Fracture Film Flow in the Unsaturated Zone: A Porous Matrix Bypass Mechanism, 20-9208

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Principal Investigators
Stefan Mayer
Derrick Coffin (Chandrika Manepally)

Inclusive Dates: 10/01/00 - 04/01/02

Background - Classical theory predicts that strong capillary forces cause excess water in rock fractures to be imbibed into the rock matrix and that overall flow and transport are governed by the properties of the porous matrix. However, new experimental results show 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. This 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. Newly developed computational fluid dynamics (CFD) codes can be used to examine the physics of the bypass at the pore scale.

We conducted this work to improve our ability to model flow in unsaturated fractured rocks, which will support  assessing the performance of deep geologic repositories.

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, film thickness) of the problem. At these length scales, the principles of continuum mechanics are valid to model flow and transport, as well as 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 was 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, significantly contribute to bypassing the porous matrix. The results and insights gained from these simulations will be used to estimate average interaction parameters between fractures and the porous matrix.

Accomplishments - The project was successful in addressing imbibition into fractures and pores over a range of scales. This study used a combination of analytical approximations, numerical solutions to a 1D approximation of the imbibition, and 2D and three dimensional (3D) numerical simulations using FLOW3D®. The results of detailed numerical simulations show that the theoretical imbibition rate from larger into smaller scale fractures can be reduced substantially in the case of film flow over a porous rock surface. Simulated reduction of film flow near the inlet of the imbibing pore, a result of local film thinning, effectively chokes local imbibition. Existence of the hypothesized porous matrix bypass mechanism is supported by the results; however, they are not sufficient to conclude to what extent a fraction of the thick film flow will percolate without being subjected to local thinning or imbibed into smaller fractures. For instance, the onset of the imbibition choking mechanism appears to be a function of the relative flux in the film and the imbibition. Further studies are needed to clarify whether the local thinning of film flow is a transient phenomenon or whether it is sustained by the capillary pressure in the pore.

SwRI investigators gained substantial experience in modeling micron-scale flows driven by surface forces. The FLOW3D code was useful in describing phenomena that cannot readily be approximated analytically, such as surface deformation and film flow imbibition interaction. For all but short time scale simulations, however,  a code using an explicit solver such as FLOW3D is not practical. Consequently, the original plan of meeting all objectives by performing parametric studies using FLOW3D simulations had to be modified, which led to the development of lower dimensional, analytical approximations. This, in turn, allowed the results to be obtained for a broader range of time and length scales. It is recommended that, for the immediate future, the use of CFD be limited to exploring difficult to quantify, small scale phenomena and checking results derived by other means.

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