Geoscience Reference
In-Depth Information
Fig. 9.1 Patterns of vertical
water flow in a porous
medium with ''wet'' initial
conditions; 40 mm of water
containing Brilliant Blue FCF
as a dye were applied to the
ground surface. The left
figures show the pattern of
infiltrating water, while the
right plots show one-
dimensional (averaged)
profiles of dye coverage with
depth. Reproduced by
permission of American
Geophysical Union. Flury
et al. (
1994
), doi:
10.1029/
94WR00871
. Copyright 1994
American Geophysical Union
Immiscible two-phase displacement occurs in two forms: drainage, which
occurs when a wetting phase is displaced by a nonwetting phase, and imbibition,
which is the opposite process, i.e., a wetting phase displacing a nonwetting phase.
The contact angle that each liquid phase forms with the grains of the solid matrix
determines which of the two liquid phases is the wetting one. The basic mecha-
nisms of immiscible displacement during drainage and imbibition are different
(Lenormand
1990
; Blunt and Scher
1995
). In drainage, the nonwetting fluid must
exceed the pressure in the wetting fluid within the throat by a value equal to the
capillary pressure. Subsequently, it spontaneously invades the adjacent pore. This
invading behavior is defined as piston-like invasion. In imbibition, the invading
behavior is affected by the pore geometry, the number of filled throats attached to a
pore, and film flow controlled by the roughness of the solid. Wetting invasion
therefore is characterized by pinch-off behaviors (Vizika and Payatakes
1989
)in
addition to piston-like invasion.
Important studies of field-scale, water infiltration patterns are provided by Flury
et al. (
1994
). Figures
9.1
and
9.2
show typical infiltration patterns of water, in
various soils, for different degrees of initial saturation. A wide range of infiltration
patterns is exhibited by these soils, from strong preferential flow to relatively
stable, uniform flow (e.g., Fig.
9.1
, lower plot).
A key control on preferential flow is the heterogeneity of soils in the vadose
zone. This heterogeneity is omnipresent, at scales ranging from the pore level to
