those within smooth fractures.The proppant migration capacity is increased within the fracture at low inclination angles(<60°)and low approach angles(<90°),and the proppant placement area is larger in the inclined fracture than that in the vertical fracture.The rise of injection rate and fracturing fluid viscosity causes more proppants migrate to far-end or secondary fractures,resulting in a non-proppant area within the near-wellbore fracture.An increase by 1.3 times in the injection rate and 3 times in the fracturing fluid viscosity leads to a decrease by 26.6%and 27%,respectively,in the proppant placement area within the near-wellbore fracture.The staged injection with small size proppants followed by large size proppants increases the proppant placement area in the primary fracture by 13%-26%,and that with large size proppants followed by small size proppants increases the proppant placement area by 19%-25%,which is due to that the latter method facilitates filling of the secondary branched fracture.The injection location mainly affects the proppant filling degree within the near-wellbore fractures.Compared with the upper injection,the middle and lower injection is not beneficial to filling of proppants within the near-wellbore fracture.
基金Supported by National Key Research and Development Program of China(2022YFE0137200)Outstanding Youth Natural Science Fund of Shaanxi Province(2022JC-37)+2 种基金Innovation Capability Support Program of Shaanxi(2023-CX-TD-31)Natural Science Basic Research Project of Shaanxi Province(2024JC-YBQN-0381)National Natural Science Foundation of China(51874240,52204021)。
文摘A three-dimensional reconstruction of rough fracture surfaces of hydraulically fractured rock outcrops is carried out by casting process,a large-scale experimental setup for visualizing rough fractures is built to perform proppant transport experiments.The typical characteristics of proppant transport and placement in rough fractures and its intrinsic mechanisms are investigated,and the influences of fracture inclination,fracture width and fracturing fluid viscosity on proppant transport and placement in rough fractures are analyzed.The results show that the rough fractures cause variations in the shape of the flow channel and the fluid flow pattern,resulting in the bridging buildup during proppant transport to form unfilled zone,the emergence of multiple complex flow patterns such as channeling,reverse flow and bypassing of sand-carrying fluid,and the influence on the stability of the sand dune.The proppant has a higher placement rate in inclined rough fractures,with a maximum increase of 22.16 percentage points in the experiments compared to vertical fractures,but exhibits poor stability of the sand dune.Reduced fracture width aggravates the bridging of proppant and induces higher pumping pressure.Increasing the viscosity of the fracturing fluid can weaken the proppant bridging phenomenon caused by the rough fractures.
基金the funding support from the National Natural Science Foundation of China (Grant No. 51904359)the Guangdong Provincial Department of Science and Technology (Grant No. 2019ZT08G090)the Open Research Fund of the State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (CUMT) (Grant No. SKLCRSM20KF002)
文摘The dynamic frictional behaviors of natural discontinuities(joints,fractures,faults)play an important role in geohazards assessment;however,the mechanisms of the dynamic fault weakening/strengthening are still unclear.In this paper,a dynamic shear box was used to perform direct shear tests on saw-cut(planar)and natural(rough)granite fractures,with different normal load oscillation amplitudes.Based on the recorded shear forces and normal displacements,the shear forces,apparent friction coefficients and normal displacements are found to change periodically with oscillated normal loads and are characterized by a series of time shifts.The observed changing patterns are similar for the rough and planar fractures.Compared with the test data under constant normal load(CNL),small/large normal load oscillation amplitude enhances/reduces the peak shear strength,with a critical point.The magnitude of critical normal load oscillation for the rough fractures is smaller than the planer fractures.The results imply that dynamic fault weakening/strengthening can be achieved by both normal load oscillation amplitudes and slip surface topography.The rough fractures with larger normal oscillation amplitude can easily cause frictional weakening under stress disturbance.
文摘The effect of roughness on flow in fractures was investigated using lattice Boltzmann method(LBM).Simulations were conducted for both statistically generated hypothetical fractures and a natural dolomite fracture. The effect of increasing roughness on effective hydraulic aperture, Izbash and Forchheimer parameters with increasing Reynolds number(Re) ranging from 0.01 to 500 was examined. The growth of complex flow features, such as eddies arising near the fracture surface, was directly associated with changes in surface roughness. Rapid eddy growth above Re values of 1, followed by less rapid growth at higher Re values, suggested a three-zone nonlinear model for flow in rough fractures. This three-zone model, relating effective hydraulic conductivity to Re, was also found to be appropriate for the simulation of water flow in the natural dolomite fracture. Increasing fracture roughness led to greater eddy volumes and lower effective hydraulic conductivities for the same Re values.
