Fracture Film Flow in the Unsaturated Zone: A Porous Matrix Bypass Mechanism, 20-9208Printer Friendly Version
Inclusive Dates: 10/01/00 - Current
Background - Modeling flow and transport in unsaturated, heterogeneous porous media presents a significant challenge. In particular, the effect of fractures in rock on overall flow and transport is not well understood. Classical theory predicts that strong capillary forces cause excess water in fractures to be imbibed into the rock matrix and that overall flow and transport rate are dominated by the properties of the porous matrix. However, a growing number of experimental results have shown that solutes can be transported vertically through unsaturated fractured porous media at a rate that far exceeds the average travel time for water in the porous matrix. Furthermore, recent reports show that fracture surfaces remain covered by a thin water film even in unsaturated conditions, which suggests that fracture film flow provides a mechanism to bypass the porous matrix, and thus may have a significant effect on the macro scale interaction between fractures and porous matrix.
Despite the potential importance of this fracture film flow bypass mechanism to unsaturated zone flow and transport modeling, previous studies have been restricted to qualitative and scaling arguments, the use of conceptual models, and a general discussion of the physical processes likely to have a significant influence. To the team's knowledge, nobody has performed a detailed, pore-scale numerical study of this phenomenon. This work is performed to improve SwRI's ability to model flow in, and specifically to understand the physical processes that allow rapid flow in unsaturated fractured rocks, which will help support the NRC in assessing the performance of deep geologic repositories. In the process, SwRI is developing expertise in micrometer-scale modeling in preparation of pursuing externally funded work.
Approach - Detailed numerical simulations are performed based on first principles, to uncover the physics of the fracture film flow. Multiphase flow is simulated at length scales of micrometers and higher, consistent with the natural length scales (e.g., pore sizes and film thickness) of the problem. At these length scales, the principles of continuum mechanics are valid to model flow, transport, and surface tension. Focus is on the details of the fracture film flow and on its interaction with the porous matrix (i.e., with capillary pores). The influences of spatially variable film thicknesses and intermittent infiltration events on the dynamic exchange processes between fractures and porous matrix are explored. The hypothesis will be explored, that competing, spatially distributed surface tensions will limit (choke) local imbibition into capillary pores while rivulets of thicker film, possibly contained within surface roughness channels, will significantly contribute to bypassing the porous matrix. The results and insights gained from these simulations will be used to provide estimates of average interaction parameters between fractures and the porous matrix.
Accomplishments - The capability of the commercial software FLOW3D to address the objectives were explored. The main limitations are due to the required very small time steps associated with small length scales of the flow domains considered. Grid resolutions of a fraction of a micrometer require time steps as small as nanoseconds. For practical purposes, a flow field resolved at that scale cannot be simulated for longer than a few milliseconds. However, this is sufficient time for a useful simulation if the overall flow domain scale is less than a few millimeters. Numerical results were obtained for basic flow simulations, such as capillary rise in a thin gap, or free surface film flow. Two-dimensional simulations to study the interaction between film flow and capillary gaps were performed. An initial parameter analysis shows that the relation of scales between film thickness and capillary gap width has a significant influence on the imbibition flux into the capillary gaps. It was found that if the film is sufficiently thin, the capillary flux is choked at the inlet, which leads to an imbibition rate that is significantly less than predicted by the capillary tension. Although results are still too limited to be conclusive, this nevertheless suggests that a sustained rapid film flow may exist in unsaturated conditions, without being significantly affected by imbibition.