Water
Saturation SENSITIVITY ANALYSIS
Water saturation varies with many parameters, such as shale volume,
porosity, rock resistivity, water resistivity, and the electrical
properties of the rock. Many arguments occur because one analyst
has used slightly different parameters or method than another.
Most of these arguments are futile and needless as the end result,
namely oil-in-place, is often relatively insensitive to the water
resistivity and electrical properties, but is usually very sensitive
to the shale volume and porosity. A reasonable sensitivity analysis
can be run in an hour or so with a spreadsheet or computer aided
log analysis program.
Even
more sophisticated Monte Carlo statistical modeling can be performed
in a spreadsheet. But remember, the final test is the sensitivity of
hydrocarbon volume, not water saturation. A 50% error in SW, when SW
= 20%, is pretty trivial in terms of oil volume, when you consider
all the other parameters. A 50% error in SW, when SW = 60%, is a
little more critical - now the possibility of zero hydrocarbon
volume is quite real and some calibration with ground truth is
appropriate.
Below
are three such studies based on reasonable assumptions from three
different reservoir types. Note that the comparison is based on
hydrocarbon pore volume (HPV) and not on water saturation (SW).
SHALLOW
TAR SAND - SENSITIVITY ANALYSIS |
|
|
|
|
|
|
|
|
|
|
|
Hydrocarbon
Pore Volume |
|
A |
M |
N |
RW@25C |
RSH |
Vsh |
PHIe |
Change
From Pore Volume |
|
|
|
|
|
|
|
|
RESD=20 |
RESD=80 |
RESD=180 |
|
|
|
|
|
|
|
|
|
|
|
BASE |
1.00 |
2.00 |
2.00 |
0.75 |
4.00 |
0.00 |
0.35 |
0% |
0% |
0% |
2 |
1.00 |
2.00 |
2.00 |
1.00 |
4.00 |
0.00 |
0.35 |
-26% |
-7% |
-4% |
3 |
1.00 |
2.00 |
2.00 |
0.50 |
4.00 |
0.00 |
0.35 |
31% |
8% |
5% |
4 |
1.00 |
2.00 |
1.80 |
0.75 |
4.00 |
0.00 |
0.35 |
8% |
6% |
4% |
5 |
0.62 |
2.15 |
2.00 |
0.75 |
4.00 |
0.00 |
0.35 |
25% |
7% |
4% |
6 |
1.00 |
2.00 |
2.00 |
0.75 |
4.00 |
0.13 |
0.31 |
10% |
1% |
-2% |
7 |
1.00 |
2.00 |
2.00 |
0.75 |
8.00 |
0.13 |
0.31 |
-4% |
-6% |
-6% |
8 |
1.00 |
2.00 |
2.00 |
0.75 |
3.00 |
0.13 |
0.31 |
27% |
8% |
3% |
9 |
1.00 |
2.00 |
2.00 |
0.75 |
4.00 |
0.26 |
0.27 |
6% |
-10% |
-14% |
10 |
1.00 |
2.00 |
2.00 |
0.75 |
8.00 |
0.26 |
0.27 |
-17% |
-18% |
-19% |
11 |
1.00 |
2.00 |
2.00 |
0.75 |
3.00 |
0.26 |
0.27 |
18% |
-6% |
-13% |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
TAR
MASS |
|
|
|
M |
N |
RW@25C |
RSH |
Vsh |
PHIe |
Change
From Base Case |
|
|
|
|
|
|
|
|
RESD=20 |
RESD=80 |
RESD=180 |
|
|
|
|
|
|
|
|
|
|
|
BASE |
1.00 |
2.00 |
2.00 |
0.75 |
4.00 |
0.00 |
0.35 |
0% |
0% |
0% |
2 |
1.00 |
2.00 |
2.00 |
1.00 |
4.00 |
0.00 |
0.35 |
-25% |
-6% |
-4% |
3 |
1.00 |
2.00 |
2.00 |
0.50 |
4.00 |
0.00 |
0.35 |
28% |
7% |
4% |
4 |
1.00 |
2.00 |
1.80 |
0.75 |
4.00 |
0.00 |
0.35 |
8% |
5% |
4% |
5 |
0.62 |
2.15 |
2.00 |
0.75 |
4.00 |
0.00 |
0.35 |
23% |
6% |
3% |
6 |
1.00 |
2.00 |
2.00 |
0.75 |
4.00 |
0.13 |
0.31 |
6% |
-3% |
-5% |
7 |
1.00 |
2.00 |
2.00 |
0.75 |
8.00 |
0.13 |
0.31 |
-7% |
-8% |
-9% |
8 |
1.00 |
2.00 |
2.00 |
0.75 |
3.00 |
0.13 |
0.31 |
21% |
3% |
-1% |
9 |
1.00 |
2.00 |
2.00 |
0.75 |
4.00 |
0.26 |
0.27 |
-3% |
-16% |
-19% |
10 |
1.00 |
2.00 |
2.00 |
0.75 |
8.00 |
0.26 |
0.27 |
-23% |
-23% |
-23% |
11 |
1.00 |
2.00 |
2.00 |
0.75 |
3.00 |
0.26 |
0.27 |
7% |
-13% |
-18% |
|
|
|
|
|
|
|
|
|
|
|
The
sensitivity study shown above indicates that RW has a strong effect
when formation resistivity is low, but much less effect at high
formation resistivity. You should only argue about water resistivity
when it matters. Lowering M or N always gains some oil or gas,
regardless of rock resistivity. Changing A and M matters a lot
at low rock resistivity but not much is gained at high resistivity.
