Fluid motion through an unsaturated fracture junction

Abstract

Fluid dynamics in unsaturated fracture intersections were experimentally investigated in a glass fracture-analog at four different flow rates and compared with theoretical behavior estimates. The experimental system consisted of a vertical fracture that divided into two fracture branches. Visual data of fluid behavior were collected using digital stills and video. Two flow modes were analyzed: film flow and capillary droplets accompanied by snapping rivulets. Even under almost ideal experimental conditions, fluid dynamics at the intersection were extremely complicated and strongly influenced by flow mode, dynamics in the main fracture, contact angle of the advancing meniscus, and the presence of a capillary barrier at the intersection. Film flow moved into one or both branches through a capillary bridge that formed above the intersection. In droplet mode, rivulets reached lengths of up to 70 cm prior to snapping, and often stretched contiguously across the intersection and into both branches while maintaining its connection to the two droplet halves. Transition from droplet to film flow mode was observed at the intersection. There was a strong correlation between the length of the incoming droplet and the potential for a switch to occur. Liquid bridges were observed to form occasionally at the entrance of a fracture branch, which blocked further liquid entry. The temporal nature of these blockages further complicates temporal and spatial variability observed in flow in fracture systems.