Determination of connate water saturation
using gas displacement method
Dócs Roland
doktorandusz, tanársegéd
Miskolci Egyetem, Kőolaj és földgáz intézet
Introduction:
In case of the determination of Original Hydrocarbon In Place (OOIP, OGIP) one of
the most crucial parameter is the connate water saturation (Swc). This saturation
determines the given reservoirs initial hydrocarbon saturation due to the simple fact
that the cumulate saturation values of different fluids present in its porous network
always equal S=1. Predicted OOIP and OGIP volumes for a given reservoir are
therefore highly affected by the error of connate water saturation determined. Reservoir
engineers all over the world are working on different methods which could increase
accuracy in these measured saturation values.
Description of the measurements taken:
As part of my thesis work I have made measurements in the form of displacement
method where by the application of different pressure steps I have displaced water
from previously saturated samples using Nitrogen gas. The reason of these
measurements was to define that which scenario is more preferable, if lower starter and
more pressure steps, or a higher starter and less pressure steps are used at given
samples. For final displacement pressure at a given sample that value was used, where
no matter of elapsed time further increase, was not recorded in displaced volume. The
samples used were chosen in a way that their porosity were identical (15-20%), but had
different permeability, the reason was to check that how does permeability effect
displacement at these circumstances. The parameters of samples used at these
displacement measurements were following.
Table 1
Parameters of measured samples
Sample:
Am-12
A-12/1
A-12/3
Solid
volume
cm3
62.4828
62.3235
66.4842
Pore
volume
cm3
14.2053
15.5179
11.1864
Porosity
%
18.52
19.94
14.14
Dry
weight
g
149.69
168.25
179.9
Gas
Water
permeability permeability
mD
mD
294,25
264,85
56,12
45,6
15,16
13.47
For all 3 samples four measurements were made as following for the 1st and 3rd
starting pressures were equal to the lowest pressure where displacement had started for
that given core, difference was in the number of applied pressure steps. Higher starting
pressures and lesser steps were used at the 2nd and 4th measurements than at the first
two cases. Pressures that were used at these measurements are covered in Table 2
below.
Table 2
Applied pressures at each measurements
1st
bar
0.15
0.27
0.53
Am-128
Measurement
2nd 3rd
4th
bar
bar
bar
0.12
0.21
0.44
0.88
0.47
1.05 1.02
1.02
1.05
1st
bar
0.4
0.82
1.6
3.2
Sample:
A 12/1
Measurement
2nd 3rd
bar
bar
0.3
0.8
0.46
0.82
1.62
1.62
3.15 3.2
4th
bar
2.27
1st
bar
0.39
0.82
1.6
3.38
2.01
A 12/3
Measurement
2nd 3rd
bar
bar
0.31
0.8
0.46
0.82
1.6
1.6
2.05 2.01
4th
bar
1.57
2.01
In all four measurements the volumes of displaced water were different for each
sample. In order to describe the displacement mechanism at each measurement the
percentage of displaced volume to total pore volume in function of applied pressure
was taken into consideration. The results of all these displacements for each sample
were as following.
Figure 1
Percentage of displacements at sample Am-12
As it can be seen in Figure 1 that displaced volumes at each measurement were
different, only similarities that can be seen are at measurements number 1 and 3 where
the pressure steps used were close to each other, but the maximum displaced volumes
were different in these cases as well. From these values it can be determined that the
largest displaced volume was present at measurement number 1, close to it the 3rd
measurement as second, and at displacements number 2 and 4 values were similar, but
better result was acquired in case of 4th measurement. For all measurements
corrections were used at the determination of displaced volumes in order to take into
consideration those water volumes that might have accumulated on the inner surface of
the core holder's outlet pipe side. The correction was made by comparing weights
measured at the end of displacements with saturated weights at start. In order to gain
better accuracy I have also calculated displaced percentages in function of not total
pore volume but actual saturated volume (Vsat). At this correction those saturation
values of the given sample were taken into consideration where they highly differed
from the 100% saturated (GT) condition.
(1)
This correction was taken because of the differences in saturated weights at each
measurement. As result of these corrections the orders of displacements were as
following.
Table 3
Order of displacements at sample Am-12
3rd
53.9007%
1st
53.3953%
2nd
52.2325%
4th
51.0653%
The displacements at sample A-12/1 were as following as shown in Figure 2.
Figure 2
Percentage of displacements at sample A-12/1
After the applied corrections that were described at sample Am-12 the order of
displacements measured at sample A-12/1 are shown in the table below.
Table 4
Order of displacements at sample A-12/1
3rd
69.7896%
1st
67.9513%
4th
64.3003%
2nd
62.7797%
As it can be seen from those values that were measured at sample A-12/1 some
similarities can be recognized with those that were present at sample Am-12. The order
in both cases for 1st and 2nd places were the same, there were only difference in the
3rd and 4th places. The displaced values at the third and final sample were the
following.
Figure 3
Percentage of displacements at sample A-12/3
The order of final displacement values gained from measurements made on sample
A-12/3 is shown in Table 5.
Table 5
Order of displacements at sample A-12/3
3rd
49.2034%
1st
48.4622%
4th
48.3660%
2nd
47.7038%
As the results show the order of displacements were similar than they were at the first
two samples, showing the 3rd measurement as best scenario for all three samples. As
next step calculation of connate water saturation (Swc) was needed from displacement
values of all four measurements at each sample. Changes in these saturation values in
function of applied pressure at each measurement can be seen in the following graphs.
Figure 4
Changes in connater water saturaturation at sample Am-12
Figure 5
Changes in connater water saturaturation at sample A-12/1
Figure 6
Changes in connater water saturaturation at sample A-12/3
The final connate water saturation values that were reached at the end of each
displacement at all three cores are shown in Table 6.
Table 6
Connate water saturations measured at each experiment
Measurement
1
2
3
4
Minimum:
Am-12
Swc
%
47.42
48.118
46.499
49.385
46.499
Sample
A-12/1
Swc
%
32.0487
37.2203
30.2104
35.6997
30.2104
A-12/3
Swc
%
51.5378
52.2962
50.7966
51.6340
50.7966
Conclusion:
As it can be seen from the measurements most liquid volumes were displaced at the 3rd
measurements in all three cases and second largest displacements were reached in all
cores at the 1st measurements. Measurements number 2 and 4 occurred in mixed order
for 3rd and 4th places. The conclusion that can be made from the gained data is that in
all cases at these chosen cores most effective displacements occurred when lowest
starting pressures and more steps were used. Although it is an interesting fact that the
lowest water saturation was reached in case of sample A-12/1 which has permeability
smaller than Am-12 and larger than A-12/3. As it is shown in previous table the value
of connate water saturation is not only influenced by porosity and permeability
parameters of a given sample there are other influencing factors above these. As an
advantage of this method above for example displacement by rock centrifuge is that the
displacement can be observed throughout the entire measurement personally, when this
at the centrifuge cannot be said, although as a disadvantage it can be recorded that this
procedure takes several hours. As suggestion connate water saturation determination of
porous rock systems by gas displacement method is highly recommended because of
its simple nature and effectiveness.
Bibliography:
Roland Dócs, 2014. Ms thesis: Role of petrophysical measurements in reservoir
engineering calculation, Miskolc