Environmental Engineering Reference
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where V i is the total volume of phase i , V p is the pore volume. In the sec-
ond equation,
is the porosity and V T is the total volume of the rock.
The simplest model for two phase fl ow is to assume that brine
and CO 2 fl ow completely independently. Suppose in our rock we have
S g
φ
0.1, i.e., 10% of the pore volume is occupied by CO 2 and 90% by
brine. If the fl ow can be seen as two independent single fl ows, we have
10% of the rock in which CO 2 permeates as if it were a single phase and
in 90% of the rock, single phase fl ow of brine. Or, in more general
terms, if S i is the saturation of phase i , we have a relative permeability,
simply given by:
=
k
=
S
ri
,
i
For most fl uids k r,i is smaller than 1. Let us now see how differences
in wettability of the two phases change this simple picture of two-phase
fl ow.
We consider the two phase fl ow of brine and CO 2 . As we have seen
before, brine is the wetting phase. Figure 9.7.1 presents two scenarios
for a high (a) and low (b) brine saturation. In Figure 9.7.1 (a) , the brine
phase occupies most of the pore space, and thereby forms a connected
Figure 9.7.1 Connected phase in pores
(a) Groundwater forms the connected phase and is relatively mobile while CO 2 exists
only as discrete blobs.
(b) CO 2 forms the connected phase while water coats the grains.
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