Unsaturated flow through fracture networks: Evolution of liquid phase structure, dynamics, and the critical importance of fracture intersections

Abstract

Experiments and analyses are presented to elucidate the critical control of fracture intersections on the evolution and dynamics of the liquid phase structure within unsaturated fracture networks in impermeable media. Phase structure was visualized within a thick vertical sheet of broken glass where the breaks constituted the fracture network. The critical system parameters, applied flow rate (viscous forces) and initial condition, were varied in a series of experiments. When initially dry, individual fracture intersections acted as capillary barriers and created a repeated dynamic from which a network-scale "slender-ladder" phase structure emerges that is composed of pools above each intersection joined by fingers or "tendrils" below. At low-flow rates, pulsation is found at intersections, within fingers, and along horizontal fractures. In some cases, pulsation extends to larger volume "cascade" events where several intersections act in concert. At higher-flow rates, viscous forces remove pulsation. Reinvasion upon drainage demonstrates that when initially wet, the capillary barrier behavior of the individual fracture intersections vanishes and intersections are rapidly spanned. This marked hysteretic response tends to guide flow and cause pathway persistence from one event to the next.