EGU2020-22593
https://doi.org/10.5194/egusphere-egu2020-22593
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Analogue laboratory experiments of preferential flow dynamics in porous fractured media: Importance of fracture intersections and porous matrix imbibition processes

Torsten Noffz1, Florian Rüdiger1, Marco Dentz2, and Jannes Kordilla1
Torsten Noffz et al.
  • 1University of Göttingen, Dept. Applied Geology, Germany
  • 2Institute of Environmental Assessment and Water Research (IDAEA), Spanish National Research Council (CSIC), Barcelona, Spain

Infiltration processes in fractured consolidated aquifer systems often exhibit complex gravity-driven flow features and hence tend to develop preferential flow paths along fracture network which contribute to rapid mass fluxes. This behavior is often difficult to model with classical methods such as the Richards equation, as a variety of interacting flow modes, ranging from free-surface flows over droplet and and rivulet flows, control the mass partitioning processes at fracture intersections and within fractures. Here we demonstrate with two different types of laboratory experiments how the complexity of such flows affects the discharge behavior: (1) In order to isolate the mass partitioning process at fracture intersections we use custom-made acrylic cubes to establish a set of vertical fractures (free-surface, bounded by one side only) intersected by horizontal fractures. In order to control the prevailing flow mode we use a multichannel dispenser and set flowrates to critical thresholds for each regime. We then calculate normalized horizontal fracture inflow rates and delineate classical Washburn-type behavior in order to obtain an analytical transfer function for the given system and extended fracture cascades. (2) In order to study the effect of a porous matrix adjacent to the fractures we carried out quasi-2D laboratory experiments of infiltration into complex fracture networks using Seeberger sandstone slices. The system allows to study both the onset of preferential fracture flow dynamics as well as the porous matrix imbibition under dynamics conditions. To study the effect of geometry on discharge dynamics we modify fracture apertures as well as fracture offsets, i.e., the geometry of the fracture intersections. Results show that, despite the complex internal flow dynamics, clear scaling patterns can be observed and the geometrical characteristics are imprinted into the outflow behavior.

How to cite: Noffz, T., Rüdiger, F., Dentz, M., and Kordilla, J.: Analogue laboratory experiments of preferential flow dynamics in porous fractured media: Importance of fracture intersections and porous matrix imbibition processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22593, https://doi.org/10.5194/egusphere-egu2020-22593, 2020

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Presentation version 2 – uploaded on 04 May 2020
- figure despriction updated
  • CC1: Comment on EGU2020-22593, Dani Or, 05 May 2020

    Nice work and experiments!

    I wonder how important the outcome to the inlet boundary conditions? Imagine similar fluxes introdcuded via intervening "sand layer" at the top of the fracture network

    Also, how is your continuum model handles the discrete nature of the internal dripping? 

  • AC1: Comment on EGU2020-22593, Jannes Kordilla, 05 May 2020

    This is a good question and we in fact have experimented with different inlet conditions (sponge type) in order to emulate a diffuse boundary condition such as a sand. However, we then went for boundary conditions that allow a clear distinction between droplet, and rivulet flow modes to isolate the partitioning processes

    The process of dripping is extremely difficult to handle due to its chaotic nature (even under controlled conditions). However, we found that the systems tends to favor the formation of rivulets when partitioning at subsequent fracture intersections occurs and hence replaced the measured ( transfer function with a Gaussian-type approximation. This works reasonably well, yet it is likely that some tailing of the breakthrough is not properly captured, as droplets posess a higher bypass capacity at the intersections.

Presentation version 1 – uploaded on 30 Apr 2020 , no comments