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.