GRAVITY-DRIVEN INFILTRATION INSTABILITY IN INITIALLY DRY NONHORIZONTAL FRACTURES

Experimental evidence demonstrating gravity-driven wetting front insta bility in an initially dry natural fracture is presented. An experimen tal approach is developed using a transparent analog rough-walled frac ture to explore gravity-driven instability. Three different boundary c onditions were observed to produce unstable fronts in the analog fract ure: application of fluid at less than the imbibition capacity, invers ion of a density-stratified system, and redistribution of flow at the cessation of stable infiltration. The redistribution boundary conditio n (analogous to the cessation of ponded infiltration) is considered in a series of systematic experiments. Gravitational gradient and magnit ude of the fluid input were varied during experimentation. Qualitative observations imply that finger development is strongly correlated to the structure of the imbibition front at the onset of flow redistribut ion. Measurements of fingertip velocity are used to develop a first-or der relationship with fingertip length. Measured finger width is compa red to theoretical predictions based on linear stability theory.