Adding shale almost always lowers porosity so HPV usually drops
unless RSH is very low AND RESD is also low, in which case HPV
can rise dramatically over the base case with no shale.
Although
most of these effects are fairly intuitive, the absolute value
of the change is not easy to predict unless a numerical sensitivity
study is run. If you are forced to argue for one set of parameters
over another, always run the sensitivity to see what matters and
what doesn’t.
MEDIUM
DEPTH - SENSITIVITY ANALYSIS |
|
|
|
|
|
|
|
|
|
|
|
Hydrocarbon
Pore Volume |
|
A |
M |
N |
RW@25C |
RSH |
Vsh |
PHIe |
Change
From Pore Volume |
|
|
|
|
|
|
|
|
RESD=20 |
RESD=80 |
RESD=180 |
|
|
|
|
|
|
|
|
|
|
|
BASE |
1.00 |
2.00 |
2.00 |
0.27 |
4.00 |
0.00 |
0.250 |
0% |
0% |
0% |
2 |
1.00 |
2.00 |
2.00 |
0.36 |
4.00 |
0.00 |
0.250 |
-9% |
-3% |
-2% |
3 |
1.00 |
2.00 |
2.00 |
0.18 |
4.00 |
0.00 |
0.250 |
11% |
4% |
3% |
4 |
1.00 |
2.00 |
1.80 |
0.27 |
4.00 |
0.00 |
0.250 |
6% |
4% |
3% |
5 |
0.62 |
2.15 |
2.00 |
0.27 |
4.00 |
0.00 |
0.250 |
7% |
3% |
2% |
6 |
1.00 |
2.00 |
2.00 |
0.27 |
4.00 |
0.13 |
0.225 |
-21% |
-18% |
-18% |
7 |
1.00 |
2.00 |
2.00 |
0.27 |
8.00 |
0.13 |
0.225 |
-25% |
-21% |
-20% |
8 |
1.00 |
2.00 |
2.00 |
0.27 |
3.00 |
0.13 |
0.225 |
-19% |
-17% |
-17% |
9 |
1.00 |
2.00 |
2.00 |
0.27 |
4.00 |
0.26 |
0.200 |
-42% |
-38% |
-37% |
10 |
1.00 |
2.00 |
2.00 |
0.27 |
8.00 |
0.26 |
0.200 |
-49% |
-41% |
-40% |
11 |
1.00 |
2.00 |
2.00 |
0.27 |
3.00 |
0.26 |
0.200 |
-38% |
-36% |
-36% |
|
|
|
|
|
|
|
|
|
|
|
In
the medium depth cases where porosity and RW are lower, changing
the RW or the electrical properties has very little effect unless
RESD is low. In the shaly cases, RSH has only a small effect,
even when RESD is relatively low. The lower porosity due to shaliness
has the largest effect, so optimizing shale parameters is important.
DEEP
RESERVOIR - SENSITIVITY ANALYSIS |
|
|
|
|
|
|
|
|
|
|
|
Hydrocarbon
Pore Volume |
|
A |
M |
N |
RW@25C |
RSH |
Vsh |
PHIe |
Change
From Pore Volume |
|
|
|
|
|
|
|
|
RESD=20 |
RESD=80 |
RESD=180 |
|
|
|
|
|
|
|
|
|
|
|
BASE |
1.00 |
2.00 |
2.00 |
0.03 |
4.00 |
0.00 |
0.150 |
0% |
0% |
0% |
2 |
1.00 |
2.00 |
2.00 |
0.04 |
4.00 |
0.00 |
0.150 |
-3% |
-2% |
-1% |
3 |
1.00 |
2.00 |
2.00 |
0.02 |
4.00 |
0.00 |
0.150 |
4% |
2% |
1% |
4 |
1.00 |
2.00 |
1.80 |
0.03 |
4.00 |
0.00 |
0.150 |
4% |
2% |
2% |
5 |
0.62 |
2.15 |
2.00 |
0.03 |
4.00 |
0.00 |
0.150 |
2% |
1% |
1% |
6 |
1.00 |
2.00 |
2.00 |
0.03 |
4.00 |
0.13 |
0.115 |
-24% |
-22% |
-22% |
7 |
1.00 |
2.00 |
2.00 |
0.03 |
8.00 |
0.13 |
0.115 |
-25% |
-23% |
-22% |
8 |
1.00 |
2.00 |
2.00 |
0.03 |
3.00 |
0.13 |
0.115 |
-24% |
-22% |
-22% |
9 |
1.00 |
2.00 |
2.00 |
0.03 |
4.00 |
0.26 |
0.083 |
-48% |
-44% |
-44% |
10 |
1.00 |
2.00 |
2.00 |
0.03 |
8.00 |
0.26 |
0.083 |
-49% |
-45% |
-44% |
11 |
1.00 |
2.00 |
2.00 |
0.03 |
3.00 |
0.26 |
0.083 |
-47% |
-44% |
-43% |
|
|
|
|
|
|
|
|
|
|
|
In
the deeper, lower porosity, lower RW case, changing RW or electrical
properties is noise level stuff. Likewise, changing RSH in the
shaly cases has no significant effect. Everything depends on porosity
and a sensitivity study on porosity parameters would be a useful
tool.
Clearly,
larger changes in parameters are possible, and additive effects
must be considered. Run the sensitivity with a realistic range
of values and then use good judgment. Don’t argue unless
it really matters and you have the backup data to support your
position.
You
will need good water analysis data and useful special core analysis
electrical properties data to bound your study. However, remember
that special core studies are not perfect and some are useless
because they do not reflect all the rock types present in the
reservoir.
|