Page 358 - gas transport in porous media

360
Gas flow through local channels
in water-filled porous media Falta
Air
1mm Air
Water
filled

Water
filled

Air Air

Figure 22.5. Conceptual model of local scale gas flow during air sparging (from Falta, (2000a))

material. Therefore, Leverett scaling could also be expected to apply to the gas entry
pressure:
1
p entry ∝ √ (22.5)
k
Agas phase will only penetrate a water filled material when the gas pressure exceeds
the sum of the water pressure and the entry pressure. The high entry pressure of lower
permeability materials often diverts the gas flow. These types of capillary barrier
effects during air sparging can be clearly seen in the experiments of Ji et al. (1993)
and in the numerical simulations by McCray and Falta (1996, 1997).
Injection of noncondensible gases below the water table tends to cause unstable
displacements that lead to undampened fingering at various scales. The low viscosity
of gases compared to water (about a factor of 50) leads to viscous instability when
the gas is displacing water, while the low density of gas compared to water leads
to buoyancy instability when the gas is moving upward through the water. The fact
that both stability criteria are not met means that the gas flow during air sparging is
theoretically unconditionally unstable (Slobod and Howlett, 1964). However it should
be noted that several numerical modeling attempts have been able to reproduce the
experimentally observed gas flow patterns (Larson and Falta, 1996; Hein et al., 1997;
McCray and Falta, 1997).

22.3.3 Expected Average Gas Saturations during Sparging
It has been observed in both laboratory and field studies that sparging gas satura-
tions tend to be higher in fine grained, low permeability formations, and lower in
coarse grained, high permeability formations (Clayton, 1998). This observation can
be explained by an analysis of one-dimensional vertical flow of gas at steady state
(Falta, 2000). Numerical simulations by McCray and Falta (1996, 1997) have shown
that once the sparging system stabilizes, the water inside the sparge zone is essentially