工程科学与技术2026，Vol.58Issue(1)：31-45,15.DOI:10.12454/j.jsuese.202500156
恒定法向刚度下裂隙网络剪切-渗流特性数值模拟研究
Numerical Simulation Study of Shear-Seepage Characteristics of Fractured Network Under Constant Normal Stiffness
摘要
Abstract
Objective Accurately predicting permeability evolution during fault shear in fractured rock masses under constant normal stiffness(CNS)bound-ary conditions is of fundamental importance for the design and optimization of geothermal reservoir stimulation,subsurface energy extraction,and fluid management in deep rock engineering.In enhanced geothermal systems(EGS),hydraulic stimulation commonly induces shear slip along pre-existing faults and fracture networks,resulting in complex and competing hydraulic responses.On one hand,fault shear dilation can sig-nificantly enhance permeability by generating preferential flow channels;on the other hand,the associated stress redistribution and normal stress amplification may induce closure and damage of surrounding fracture networks,leading to permeability degradation.Despite extensive experi-mental and numerical studies on shear-flow coupling in single fractures,the permeability evolution of three-dimensional discrete fracture net-works(DFNs)interacting with a shearing fault under CNS conditions remains poorly understood.Existing studies often focus on either fault per-meability enhancement or fracture closure effects in isolation,while neglecting the coupled competition between shear-induced dilation and network-scale compression damage.Moreover,the influence of aperture anisotropy,boundary stiffness,and stress constraints on flow anisotropy and channelization patterns has not been systematically quantified.The objective of this study is therefore to quantitatively investigate the dual mechanisms governing permeability evolution during fault shear under CNS conditions:1)permeability enhancement driven by fault shear dila-tion and channelization,and 2)permeability reduction induced by compression damage of the surrounding fracture network.Particular emphasis is placed on evaluating the effects of fracture aperture anisotropy,normal stiffness,and boundary stress on damage evolution,permeability anisot-ropy,and flow partitioning between faults and fracture networks. Methods A three-dimensional DFN model incorporating a through-going fault was developed to simulate shear-flow coupling processes under CNS boundary conditions.The DFN consists of multiple randomly oriented fractures with statistically prescribed aperture distributions,embed-ded within a cubic rock domain.Fracture apertures were assumed to follow a truncated Gaussian distribution with spatial correlations,character-ized by a mean aperture μ0 ranging from 1 to 4 mm and a standard deviation σ0 ranging from 0.3 to 1.2 mm.This formulation captures the inher-ent heterogeneity and anisotropy of natural fracture apertures while avoiding unrealistically negative values.Fault shear was simulated by impos-ing incremental shear displacements(uₛ:0 to 200 mm)under CNS constraints,such that normal stress evolved dynamically in response to shear-induced dilation.The mechanical response of fractures to normal stress was governed by a nonlinear hyperbolic closure relationship,enabling pro-gressive aperture reduction and damage accumulation in the DFN during shear.A fracture damage rate(RD),defined based on the reduction of av-erage fracture aperture relative to the maximum allowable closure,was introduced to quantify the degree of DFN damage at each shear stage.Fluid flow within the DFN and fault was modeled using the Reynolds equation,assuming laminar flow through rough-walled fractures.Numeri-cal simulations were conducted in COMSOL Multiphysics,with flow applied along the x-,y-,and z-directions to evaluate permeability anisot-ropy.Equivalent permeability coefficients were calculated using the cubic law,and permeability evolution ratios were derived relative to the ini-tial,undeformed state.To assess the influence of mechanical boundary conditions,a parametric study was conducted by varying the normal stiff-ness(kₙ:0.25 to 1.00 GPa/m)and boundary stress(σy:1 to 4 MPa).Flow partitioning between the fault and the DFN was quantified by calculating the proportion of total volumetric flux transmitted through each component,allowing for detailed analysis of channelization and hydraulic dominance. Results and Discussions Simulation results reveal that permeability evolution during fault shear is governed by a pronounced competition be-tween fault-induced permeability enhancement and DFN compression-induced permeability reduction.This competition is strongly modulated by fracture aperture anisotropy,mechanical boundary conditions,and flow direction.Increasing aperture standard deviation σ0 significantly reduced the DFN damage rate RD across all shear stages.For example,at μ0=1 mm and us=200 mm,RD decreased from 34.68%for σ0=0.3 mm to 12.35%for σ0=1.2 mm,representing a reduction of approximately 64%.This effect arises from the increased heterogeneity of fracture apertures,which promotes stress redistribution and limits the spatial extent of fracture closure.Highly heterogeneous apertures also facilitate localized chan-nelized flow,resulting in pronounced groove flow patterns within both the DFN and the fault.Normal stiffness kₙ and boundary stress σy exerted a strong control on DFN damage evolution.Higher kₙ and σy amplified normal stress accumulation during shear,leading to accelerated fracture clo-sure and increased RD.The most significant damage increments,reaching up to 45%,occurred during the initial shear stage(uₛ:0 to 40 mm),cor-responding to rapid stress build-up under CNS conditions.Beyond this stage,damage accumulation gradually stabilized as shear progressed into a residual regime.Flow simulations demonstrated marked permeability anisotropy induced by fault shear.Permeability along the shear-parallel z-axis increased by two to three orders of magnitude due to fault dilation and the development of continuous high-aperture channels.In contrast,permeability along the x-and y-directions decreased by approximately 60%to 80%,reflecting dominant DFN compression and loss of intercon-nected flow pathways.Flow partitioning analysis showed that fluid progressively concentrated within the fault as shear displacement increased.At uₛ=200 mm,more than 94%of the total flow was transmitted through the fault for most boundary conditions,rendering the surrounding DFN hydraulically negligible.This dominance of fault channel flow was further enhanced by higher kₙ and σy,which suppressed DFN permeability while promoting fault-controlled transport.Notably,at large shear displacements(us>160 mm),shear-induced stresses significantly exceeded the imposed boundary stress σy,resulting in a diminished influence of σy on permeability evolution.This indicates a transition from boundary-controlled to shear-dominated hydraulic behavior,with important implications for long-term reservoir performance. Conclusions This study provides a comprehensive quantitative framework for understanding permeability evolution in fractured rock masses dur-ing fault shear under CNS conditions.The results highlight that permeability enhancement and reduction mechanisms coexist and compete throughout the shear process,with their relative dominance controlled by fracture aperture anisotropy,normal stiffness,and boundary stress.From an engineering perspective,the findings suggest that stimulation strategies in geothermal reservoirs should explicitly account for DFN damage in-duced by shear-related stress amplification,rather than assuming monotonic permeability enhancement.High aperture heterogeneity and con-trolled shear displacements may be leveraged to promote stable channelized flow while minimizing network-scale permeability loss.The strong localization of flow within faults at large shear displacements also underscores the need to manage fault-dominated flow paths to avoid premature thermal breakthrough or uneven reservoir depletion.关键词
裂隙岩体/渗透率/剪切/离散裂隙网络Key words
fractured rock mass/permeability/shear/discrete fracture network分类
建筑与水利引用本文复制引用
蔚立元,王晓琳,杨瀚清,刘日成,李树忱..恒定法向刚度下裂隙网络剪切-渗流特性数值模拟研究[J].工程科学与技术,2026,58(1):31-45,15.基金项目
国家自然科学基金项目(U24B2040 （）
52179118) （）
江苏省基础研究计划重点项目(BK20243050) （BK20243050）
