Calculating formation porosity from a Microlog
To get porosity from a microlog, you back-calculate it from an assumed formation model (a set of variables for Archie's equation, sw=sqrt((a/porosity^m)*Rw/Rt). The scheme used here relies upon an empirical relationship between Rm and the 2-inch normal curve, which seems to give better answers in practice than other approaches which employ mud-cake resistivity (always difficult to measure). Be careful taking quantitative answers from micrologs unless you have newer porosity tools from offset wells, core data, etc., for comparison.
The type correction which this program applies is also empirical--it adds 6% to the Humble model porosity for tools that used hydraulic pads instead of solid pads. This change propagates through the Archie and Optional model calculations. Sometimes the engineer was kind enough to list an "H" or an "S" under "Pad type" on the log's header, sometimes not. Solid pads came first, so you can go by age if there is no other indication: a microlog recorded in the mid-1950s or earlier probably used solid pads.
My experience has been that hydraulic tools give porosities an AVERAGE of 6% high, but this is based on the midcontinent USA. Your mileage may vary. If the 6% correction doesn't seem appropriate in your part of the world, choose the solid pad tool option and then develop local field correlations for how many porosity units to add to the final answer.
Micrologs can be used quantitatively with a bit of care. Try to calibrate them to better porosity-measuring tools, like neutron-density logs. When comparing micrologs look for similar-vintage tools, run by the same logging company, using the same pad configurations.
Do you want to save your calculations? The input box at the very bottom of the screen records all the inputs and outputs for each calculation run. To save this information, select all the text in the box and copy it, then open a spreadsheet and paste it in as comma-separated values. Each data type will land in its own column, and each calculation run, or depth, will occupy a row. Format the spreadsheet to separate rows into different geologic formations, and you're done. Isn't that easier than writing everything down?
Don't have a spreadsheet handy? If you are working on a phone or a tablet, you can still copy the text and paste it into a note or an email.
The Recording box will reset if you press the "Help" or "Reset" buttons, or if you navigate to a different page.