Calculating Rw from the SP Curve
If you have fairly fresh mud and a good SP response, in a thick zone with low resistivity (i.e.--it looks wet), then you can try using this program to calculate the formation water resistivity, or Rw.
In less than ideal cases, where you have a salty mud or poor SP response, use a different method. Find an adjacent set of logs recorded under better conditions, check with a well logging company (they should have tables of water resistivities for different formations and regions), or try using logs from multiple wells on a Pickett plot. You can also estimate an Rw value by performing water saturation calculations on obviously wet zones (with good porosity and extremely low resistivity) in different wells, to see the value of Rw required to make the zones calculate 100% wet. Of course you also have to assume that all of your zone correlations are accurate, that the formation retains the same mineralogy, and that Rw doesn't change.
DATA LOGGING
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.