Partitioning Dynamics of Gravity‐Driven Unsaturated Flow Through Simple T‐Shaped Fracture Intersections

A fundamental understanding of the fluid movement and dynamic partitioning process at fracture intersections is important to accurately predict water infiltration and contaminant transport in networks of fractures. We present an experimental study on the flow‐splitting behavior at a T‐shaped intersection. Different combinations of apertures of the vertical (bv) and horizontal (bh) fractures are considered. Experimental results confirm that the gravity‐driven flow in the vertical fracture transitions from droplet to rivulet mode as the flow rate increases. We quantify the flow dynamics through the intersection and especially focus on the partitioning efficiency (η) defined as the percentage of flow partitioned into the horizontal fracture. We identify three regimes of flow partitioning at the intersection for the case of bv < bh: total partitioning (η → 1), splitting or partial bypass (0 < η < 1), and total bypass (η → 0). The total bypass regime is associated with the rivulet mode with a flow rate higher than ~1.5 ml/min. We find a simple relationship between η and the flow rate Q for droplet flow, η = min(1, ChQ−1), where Ch is a threshold flow rate below which droplets almost completely imbibe into the horizontal fracture, leading to η → 1. A force balance analysis links Ch to a critical droplet length for the transition from complete partitioning to path splitting. The obtained relationship is further supported by numerical simulations of droplet flow through intersections. The results and analysis from this study may provide insights and physical constraints on construction of reduced order unsaturated flow models based on simplified discrete fracture networks. Key Points We experimentally study unsaturated flow through fracture intersections with a focus on the flow‐partitioning behavior We identify three regimes of partitioning behavior, including total partitioning, partial bypass, and total bypass, depending on flow rate We find a simple relationship between partitioning efficiency and flow rate for droplet mode and elucidate the mechanism of partitioning