Environmental Engineering Reference
In-Depth Information
4.3.1.1 Willapa Bay (Pleistocene), Washington, USA
The only detailed permeability study of SCD, by
Gingras et al. (1999)
, inves-
tigated Pleistocene examples of
Glossifungites
-Ichnofacies-enhanced perme-
ability using field slug tests, laboratory Darcy tests, and numerical models.
They reported that three variables determine the flow characteristics of a
Glossifungites
surface: (1) the degree of contrast between burrow permeability
(
K
b
) and matrix permeability (
K
m
), (2) the amount of burrow interpenetration,
and (3) the burrow density. Their computer simulations indicated that the dom-
inant influences are the permeability contrast (
K
b
/
K
m
) and the degree of burrow
connectivity.
Gingras et al. (1999)
further showed that empirical and analytical
formulae could be derived to determine the effective (bulk) permeability (
Fig. 7
).
This was determined from
K
b
,
K
m
, and the degree of connectivity. The general
formula can be applied in any geological medium as a simple, first-run
approximation:
V
b
K
b
;
where
V
b
is the fractional volume of burrows within a block.
log
K
bulk
¼
log
½ð
1
V
b
Þ
K
m
þ
10
8
10
7
10
6
10
5
10
4
10
3
1%
10
2
10
Limit as burrow % approaches 0%
10
8
1
10
2
10
4
k
b
/
k
m
(=
k
b
¢
)
10
6
1
FIGURE 7
Curves showing the relationship log
K
bulk
¼
V
b
K
b
] for volumes of
1%, 25%, 50%, and 100% bioturbation. In this instance, enhancement of the vertical permeability
is shown. The
x
-axis shows how the ratio of burrow (
k
b
) and matrix (
k
m
) permeabilities diverts the
behavior from the predicted permeability. The
y
-axis indicates how much bulk permeability is
enhanced of the matrix permeability (
k
v
/
k
m
), depending on (1) the burrow and matrix permeability
ratio, and (2) the proportion of medium that is bioturbated. This relationship was derived empirically
using Darcy experiments, slug tests, and numerical models. The formula is best applied to surface-
constrained discrete heterogeneities and is useful for the estimation of the bulk permeability.
log[(1
V
b
)
K
m
þ
Search WWH ::
Custom Search