Old Electric Log Technique

Old Electric Log Technique

There are many resistivity-only interpretation techniques dating from the dawn of well logging. All that they ask you to do is make a few guesses: about invasion profiles, the ratios of mud and mud filtrate resistivities to formation water resistivity, the expected permeability and porosity ranges for the formation, the degree of flushing that has occurred in the invaded zone, etc. Even armed with the appropriate correction charts and departure curves for the specific tools in use, errors due to these assumptions could propagate through your nomographs and calculations to smother the final answer with uncertainty.

If all you have is an old electric log, with an SP curve, and two resistivity curves, and no information on the log header about mud or mud filtrate resistivities, then you might try this program. It makes the following assumptions:

1) The short-spaced normal resistivity curve is measuring the flushed zone and hence is a proxy for the formation factor.

2) The long-spaced normal resistivity curve reads the uninvaded formation and is a proxy for true formation resistivity.

The program doubles the short normal reading to estimate the formation factor in uniformly porous and saturated formations. In high-porosity, shallow-invasion rocks, the first assumption will be off. In low-porosity, deep-invasion rocks the second assumption will be off.

Rw, the formation water resistivity, is either estimated or pre-calculated from the SP curve.

Having F, Rw, and Rt, the program calculates water saturation, Sw, and then back-calculates porosities by assuming different models. The program computes Archie and Humble porosities, or you can enter your own values for the correlation coefficient 'a' and the cementation factor 'm'.

As simple as this approach is, I've found it accurate enough for some non-shalely sandstones and intergranular-porosity carbonates in the midcontinent USA. Be sure that you pick resistivity points in beds that are thick enough to give a true resistivity (ideally thicker than the tool's electrode spacing), and compare the answers to offset wells logged with newer tools. Plot some data. You may find that the answers from this program are off, but that they vary in a linear relationship to the numbers that you get from better logging suites...which gives you enough information to develop your own fudge factors for old elogs run in specific formations and fields.

You can also try the Tixier program, or the Sw Ratio program, or go find yourself a decent porosity log.


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.