Steady fluid flow and travel times in partially saturated fractures using a discrete air‐water interface

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Abstract
Fracture flow under conditions of partial fluid saturation is studied where a portion of the fracture is filled with water and the remaining portion is filled with air. A discrete air‐water interface is used to separate the saturated from the drained regions within the fracture. A laboratory flow experiment is presented which demonstrates the interface concept. The steady state air‐water interface position is observed by maintaining a circular water source at constant positive pressure head between two vertical glass plates. An analytic solution to the flow problem is compared to the observed interface position and to simulated results which assume that a constant capillary pressure head can be assigned along the interface. Simulations performed for a hypothetical vertical fracture indicate zones of positive and negative pressure head in the saturated zone as well as an air‐filled zone. For the hypothetical fracture examined, fluid travel times are not substantially changed as fracture saturation decreases. This result may not hold generally for all fractures.