基金funded by National Key R&D Program of China(No.2022YFE0128300)National Natural Science Foundation of China(Grant Nos.52379114 and 52379113)+2 种基金Natural Science Foundation of Jiangsu Province,China(No.BK20211584)the Assistance Program for Future Outstanding Talents of the China University of Mining and Technology(No.2023WLKXJ187)the Postgraduate Research&Practice Innovation Program of Jiangsu Province(No.KYCX23_2746).
文摘This study aims to propose an empirical prediction model of hydraulic aperture of 2D rough fractures through numerical simulations by considering the influences of fracture length,average mechanical aperture,minimum mechanical aperture,joint roughness coefficient(JRC)and hydraulic gradient.We generate 600 numerical models using successive random additions(SRA)algorithm and for each model,seven hydraulic gradients spanning from 2.5×10^(-7)to 1 are considered to fully cover both linear and nonlinear flow regimes.As a result,a total of 4200 fluid flow cases are simulated,which can provide sufficient data for the prediction of hydraulic aperture.The results show that as the ratio of average mechanical aperture to fracture length increases from 0.01 to 0.2,the hydraulic aperture increases following logarithm functions.As the hydraulic gradient increases from 2.5×10^(-7)to 1,the hydraulic aperture decreases following logarithm functions.When a relatively low hydraulic gradient(i.e.,5×10^(-7))is applied between the inlet and the outlet boundaries,the streamlines are of parallel distribution within the fractures.However,when a relatively large hydraulic gradient(i.e.,0.5)is applied between the inlet and the outlet boundaries,the streamlines are disturbed and a number of eddies are formed.The hydraulic aperture predicted using the proposed empirical functions agree well with the calculated results and is more reliable than those available in the preceding literature.In practice,the hydraulic aperture can be calculated as a first-order estimation using the proposed prediction model when the associated parameters are given.
基金funding support from the National Natural Science Foundation of China(Grant No.52274082)the Program of Qingjiang Excellent Young Talents,Jiangxi University of Science and Technology(Grant No.JXUSTQJBJ2020003)the Innovation Fund Designated for Graduate Students of Jiangxi Province(Grant No.YC2023-B215).
文摘The roughness of the fracture surface directly affects the strength,deformation,and permeability of the surrounding rock in deep underground engineering.Understanding the effect of high temperature and thermal cycle on the fracture surface roughness plays an important role in estimating the damage degree and stability of deep rock mass.In this paper,the variations of fracture surface roughness of granite after different heating and thermal cycles were investigated using the joint roughness coefficient method(JRC),three-dimensional(3D)roughness parameters,and fractal dimension(D),and the mechanism of damage and deterioration of granite were revealed.The experimental results show an increase in the roughness of the granite fracture surface as temperature and cycle number were incremented.The variations of JRC,height parameter,inclination parameter and area parameter with the temperature conformed to the Boltzmann's functional distribution,while the D decreased linearly as the temperature increased.Besides,the anisotropy index(Ip)of the granite fracture surface increased as the temperature increased,and the larger parameter values of roughness characterization at different temperatures were attained mainly in directions of 20°–40°,60°–100°and 140°–160°.The fracture aperture of granite after fracture followed the Gauss distribution and the average aperture increased with increasing temperature,which increased from 0.665 mm at 25℃to 1.058 mm at 800℃.High temperature caused an uneven thermal expansion,water evaporation,and oxidation of minerals within the granite,which promoted the growth and expansion of microfractures,and reduced interparticle bonding strength.In particular,the damage was exacerbated by the expansion and cracking of the quartz phase transition after T>500℃.Thermal cycles contributed to the accumulation of this damage and further weakened the interparticle bonding forces,resulting in a significant increase in the roughness,anisotropy,and aperture of the fracture surface after five cycles.
基金supported by the National Natural Science Foundation of China(Nos.52034010 and 52479113)the Natural Science Foundation of Shandong Province,China(No.ZR2024ME165)the Postgraduate Education and Teaching Reform Project of China University of Petroleum(East China)(No.YJG2024005).
