Calculating water saturation -- Tixier method
Tixier approached the problem of having only resistivity logs by estimating what percentage of formation water was not flushed out of the invasion zone. He generated a mixed-water resistivity using Rw and Rmf, and then used it, along with the deep and shallow resistivity values, to estimate water saturation.
The program will estimate Rmf for you, or let you enter it. Try it both ways, if you have the data, but keep in mind that field-measured Rmf values are often inaccurate.
Enter the percent of formation water remaining in the flushed zone, using as a guide the expected porosity range of the formation (see the chart of recommended values). Low-porosity formations usually have deep mud filtrate invasion, high-porosity zones shallow invasion. Try several different z% values to see how it affects the results.
After computing Sw, porosities are then back-calculated from it using the Archie, Humble, and an optional formation factor, if you specify both 'a' and 'm.'
Caveats:
You need an invaded zone broad enough for the short-spaced resistivity tool to see it, but narrow enough for the long-spaced resistivity tool not to see it. There needs to be a resistivity contrast between the mud filtrate and the formation fluid, which sometimes doesn't exist in oil and gas zones. The bed should also be thick enough for both curves to give valid readings. If both resistivity curves are slammed together then you probably have very deep invasion and this program can't help you!
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.