Influence of fracture intersections under unsaturated, low‐flow conditions

Abstract

[1] Recent experimental evidence suggests that the capillary heterogeneity associated with fracture intersections can act to impose temporal and spatial structure on network-scale flows. A simple intersection between orthogonal fractures, one horizontal and the other vertical, has been shown to integrate unsaturated flows. At low flows the intersection forms a capillary barrier that accumulates water in a growing pool. Eventually, the retaining meniscus snaps, discharging a pulse of water. Here we develop a mechanistic explanation for this observed behavior and experimentally consider three perturbations to the geometry of the simple orthogonal intersection. Two of the perturbations also act as capillary barriers, while the third formed a capillary bridge across the intersection. At low flow, all of our experimental intersections imposed a temporal signal, with the nature of that signal dependent on intersection geometry and participation by the horizontal fractures in dynamic storage. At high flow a continuous fluid tendril spanned the system from inlet to outlet with water pooled above the intersection caused by a narrow fluid connection that restricted flow across the intersection. Results from all experiments suggest that pulsation is critically sensitive to small variations in the geometry of fracture intersections and storage in the horizontal fractures. When combined with dependency on supply rate, this sensitivity can generate pulsation of flow across a wide range of time periods and discharge volumes.

Analogue Fracture Experiments and Analytical Modeling of Unsaturated Percolation Dynamics in Fracture Cascades

“…The aforementioned study focused on the effect of free‐surface flow on the partitioning dynamics at a single fracture intersection. Flow on the vertical surface, since it has no opposing fracture wall, is dominated by inertial forces (Wood et al, 2005). In natural settings, free‐surface flow may occur along dissolution shafts and wide‐aperture fractures with widths of up to several millimeters, which have been measured in field experiments (e.g., Dahan et al, 2000; Salve et al, 2004).…”

“…Different flow regimes, e.g., droplet, rivulet, and film flow, can coexist even in controlled settings and are difficult to cast into unified conceptual frameworks (Ghezzehei, 2004). In well-controlled analog percolation experiments, the fracture-specific formation of flow modes and instabilities (Jones et al, 2018;Li et al, 2018) and the role of unsaturated fracture intersections on partitioning behavior has been investigated (e.g., Ji et al, 2006;Kordilla et al, 2017;LaViolette et al, 2003;Nicholl and Glass, 2005;Wood et al, 2002Wood et al, , 2005Wood and Huang, 2015), where the applied flow rate controls the stability of a desired flow regime (Towell and Rothfeld, 1966;Schmuki and Laso, 1990). These studies emphasize the importance of fracture intersections as capillary barriers (until steady-state conditions are established), which may induce pulsating flows and act as important integrators for dispersion and recharge processes.…”

Advanced Conceptual Models for Unsaturated and Two-Phase Flow in Fractured Rock

“…The release of fluid from the pools was seen to be sensitive to both intersection geometry and steady flow rate. A manuscript documenting these results has been submitted to Water Resources Research [Wood et al, 2004b]. 11) Slender transport pathways have been found in laboratory and field experiments within unsaturated fractured rock.…”

Benchmarking NAPL Redirection and Matrix Entry at Fracture Intersections Below the Water Table

“…These groups investigated flow redirection at simple fracture intersections in impermeable glass at various inflow rates and intersection geometries. Wood et al () found that variations in fracture aperture had a great influence on the flow path of water invading air‐filled fracture intersections, and furthermore calculated that capillary pressure ( P c ) was indeed the dominant process for redirecting inflowing water at heads greater than 1 cm and with branch apertures less than 800 μm. Kordilla et al () recently reported on the dynamics of droplet and rivulet‐flowing water down a surface intersected by a perpendicular horizontal fracture, and how velocities and apertures affected the volume of fluid that “bypassed” the horizontal fracture.…”