When there is only one type of fluid
flowing through porous media, the
permeability for this case is called
“absolute permeability.” However,
when there is more than one type of
fluids present in a rock, a
permeability of each fluid to flow
is decreased because another fluid
will be moving in the rock as well.
A new term of permeability called “effective
permeability”
is a permeability of a rock to a
particular fluid when more than one
type of fluid is in a rock
Reservoir consists of three
fluids (gas, oil, and water) so
these are commonly used
abbreviations for effective
permeability for each fluid.
•kg = effective permeability to
gas
•ko = effective permeability to
oil
•kw = effective permeability to
water
Normally, it is
common to state effective
permeability as a function
of a rock’s absolute
permeability. Relative
permeability is defined as a
ration of effective
permeability to an
absolute permeability of
rock. The relative
permeability is widely used
in reservoir engineering.
These functions below are
the relative permeability of
gas, oil, and water.
•krg
= kg ÷ k
•kro
= ko ÷ k
•krw
= kw ÷ k
k = absolute
permeability
Relative permeability is
normally plotted as a
function of water
saturation in a rock .
Figure 1
demonstrates a plot of
oil-water relative
permeability curves.
As water saturation (Sw)
decreases, relative
permeability of oil
(Kro) decreases and
relative permeability of
water increases (Krw).
If water saturation is
below connate water
saturation (Swc), only
oil will flow, but water
will not flow (Figure
2)
When water saturation (Sw) in a rock is
equal to connate water saturation (Swc),
water starts to flow (Figure
3)
Water starts to
flow at Swc.
Oil flow continues to decrease and water
flow continues to decrease because the
water saturation goes up. If water
saturation (Sw) is between connate water
saturation (Swc) and 1 minus Sor
(irreducible oil saturation), both oil
and water flow (Figure
4).
Once water saturation in a rock
increases to 1 minus Sor (irreducible
oil saturation), oil will not flow, but
only water will flow. Beyond this point
oil will not move at all but water will
continue to increase as water saturation
(Sw) in a rock increases (Figure
5).
|
|
|
|
|
figure 3 |
figure 4 |
figure 5 |
|
.jpg)
.jpg)
.jpg)
.jpg)
.jpg)