工程科学与技术 (Jan 2025)
Numerical simulation study of shear-seepage characteristics of fractured network under constant normal stiffness
Abstract
ObjectiveAccurately predicting permeability evolution during fault shear in fractured rock masses under constant normal stiffness (CNS) conditions is critical for optimizing geothermal reservoir stimulation and subsurface fluid management. Existing studies often neglect the interplay between fracture network damage and fluid channeling under varying mechanical constraints. This study aims to quantify the dual mechanisms of fault shear-driven permeability enhancement and fracture network compression-induced permeability reduction, while evaluating the impacts of aperture anisotropy, normal stiffness, and boundary stress on fluid flow anisotropy. MethodsA three-dimensional discrete fracture network (DFN) model incorporating faults was developed to simulate shear processes under CNS boundary conditions. Fracture apertures followed a truncated Gaussian distribution with spatial correlations, characterized by mean (μ₀: 1.0 – 4.0 mm) and standard deviation (σ0: 0.3 – 1.2 mm). Mechanical shear displacements (uₛ: 0 – 200 mm) were applied to analyze fracture network damage dynamics. Fluid flow simulations across multiple directions (x-, y-, z-axes) were conducted using the Reynolds equation in COMSOL Multiphysics, with permeability coefficients calculated via cubic law. Boundary stiffness (kₙ: 0.25 – 1.0 GPa/m) and stress (σᵧ: 1.0 – 4.0 MPa) variations were systematically investigated to assess their influence on damage rates and flow channeling. Results and DiscussionsIncreasing σ0 reduced DFN damage rates (RD) by 18% – 32% (e.g., μ₀ = 1 mm, σ₀ = 1.2 mm: RD = 12.35% at us = 200 mm vs. σ0 = 0.3 mm: RD = 34.68%). Channelized flow became prominent in both DFN and faults under higher σ0. Higher kn and σᵧ amplified DFN damage, with maximum damage rate increments (45%) occurring at us = 0 – 40 mm. Fluid flow concentrated in faults as kₙ and σᵧ increased, with >94% of total flow localized to faults at us = 200 mm. Permeability along the z-axis increased by 2–3 orders of magnitude during shear, while x- and y-axis permeability decreased by 60%–80% due to fracture compression. At us > 160 mm, shear-induced stresses dominated over σᵧ, weakening its influence. For z-axis flow, fault channel flow accounted for >94% of total flux at us = 200 mm, rendering the DFN hydraulically negligible. ConclusionsThis study quantifies the competing mechanisms governing permeability evolution in sheared DFN systems. Aperture heterogeneity (σo) significantly mitigates DFN damage, while higher μ₀ diminishes this effect. Boundary stiffness and stress intensify damage and channelized flow, with critical thresholds identified at us = 40 mm. Permeability anisotropy is shear-displacement-dependent, with fault channelization dominating at large displacements. These insights refine predictive models for hydraulic fracturing, geothermal reservoir management, and subsurface storage under shear-dominated regimes.
