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 Vsh PHIe Sw PHINSH PHIDSH DENSMA HPV CHANGE IN Frac frac frac PV HPV 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 0.13 0.308 0.35 0.30 0.12 2646 0.201 0.13 0.308 0.13 0.30 0.12 2646 0.267 0.26 0.266 0.32 0.30 0.12 2646 0.180 0.26 0.266 0.11 0.30 0.12 2646 0.238 0.00 0.254 0.52 0.30 0.12 2646 0.123 0.00 0.254 0.23 0.30 0.12 2646 0.195 0.13 0.211 0.48 0.30 0.12 2646 0.110 0.13 0.211 0.17 0.30 0.12 2646 0.175 0.26 0.168 0.44 0.30 0.12 2646 0.094 0.26 0.168 0.13 0.30 0.12 2646 0.147 0.00 0.148 0.89 0.30 0.12 2646 0.017 0.00 0.148 0.40 0.30 0.12 2646 0.089 0.13 0.100 0.83 0.30 0.12 2646 0.017 0.13 0.100 0.25 0.30 0.12 2646 0.076 0.26 0.054 0.69 0.30 0.12 2646 0.017 0.26 0.054 0.15 0.30 0.12 2646 0.046 LOWER PHIDSH 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% HIGHER DENSMA PLUS LOWER PHIDSH 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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