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Table 9.1. Difference between gravity-driven flow of wetting and non-wetting fluids
Liquid fingering Persoff
Feature downward Gas bubble rising Factors affecting
Force causing fluid to Gravity aided by Gravity opposed by Surface tension
enter fracture capillarity capillarity. Air entry opposing gas entry
pressure must be must be exceeded.
exceeded Gas enters at the
largest pore space
Shape of moving Long fingers form String of bubbles, size
fluid – infinite limited by water
source at fracture flow to cause
edge snap-off
Shape of moving Liquid remains as a Finite number of gas Gas invades as a
fluid – finite source film on walls and bubbles, but will finger but bubble
at fracture edge slug may be keep rising forms in a
completely “snap-off” process.
consumed in this Bubble size depends
way. Water will not on properties of
penetrate liquid ρ, σ, and µ.
indefinitely deep Snap off occurs
unless replenished sooner (bubbles
from above smaller) if σ or ρ is
larger, µ is smaller
Fluid contact with Water wets fracture Gas does not wet
fracture walls walls fracture wall, film of
liquid remains
between gas and wall
Effect of constriction Constriction restricts Constriction opposes
liquid flow but does bubble rise and may
not block penetration cause blockage
(Jamin effect)

bubbles and finally to shut off (contain) the gas flow. Gas flow rates were estimated
from bubble sizes and velocities as recorded on videotape.
The following features were observed: Gas flow was not contained until the increas-
ing P (waterpressureminusgaspressure)wasapproximately2.5kPa. For P below
this value, bubbles always formed in the largest pore along the bottom edge of the
fracture, which was approximately 1.5 mm. Observations from an actual gas-storage
cavern showed that P of approximately 25 kPa was needed to prevent gas leakage,
suggesting that a fracture with a larger entry pore intersects the cavern.
In porous media, gas will not generally displace liquid from a pore until the gas
pressure exceeds the liquid pressure by the “air-entry pressure,” which is determined
by the pore diameter and liquid surface tension. The data of Kostakis (1998), however,
show, in effect, a negative air-entry pressure. This suggests that the invasion of gas