3 Saturation Overshoot

[2328.2.1] Infiltration experiments [59] on constant flux imbibition into a very dry porous medium report existence of non-monotone travelling wave profiles for the saturation [55, 62]

as a function of time t≥0 and position 0≤z≤1 along the column. [2328.2.2] Here -∞<y<∞ denotes the similarity variable

and the parameter -∞<c*<∞ is the constant wave velocity. [2328.2.3] From here on all quantitites z,t,y,S,c* are dimensionless. [2328.2.4] The relation

defines the dimensional depth coordinate 0≤z^≤L increasing along the orientation of gravity. [2328.2.5] The length L is the system size (length of column). [2328.2.6] The dimensional time is

where Q denotes the total (i.e. wetting plus nonwetting) spatially constant flux through the column in m/s (see eq.  (13)). [2328.2.7] The dimensional similarity variable reads

[2328.2.8] Experimental observations show fluctuating profiles with an overshoot region [59, 73, 54, 55, 74, 75]. [2328.2.9] It can be viewed as a travelling wave profile consisting of an imbibition front followed by a drainage front.

[2328.3.1] The problem is to determine the height S* of the overshoot region (=tip) and its velocity c* given an initial profile S⁢z,0, the outlet saturation Sout, and the saturation Sin at the inlet as data of the problem. [2328.3.2] Constant Q and constant Sin are assumed for the boundary conditions at the left boundary. [2328.3.3] Note, that this differs from the experiment, where the flux of the wetting phase and the pressure of the nonwetting phase are controlled.

[2328.4.1] The leading (imbibition) front is a solution of the nondimensionalized fractional flow equation (obtained from eq.  (17) for d=1)

[page 2329, §0]    while

must be fulfilled at the trailing (drainage) front. [2329.0.1] Here ϕ is the porosity and the variables z and t have been nondimensionalized using the system size L and the total flux Q. The latter is assumed to be constant. [2329.0.2] The functions fim,fdr are the fractional flow functions for primary imbibition and secondary drainage. [2329.0.3] They are given as

with i∈im,dr and the dimensionless gravity number [76, 37]

defined in terms of total flux Q, wetting viscosity μW, density ϱW, acceleration of gravity g and absolute permeability k of the medium. [2329.0.4] The functions kWri⁢S,kOri⁢S with i∈im,dr are the relative permeabilities. [2329.0.5] The capillary flux functions for drainage and imbibition are defined as

with i∈im,dr and a minus sign was introduced to make them positive. [2329.0.6] The functions Pci are capillary pressure saturation relations for drainage and imbibition. [2329.0.7] The dimensionless number

is the macroscopic capillary number [76, 37] with Pb representing a typical (mean) capillary pressure at S=0.5 (see eq.  (34)). [2329.0.8] For Ca=∞ one has Di=0 and eqs. (23) reduce to two nondimensionalized Buckley-Leverett equations

for the leading (imbibition) front and

for the trailing (drainage) front.

[page 2330, §1]

[2330.1.1] Conventional hysteresis models for the traditional theory require to store the process history for each location inside the sample [77, 78, 79]. [2330.1.2] Usually this means to store the pressure and saturation history (i.e. the reversal points, where the process switches between drainage and imbibition). [2330.1.3] A simple jump-type hysteresis model can be formulated locally in time based on eq.  (23) as

[2330.1.4] Here Ξ⁢S denotes the left sided limit

Θ⁢x is the Heaviside step function (see eq.  (35)), and the parameter functions fi⁢S,Di⁢S with i∈dr,im require a pair of capillary pressure and two pairs of relative permeability functions as input. [2330.1.5] The relative permeability functions employed for computations are of van Genuchten form [80, 81]

with i∈im,dr and

the effective saturation. [2330.1.6] The resulting fractional flow functions with parameters from Table 1 are shown in Figure 1b. [2330.1.7] The capillary pressure functions used in the computations are

with i=dr,im and the typical pressure Pb in (27) is defined as

[2330.1.8] Equation (29) with initial and boundary conditions for S and an initial condition for ∂⁡S/∂⁡t is conjectured to be a well defined semigroup of bounded operators on L1⁢0,L on a finite interval 0,T of time. [2330.1.9] The conjecture is supported by the fact that each of the equations (23) individually defines such a semigroup, and because multiplication by Ξ or 1-Ξ are projection operators.