Page 181 - gas transport in porous media

Chapter 9: Unstable Gas Flow in Fractures
finding an alternative flow path; also, a fracture is highly permeable and can deliver
water to block and reblock the critical pore. 175
Unstable gas flow in wells as observed, for example, at the Wellenberg site in
Switzerland (Finsterle, 1994), may be an example of this type of multi-phase flow
phenomena in which a non-wetting phase cannot maintain sufficient pressure to keep
a flow path continuously open. Thunvik and Braester (1990) performed numerical
simulations of two-phase flow in networks of capillary tubes. They showed that a
constant gas pressure boundary condition in a fracture network could result in a cyclic
gas flow through the network, as a result of some fractures alternating between water
and gas occupancy. Recently, Faybishenko (2002) calculated diagnostic parameters
of deterministic chaos from time-series data from the experiments of Persoff and
Pruess (1995) and of Su et al. (1999).

9.3 BUBBLE FLOW IN FRACTURES BELOW
THE WATER TABLE
Below the water table, fractures are by definition fully saturated, so that relative
permeability to gas is zero. The presence of gas bubbles below the water table is
by definition unstable; also the presence of gas makes the fracture at least locally
unsaturated. In these cases gas exists not in continuous flow paths but as discrete
bubbles. In vertical or inclined fractures, buoyancy is a significant force driving gas
flow upward.
Water in a dry vertical or inclined fracture is gravitationally unstable and must flow
down; if the rate of water delivery to the fracture is less than the conductivity of the
fracture, the water will flow in fingers. This process has been studied for example by
Su et al. (1999). The analogous process for gas occurs when gas is injected or formed
in a fracture below the water table. This case is of interest in studying the loss of
gas from underground storage caverns (Kostakis, 1998); also bubbles of gas may be
formed below the water table by chemical reaction or exsolution of dissolved gas in
response to pressure drop. The rise of bubbles in a saturated vertical fracture is analo-
gous to the downward fingering of water in a dry fracture in that both events occur in
response to gravitational instability. The processes differ, however, as shown in Table
9.1, because of different wetting character of the fluid relative to the fracture wall.
Natural gas can be stored in excavated caverns in hard rock. To prevent leakage
of gas through fractures, water pressure in fractures surrounding the cavern is main-
tained greater than the gas pressure inside the cavern; this is called hydrodynamic
containment, or a water curtain. (i.e., one does not depend on the air-entry value of the
fracture to prevent gas from entering the fracture.) Simulating this situation, Kostakis
(1998) investigated bubble initiation and flow in vertical and inclined water-filled
fractures experimentally and with numerical simulations. A transparent replica of a
natural rough fracture was filled with water and connected to a gas-filled reservoir at
the bottom. Initially slugs of gas flowed rapidly through the fracture. Water pressure
was increased stepwise to reduce the gas flow to a stream of bubbles, then individual