On fracture structure and preferential flow in unsaturated chalk

The mechanisms controlling fluid flow through fractures intersecting chalk in the vadose zone were studied through mater percolation experiments in na tural discrete fractures and by close examination of the inner structure of fracture voids. The percolation experiments showed that the flow is focuse d in dissolution channels along the fracture plane, and that fluxes and Row trajectories within that net vary in both time and space. The locations of the dissolution channels, the main potential flowpaths within the fracture plane, were generally associated with fracture intersections, The flow thr ough. these channels was governed primarily by the mineralogical compositio n of the filling material and the inner structure of the fracture voids. Sa lt dissolution, solid-particle migration, and clay smelling were found to b e the predominant processes controlling flow through the dissolution channe ls, These physical changes in the structure of the filling material in the dissolution channels accounted for the observed unstable flow behavior. Our results suggest that models aimed at simulating water percolation through fractures in unsaturated chalk should consider the mapping of fracture inte rsections in addition to the commonly used mapping of fracture lineaments, Moreover, the detailed characterization of fracture apertures may not he th e key parameter determining fracture Row, because in such formations the fl ow. is controlled primarily. by filling material. These materials undergo s ignificant physical variations during wetting and drying cycles.