文摘Understanding the complex flow behavior along a rough rock fracture under high-temperature,high-stress,and high-seepage pressure(HTHM)coupling conditions is of great significance for optimizing deep resource extraction.This study investigates the complex flow behavior of a single rock fracture under coupled HTHM conditions using a self-developed multi-field coupling experimental system,considering real-time high temperatures(20–90℃),confining pressures(30–120 MPa),and seepage pressures(5–60 MPa).Experimental results show that as confining pressure increases,two typical nonlinear flow behaviors are observed,which are Forchheimer flow and low-velocity nonlinear flow.The increase in temperature and decrease in roughness significantly promote the fluid flow and enhance the nonlinear relationship between the volumetric flow rate and the hydraulic gradient at lower confining pressures(30 MPa).However,the change in temperature and fracture surface roughness does not affect the nonlinear type of fluid flow.Under a given hydraulic gradient,the influence of temperature and fracture roughness on the volumetric flow rate varies with changes in confining pressure.Additionally,this study considers both the viscous and inertial terms,and a modified Forchheimer equation is proposed using two parameters:the contact area ratio and the thermal expansion coefficient of the rock.The proposed model can effectively predict the nonlinear flow behavior of fluid along rough fractured rocks under varying temperatures and surface roughness.The experimental results and the proposed model provide valuable data and theoretical guidance for deep oil and gas exploration as well as hydraulic fracturing design.
基金support from the National Natural Science Foundation of China(Grant Nos.52174092 and 52104125)the Fundamental Research Funds for the Central Universities,China(Grant No.2022YCPY0202)is gratefully acknowledged.
文摘Through high-precision engraving,self-affine sandstone joint surfaces with various joint roughness coefficients(JRC=3.21e12.16)were replicated and the shear sliding tests under unloading normal stress were conducted regarding various initial normal stresses(1e7 MPa)and numbers of shearing cycles(1 e5).The peak shear stress of fractures decreased with shear cycles due to progressively smooth surface morphologies,while increased with both JRC and initial normal stress and could be verified using the nonlinear Barton-Bandis failure criterion.The joint friction angle of fractures exponentially increased by 62.22%e64.87%with JRC while decreased by 22.1%e24.85%with shearing cycles.After unloading normal stress,the sliding initiation time of fractures increased with both JRC and initial normal stress due to more tortuous fracture morphologies and enhanced shearing resistance capacity.The surface resistance index(SRI)of fractures decreased by 4.35%e32.02%with increasing shearing cycles due to a more significant reduction of sliding initiation shear stress than that for sliding initiation normal stress,but increased by a factor of 0.41e1.64 with JRC.After sliding initiation,the shear displacement of fractures showed an increase in power function.By defining a sliding rate threshold of 5105 m/s,transition from“quasi-static”to“dynamic”sliding of fractures was identified,and the increase of sliding acceleration steepened with JRC while slowed down with shearing cycles.The normal displacement experienced a slight increase before shear sliding due to deformation recovery as the unloading stress was unloaded,and then enhanced shear dilation after sliding initiation due to climbing effects of surface asperities.Dilation was positively related to the shear sliding velocity of fractures.Wear characteristics of the fracture surfaces after shearing failure were evaluated using binary calculation,indicating an increasing shear area ratio by 45.24%e91.02%with normal stress.
基金support provided by United States Department of Energy Grant DE-FE0023354。
文摘Subsurface fluid injections can disturb the effective stress regime by elevating pore pressure and potentially reactivate faults and fractures.Laboratory studies indicate that fracture rheology and permeability in such reactivation events are linked to the roughness of the fracture surfaces.In this study,we construct numerical models using discrete element method(DEM)to explore the influence of fracture surface roughness on the shear strength,slip stability,and permeability evolution during such slip events.For each simulation,a pair of analog rock coupons(three-dimensional bonded quartz particle analogs)representing a mated fracture is sheared under a velocity-stepping scheme.The roughness of the fracture is defined in terms of asperity height and asperity wavelength.Results show that(1)Samples with larger asperity heights(rougher),when sheared,exhibit a higher peak strength which quickly devolves to a residual strength after reaching a threshold shear displacement;(2)These rougher samples also exhibit greater slip stability due to a high degree of asperity wear and resultant production of wear products;(3)Long-term suppression of permeability is observed with rougher fractures,possibly due to the removal of asperities and redistribution of wear products,which locally reduces porosity in the dilating fracture;and(4)Increasing shear-parallel asperity wavelength reduces magnitudes of stress drops after peak strength and enhances fracture permeability,while increasing shear-perpendicular asperity wavelength results in sequential stress drops and a delay in permeability enhancement.This study provides insights into understanding of the mechanisms of frictional and rheological evolution of rough fractures anticipated during reactivation events.
