Novel fluid-fluid interface domains in geologic media

Abstract

Pore-scale fluid processes in geological media are critical for numerous applications in several fields. Continued improvement of high-resolution image acquisition and processing methods has provided a means to directly characterize pore-scale fluid processes for natural geomedia, and to test the representativeness of theoretical and computational models developed to simulate them. High-resolution synchrotron X-ray microtomography (XMT) combined with advanced 3-D image visualization was used to investigate the impact of larger-scale solid-surface heterogeneity on nonwetting-wetting (air-water) interfacial area for two natural geomedia (a sand and a soil). The studies revealed the presence of air-water interfaces associated with water residing within macroscopic features such as pits and crevices on the surfaces of the solids. The diameters of the features ranged from tens to 100's of μm for the sand, and the aggregate associated air-water interfacial area was estimated to represent ∼12% of the maximum capillary interfacial area. These features and respective fluid interfaces, which are not considered in standard conceptualizations of fluid distribution in geomedia, may have an impact on pore-scale physical and biogeochemical processes.

Figure 1.

Conceptual representation of fluid distribution and interfaces in a pore sequence: (A) Conceptualization representing solely capillary interfacial area. (B) Conceptualization incorporating wetting-fluid film. (C) Conceptualization of fluid-fluid interfaces for natural porous media, including fluid-fluid interface associated with macroscopic solid-surface features.

Figure 2.

Scanning-Electron Microscopy (SEM) image. A. Sand (200 μm). B. Soil (50 μm).

Figure 3.

Image-data visualization for a 1-mm diameter glass bead pack (7.41 μm/pix) at wetting-phase saturation (S_w) ≅ 0.12: (A) Greyscale 3-D rendering without noise reducing filters. (B) Two-dimensional greyscale slice pre-processed with noise-reduction filters with false-color overlay (blue) to highlight the wetting (water) fluid. (C) Wetting-fluid distribution within a 3-D section of the imaged domain. (D) The nonwetting-wetting fluid interface distribution without the solid phase.

Figure 4:

Image-data visualization for the natural sand (3.8 μm/pix) at S_w = ~0.2: (A) 3-D rendering of the sand pack. (B) 2-D greyscale slice with false-color overlay (blue) for the wetting fluid. (C) 3-D greyscale rendering with false-color map overlay (green) for wetting fluid for two sections of the domain presented in A.

Figure 5:

Subsection of 3-D greyscale rendering of the natural sand pack at 2 μm/pix: (A) 3-D rendering of the solids. (B) 3-D rendering with false-color map overlay (green) for the wetting fluid. (C) 3-D Magnification of a ‘pit’ on the surface of a solid grain, with wetting fluid residing within.

Figure 6:

Subsection of 3-D greyscale rendering of the natural soil pack (7.3 μm/pix) at S_w = ~0.6: (A) 2-D rendering of the solids. (B) 2-D greyscale slice with false-color overlay (blue) for the wetting (water) fluid. (C) 3-D rendering with false-color overlay (blue) for the wetting fluid. (D) 3-D example of a ‘pit’ on the surface of a solid grain, with wetting fluid residing within as indicated by the red arrow. (E) 3-D Magnification of a ‘pit’ on the surface of a solid grain, with wetting fluid residing within as indicated by the red arrow.

Figure 7:

Microscope images of sand grains. Image taken using 12-mp camera through a Nikon E400 phase contrast microscope, with a 10x magnification. Lens scale (native) = 1 mm. A. Red arrows showing crevices and pits within a grain surface. B-D. Crevices and pits highlighting the textural heterogeneity within a sample. Adjacent red circle shows enhancement of pit within a grain-grain configuration. E. Microscope images of Glass Beads.

Figure 8:

Microscope images of natural soil. Image obtained using Nikon E400 phase contrast microscope, with a 10x magnification. Lens scale (native) = 1 mm. A. Red arrows showing crevices and pits within a grain surface. B-D. Crevices and pits highlighting the textural heterogeneity within a sample. B-C. Adjacent red circle shows enhancement of pit within a grain-grain configuration.