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C.J. G.Darnault, D.A. DiCarlo1, T.W.J.Bauters, J.A. Throop, T.S. Steenhuis, J.-Y. Parlange and C.D. Montemagno
Department of Agriculture and Biological Engineering, Cornell University
1Department of Petroleum Engineering, Stanford University

Accurate monitoring of flow instabilities, which can occur when water displaces non-aqueous phase liquids (NAPLs, eg. oil) in porous media, is an important component of predicting the transport and fate of these compounds in the subsurface. In particular, flow situations in which three separate phases (such as water, NAPL, and air) exist in the porous media have only begun to be studied. Unfortunately, very few methods exist that allow fast and accurate determination of fluid contents in three phase transient flow fields.

This group has developed two techniques to attack this problem: a light transmission technique and a dual-energy x-ray technique. The x-ray technique uses synchrotron x-rays from CHESS to measure three-phase fluid concentrations on the time scale of seconds. Using the harmonic content of x-ray diffraction from perfect silicon crystals, they obtain a high intensity x-ray beam consisting of two distinct tunable energies, typically 20 keV and 40 keV. Scintillation detectors installed before and after the sample chamber measured incident and transmitted intensities of both x-ray energies. Simple configurations of oil infiltrating into wetted soil were unstable at low flow rates and produced fingers. The widths and concentrations within each finger depended on the concentration of the other phase. When combined with the light transmission and simultaneous pressure measurements, a complete picture of three-phase flow instabilities can be realized.

A report of this work won an "Outstanding Student Paper" award for Ph.D. candidate T. W. J. Bauters from the American Geophysical Union Hydrology Section.

Light images through sand
Transmission light images through translucent silica sand media showing the equilibrium concentration of oil, water and air in different layers. Visual image (left), hue image (center) and intensity image (right) are shown. X-ray data on this cell were used to calibrate the absolute densities of each component.