基金financially supported by the National Key R&D Program of China(Grant No.2019YFB1504103)the China Postdoctoral Science Foundation(Grant Nos.2019TQ0174)。
文摘Numerous intersected rock fractures constitute the fracture network in enhanced geothermal systems.The complicated convective heat transfer behavior in intersected fractures is critical to the heat recovery in fractured geothermal reservoirs.A series of three-dimensional intersected fracture models is constructed to perform the flow-through heat transfer simulations.The geometry effects of dead-end fractures(DEFs)on the heat transfer are evaluated in terms of intersected angles,apertures,lengths,and the connectivity.The results indicate that annular streamlines appear in the rough DEF and cause an ellipse distribution of the cold front.Compared to plate DEFs,the fluid flow in the rough DEF enhances the heat transfer.Both the increment of outlet water temperatureΔToutand the ratio of heat production Qrpresent the largest at the intersected angle of 90°while decline with the decrease of the intersected angle between the main flow fracture(MFF)and the DEFs.The extension of the length of intersected DEFs is beneficial to heat production while enhancing its aperture is not needed.Solely increasing the number of intersected DEFs induces a little increase of heat extraction,and more significant heat production can be obtained through connecting these DEFs with the MFF forming the flow network.
基金The authors also would like to acknowledge the support of the National Basic Research Program (973 Program) (2007CB209607) of ChinaNational High-tech R&D Program (863 Program) (2007AA060502)the Fundamental Research Project (07A10303) of CNPC
文摘In fractured reservoirs, the fractures not but also form the main flow channels which connect productivity of reservoirs. However, because of the only provide the storage space for hydrocarbons, the pores of the matrix, so fractures dominate the heterogeneity and randomness of the distribution of fractures, exploration and evaluation of fractured reservoirs is still one of the most difficult problems in the oil industry. In recent years, seismic anisotropy has been applied to the assessment of fractured formations, whereas electrical anisotropy which is more intense in fractured formations than seismic anisotropy has not been studied or used so extensively. In this study, fractured reservoir models which considered multiple sets of fractures with smooth and partly closed, rough surfaces were established based on the fractures and pore network, and the vertical and horizontal electrical resistivities were derived as a function of the matrix and fracture porosities according to Ohm＇s law. By using the anisotropic resistivity equations, variations of the electrical anisotropy of three types of fractured models under the conditions of free pressure and confining pressure were analyzed through the variations of the exerted pressure, matrix porosity, fracture aperture and formation water resistivity. The differences of the vertical and horizontal resistivities and the anisotropy between the connected and non-connected fractures were also analyzed. It is known from the simulated results that an increase of the confining pressure causes a decrease of electrical anisotropy because of the elasticity of the closed fractures and the decrease of the fracture aperture. For a fixed fracture porosity, the higher the matrix porosity, the weaker the electrical anisotropy in the rock formation.
基金supported by the National Natural Science Foundation of China(Grants No.51079043,41172204,and 51109139)the Natural Science Foundation of Jiangsu Province(Grant No.BK2011110)
文摘In this study, the lattice Boltzmann method （LBM） was used to simulate the solute transport in a single rough fracture. The self-affine rough fracture wall was generated with the successive random addition method. The ability of the developed LBM to simulate the solute transport was validated by Taylor dispersion. The effect of fluid velocity on the solute transport in a single rough fracture was investigated using the LBM. The breakthrough curves （BTCs） for continuous injection sources in rough fractures were analyzed and discussed with different Reynolds numbers （Re）. The results show that the rough frac~＇e wall leads to a large fluid velocity gradient across the aperture. Consequently, there is a broad distribution of the immobile region along the rough fracture wall. This distribution of the immobile region is very sensitive to the Re and fracture geometry, and the immobile region is enlarged with the increase of Re and roughness. The concentration of the solute front in the mobile region increases with the Re. Furthermore, the Re and roughness have significant effects on BTCs, and the slow solute molecule exchange between the mobile and immobile regions results in a long breakthrough tail for the rough fracture. This study also demonstrates that the developed LBM can be effective in studying the solute transport in a rough fracture.
