POROSITY
Sensitivity Analysis
Porosity determined from a log analysis is sensitive to many factors,
such as shale volume, shale parameters, matrix rock parameters,
and hydrocarbon (gas) corrections. Borehole roughness and washouts
also play a role. Each mathematical model gives a different answer.
It is very unlikely that a perfect set of parameters could be
found that would make all methods give the same results.
Since the objective of log analysis is to obtain a
reasonable match to core analysis porosity, we often vary
one or more of the parameters listed above until this match
is achieved.
Sometimes it is worth using a spreadsheet program or a computer
aided log analysis package to run a sensitivity analysis to see
which combination of parameters might match core data with he
least difficulty. One such analysis is shown below for a variety
of rock properties. Note that PV stands for pore volume and HPV
for hydrocarbon pore volume.
From observation of the changes to PV and HPV relative to the
porosity and shale volume, you can see whether a change in parameters
has a significant effect. Although some changes create large percentage
changes in answers, bear in mind that a large change in a small
value is still pretty small.
Changing shale properties can have some surprising results, not
shown here. For example, reducing PHIDSH will increase porosity
slightly but only in shaly zones. If the VSHx and VSHmin shale
volume methods are turned on AND if VSHx becomes less than Vsh
from some other method that is turned on, porosity may increase
more dramatically. Also, such changes may create gas crossover,
again increasing porosity more than expected.
Increases in matrix density will always increase porosity and
may impose gas corrections that can also increase porosity more
than expected.
SENSITIVITY
ANALYSIS - SHALE AND MATRIX PARAMETERS |
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Vsh |
PHIe |
Sw |
PHINSH |
PHIDSH |
DENSMA |
HPV |
CHANGE
IN |
Frac |
frac |
frac |
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PV |
HPV |
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0.00 |
0.349 |
0.38 |
0.30 |
0.12 |
2646 |
0.217 |
BASE |
CASE |
0.00 |
0.349 |
0.17 |
0.30 |
0.12 |
2646 |
0.290 |
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0.13 |
0.308 |
0.35 |
0.30 |
0.12 |
2646 |
0.201 |
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0.13 |
0.308 |
0.13 |
0.30 |
0.12 |
2646 |
0.267 |
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0.26 |
0.266 |
0.32 |
0.30 |
0.12 |
2646 |
0.180 |
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0.26 |
0.266 |
0.11 |
0.30 |
0.12 |
2646 |
0.238 |
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0.00 |
0.254 |
0.52 |
0.30 |
0.12 |
2646 |
0.123 |
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0.00 |
0.254 |
0.23 |
0.30 |
0.12 |
2646 |
0.195 |
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0.13 |
0.211 |
0.48 |
0.30 |
0.12 |
2646 |
0.110 |
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0.13 |
0.211 |
0.17 |
0.30 |
0.12 |
2646 |
0.175 |
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0.26 |
0.168 |
0.44 |
0.30 |
0.12 |
2646 |
0.094 |
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0.26 |
