[1] We conducted laboratory experiments to evaluate the effects of simple fracture intersections with differing aperture on density-driven immiscible wetting (water into air) and nonwetting (Trichloroethylene into water) flows, and analyzed them quantitatively. The experimental systems consisting of vertical and horizontal fractures were fabricated with glass for easy visualization. The aperture variation between intersecting fractures and the viscous force of the injected fluid were considered to be critical system parameters. Experimental results show the critical difference between the wetting and nonwetting flows by the intersection and viscous force, and subsequent mathematical analyses explain well our observations: The intersection acts as a capillary barrier (CB) for the wetting and capillary bridge for the nonwetting flows, and the viscous force of flowing fluids reduces the strength of CBs. The results of both laboratory experiments and mathematical analyses suggest that the fracture intersection with differing aperture can be a more significant factor controlling the network-scale phase structure for the nonwetting than the wetting flows.
[1] The influence of a single fracture intersection on density-driven immiscible flow is compared between wetting (water into air) and nonwetting (Trichloroethylene into water) flows. At low supply rates, the intersection acted as a hysteretic gate to pulsed flow of the wetting phase, but had minimal influence on nonwetting phase flow. For both cases, increasing the supply rate led to the formation of continuous fluid tendrils that crossed the intersection without interruption. The wetting experiment returned to pulsed flow as the supply rate was decreased, while the nonwetting experiment maintained a continuous flow structure. Results suggest a fundamental difference between wetting and nonwetting phase flows in fracture networks.
[1] Experiments were carried out to investigate the influence of the groundwater flow regime on dense nonaqueous phase liquid (DNAPL) migration in a rough-walled fracture. Hydraulic and DNAPL migration experiments were performed at Reynolds numbers ranging from 0 to 60.8. Nonlinear groundwater flow occurred in the fracture from Reynolds number of approximately 10. Different migration paths taken by DNAPL were observed between linear and nonlinear flow regimes. Experiments were quantitatively analyzed using the modified invasion percolation (MIP) model, which was found to be attributed to the inertial force of flowing groundwater. The MIP model was reformulated to incorporate the effect of inertia to predict the DNAPL migration path in a rough-walled fracture. The model data provided a good match to the experimentally determined DNAPL migration. The studies indicated that DNAPL migration depended on the groundwater flow regime, which needs to be considered in order to better understand DNAPL migration in rock fractures.
A statistically conceptualized fracture network is generally used in modeling flow and transport in the discrete fracture network (DFN) approach. To quantify the influence of the fracture connectivity and characterization level on the uncertainty from a statistical conceptualization of fractures, the ensemble mean and variability of the equivalent permeability for stochastically generated fracture networks is analyzed with various percolation parameters ( p) for different structures following power law size distributions. The results of Monte Carlo analyses show that statistics of a fracture network can be used to estimate its hydraulic properties with an acceptable level of uncertainty when p is greater than the specific percolation parameter ( p s ) where the domain size is expected to become equal to the correlation length of a given fracture network. However, when p is smaller than the p s , the uncertainty of the hydraulic properties induced from statistical characteristics of fractures is large, thus statistical conceptualization is not recommended. Conditional simulations support them: although we have deterministic information on a significant amount of fractures in the domain, a small number of stochastically generated fractures still produce significant uncertainty in the estimated system properties when p is smaller than p s . These results suggest that the p s and correlation length of a fracture network can be criteria to evaluate the applicability of the statistical conceptualization for modeling flow in a given fractured rock.
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