基金the support from the National Key Research and Development Program of China(Grant No.2022YFE0137200)the Natural Science Basic Research Program of Shaanxi Province,China(Program No.2024JC-YBQN-0381,2023JC-QN-0403)+2 种基金the Natural Science Basic Research Program of Shaanxi Province,China(Program No.2022JC-37)the Innovation Capability Support Program of Shaanxi(Program No.2023-CX-TD31)the Funded by Open Foundation of Shaanxi Key Laboratory of Carbon Dioxide Sequestration and Enhanced Oil Recovery,and the Youth Innovation Team of Shaanxi Universities。
文摘Proppant transport within fractures is one of the most critical tasks in oil,gas and geothermal reservoir stimulation,as it largely determines the ultimate performance of the operating well.Proppant transport in rough fracture networks is still a relatively new area of research and the associated transport mechanisms are still unclear.In this study,representative parameters of rough fracture surfaces formed by supercritical CO_(2) fracturing were used to generate a rough fracture network model based on a spectral synthesis method.Computational fluid dynamics(CFD)coupled with the discrete element method(DEM)was used to study proppant transport in this rough fracture network.To reveal the turning transport mechanism of proppants into branching fractures at the intersections of rough fracture networks,a comparison was made with the behavior within smooth fracture networks,and the effect of key pumping parameters on the proppant placement in a secondary fracture was analyzed.The results show that the transport behavior of proppant in rough fracture networks is very different from that of the one in the smooth fracture networks.The turning transport mechanisms of proppant into secondary fractures in rough fracture networks are gravity-driven sliding,high velocity fluid suspension,and fracture structure induction.Under the same injection conditions,supercritical CO_(2)with high flow Reynolds number still has a weaker ability to transport proppant into secondary fractures than water.Thickening of the supercritical CO_(2)needs to be increased beyond a certain value to have a significant effect on proppant carrying,and under the temperature and pressure conditions of this paper,it needs to be increased more than 20 times(about 0.94 m Pa s).Increasing the injection velocity and decreasing the proppant concentration facilitates the entry of proppant into the branching fractures,which in turn results in a larger stimulated reservoir volume.The results help to understand the proppant transport and placement process in rough fracture networks formed by reservoir stimulation,and provide a theoretical reference for the optimization of proppant pumping parameters in hydraulic fracturing.
基金supported by the National Natural Science Foundation of China(52174071,U1903216,52004052)the National Key R&D Program of China(2022YFC2903903).
文摘In order to understand the mechanical properties and the fracture surface roughness characteristics of thermally damaged granite under dynamic splitting,dynamic Brazilian splitting tests were conducted on granite samples after thermal treatment at 25,200,400,and 600℃.Results show that the dynamic peak splitting strength of thermally damaged granite samples increases with increasing strain rate,showing obvious strain‐rate sensitivity.With increasing temperature,thermally induced cracks in granite transform from intergranular cracks to intragranular cracks,and to a transgranular crack network.Thermally induced damages reduce the dynamic peak splitting strength and the maximum absorbed energy while increasing the peak radial strain.The fracture mode of the thermally damaged granite under dynamic loads is mode Ⅱ splitting failure.By using the axial roughness index Z2 a,the distribution ranges of the wedge‐shaped failure zones and the tensile failure zones in the fracture surfaces under dynamic Brazilian splitting can be effectively identified.The radial roughness index Z_(2)^(r)is sensitive to the strain rate and temperature.It shows a linear correlation with the peak splitting strength and the maximum absorbed energy and a linear negative correlation with the peak radial strain.Z_(2)^(r)can be used to quantitatively estimate the dynamic parameters based on the models proposed.
基金the National Natural Science Foundation of China(Grant Nos.42077247 and 42107163)the Fundamental Research Funds for the Central Universities.