0.168 |
0.13 |
0.30 |
0.12 |
2646 |
0.147 |
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0.00 |
0.148 |
0.89 |
0.30 |
0.12 |
2646 |
0.017 |
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0.00 |
0.148 |
0.40 |
0.30 |
0.12 |
2646 |
0.089 |
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0.13 |
0.100 |
0.83 |
0.30 |
0.12 |
2646 |
0.017 |
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0.13 |
0.100 |
0.25 |
0.30 |
0.12 |
2646 |
0.076 |
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0.26 |
0.054 |
0.69 |
0.30 |
0.12 |
2646 |
0.017 |
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0.26 |
0.054 |
0.15 |
0.30 |
0.12 |
2646 |
0.046 |
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LOWER
PHIDSH |
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0.00 |
0.349 |
0.38 |
0.30 |
0.00 |
2646 |
0.217 |
0% |
0% |
0.00 |
0.349 |
0.17 |
0.30 |
0.00 |
2646 |
0.290 |
0% |
0% |
0.13 |
0.313 |
0.34 |
0.30 |
0.00 |
2646 |
0.205 |
2% |
2% |
0.13 |
0.313 |
0.13 |
0.30 |
0.00 |
2646 |
0.272 |
2% |
2% |
0.26 |
0.266 |
0.32 |
0.30 |
0.00 |
2646 |
0.180 |
0% |
0% |
0.26 |
0.266 |
0.11 |
0.30 |
0.00 |
2646 |
0.238 |
0% |
0% |
0.00 |
0.254 |
0.52 |
0.30 |
0.00 |
2646 |
0.123 |
0% |
0% |
0.00 |
0.254 |
0.23 |
0.30 |
0.00 |
2646 |
0.195 |
0% |
0% |
0.13 |
0.222 |
0.46 |
0.30 |
0.00 |
2646 |
0.120 |
5% |
9% |
0.13 |
0.222 |
0.17 |
0.30 |
0.00 |
2646 |
0.185 |
5% |
6% |
0.26 |
0.192 |
0.40 |
0.30 |
0.00 |
2646 |
0.114 |
14% |
21% |
0.26 |
0.192 |
0.12 |
0.30 |
0.00 |
2646 |
0.169 |
14% |
15% |
0.00 |
0.148 |
0.89 |
0.30 |
0.00 |
2646 |
0.017 |
0% |
0% |
0.00 |
0.148 |
0.40 |
0.30 |
0.00 |
2646 |
0.089 |
0% |
0% |
0.13 |
0.113 |
0.77 |
0.30 |
0.00 |
2646 |
0.027 |
13% |
54% |
0.13 |
0.113 |
0.23 |
0.30 |
0.00 |
2646 |
0.087 |
13% |
14% |
0.26 |
0.079 |
0.62 |
0.30 |
0.00 |
2646 |
0.030 |
45% |
74% |
0.26 |
0.079 |
0.15 |
0.30 |
0.00 |
2646 |
0.067 |
45% |
46% |
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HIGHER
DENSMA PLUS LOWER PHIDSH |
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0.00 |
0.351 |
0.37 |
0.30 |
0.00 |
2740 |
0.219 |
1% |
1% |
0.00 |
0.351 |
0.17 |
0.30 |
0.00 |
2740 |
0.292 |
1% |
1% |
0.13 |
0.313 |
0.34 |
0.30 |
0.00 |
2740 |
0.205 |
2% |
2% |
0.13 |
0.313 |
0.13 |
0.30 |
0.00 |
2740 |
0.272 |
2% |
2% |
0.26 |
0.266 |
0.32 |
0.30 |
0.00 |
2740 |
0.180 |
0% |
0% |
0.26 |
0.266 |
0.11 |
0.30 |
0.00 |
2740 |
0.238 |
0% |
0% |
0.00 |
0.264 |
0.50 |
0.30 |
0.00 |
2740 |
0.133 |
4% |
8% |
0.00 |
0.264 |
0.22 |
0.30 |
0.00 |
2740 |
0.205 |
4% |
5% |
0.13 |
0.239 |
0.43 |
0.30 |
0.00 |
2740 |
0.135 |
13% |
23% |
0.13 |
0.239 |
0.16 |
0.30 |
0.00 |
2740 |
0.201 |
13% |
15% |
0.26 |
0.214 |
0.38 |
0.30 |
0.00 |
2740 |
0.134 |
27% |
42% |
0.26 |
0.214 |
0.12 |
0.30 |
0.00 |
2740 |
0.189 |
27% |
29% |
0.00 |
0.177 |
0.74 |
0.30 |
0.00 |
2740 |
0.046 |
20% |
177% |
0.00 |
0.177 |
0.33 |
0.30 |
0.00 |
2740 |
0.118 |
20% |
33% |
0.13 |
0.150 |
0.63 |
0.30 |
0.00 |
2740 |
0.056 |
49% |
223% |
0.13 |
0.150 |
0.21 |
0.30 |
0.00 |
2740 |
0.119 |
49% |
57% |
0.26 |
0.124 |
0.52 |
0.30 |
0.00 |
2740 |
0.059 |
127% |
247% |
0.26 |
0.124 |
0.14 |
0.30 |
0.00 |
2740 |
0.107 |
127% |
131% |
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