文摘Fracture/fault instability induced by fluid injection in deep geothermal reservoirs could not only vary the reservoir permeability but also trigger hazardous seismicity.To address this,we conducted triaxial shear experiments on granite fractures with different architected roughnesses reactivated under fluid injection,to investigate the controls on permeability evolution linked to reactivation.Our results indicate that the fracture roughness and injection strategies are two main factors affecting permeability evolution.For fractures with dif-ferent roughnesses,a rougher fracture leads to a lower peak reactivated permeability(k_(max)),and varying the fluid injection strategy(in-cluding the confining pressure and injection rate)has a less impact on kmax,indicating that the evolution of permeability during fluid pressurization is likely to be determined by the fracture roughness along the shear direction.Both the fracture roughness and injection strategies affect the average rates of permeability change and this parameter also reflects the long-term reservoir recovery.Our results have important implications for understanding the permeability evolution and the injection-induced fracture/fault slips in granite reser-voirs during the deep geothermal energy extraction.
基金supported by Yunlong Lake Laboratory of Deep Underground Science and Engineering Project(No.104023006)National Key Research and Development Program of China(No.2024YFF0507903).
文摘For hard rock cracking induced by laser irradiation,the failure modes and fracture characteristics among rocks of different types and sizes are still unclear.Therefore,the experiments on laser-induced fracturing of limestone,sandstone,and various-sized granite specimens were conducted.Real-time acoustic emission monitoring and laser scanning were employed to capture acoustic emission signals inside rocks during laser irradiation and to reconstruct the fracture surfaces after laser irradiation.Results indicate that abundant melts in sandstone and granite dissipated laser energy,leading to lower acoustic emission peak energy compared to limestone.Larger-sized specimen delayed the occurrence of peak energy.Crystal thermal expansion and changes in pore pressure induced tensile-shear composite failure in limestone,whereas thermal expansion of minerals in sandstone and granite promoted tensile failure.Fracture surface morphology was influenced by sampling interval,anisotropy,and size effects.The joint roughness coefficient and fractal dimension of sandstone exceed granite and limestone.Asperity heights and slope angles ranged from 1–14 mm and 0–40°,respectively,with the average aspect angles exceeding 110°.Granite exhibited the highest proportion of macropores after laser irradiation,approximately 4.8%.These findings provide valuable insights for the application of laser-assisted fracturing in hard rock excavation.
基金supported by the National Natural Science Foundation of China(No.52522405)Henan Provincial Natural Science Foundation(No.252300421323).
文摘Fractures in rock strata serve as flow pathways for gas flow.The undulation of fracture channels can influence the guidance of gas flow.In this context,four-point bending experiments on prefabricated fractured rocks at different angles under stable stepped loading stress.The experiment results clarified the evolutionary law that the undulation degree of the rock tensile fracture surface is separated by an initial fracture angle of 45°.The high undulation intervals were less than 45°,whereas the low undulation intervals were more than 45°.Furthermore,the relative undulation degree,undulation frequency,and matching degree of the fracture surface were quantified.The relationship between the change in fracture surface undulation and gas flow guidance was established.Based on this,the stability,tortuosity,and uniformity of the gas flow in the fracture channel were quantitatively characterized.Subsequently,numerical models of the fracture channels were constructed to validate the indices proposed in this study.The results of the study clarified the influence of different initial fracture angles on the undulation changes of fracture surfaces,and established the relationship between these changes and gas flow,which is conducive to understanding the role of internal fracture channels in rocks in guiding the gas flow process.
基金financial support from the National Natural Science Foundation of China(Grant Nos.U1965205 and 51779251)。
文摘Large shear deformation problems are frequently encountered in geotechnical engineering.To expose the shear failure mechanism of rock tunnels,compression-shear tests for rock models with circular tunnel were carried out,including single tunnel and adjacent double tunnels.The failure process is recorded by the external video and miniature cameras around the tunnel,accompanied by real-time acoustic emission monitoring.The experiments indicate that the shearing processes of rock tunnel can be divided into four steps:(i)cracks appeared around tunnels,(ii)shear cracks and spalling ejection developed,(iii)floor warping occurred,and(iv)shear cracks ran through the tunnel model.Besides,the roughness of the sheared fracture surface decreased with the increase in normal stress.Corresponding numerical simulation indicates that there are tensile stress concentrations and compressive stress concentrations around the tunnel during the shearing process,while the compressive stress concentration areas are under high risk of failure and the existence of adjacent tunnels will increase the degree of stress concentration.
