In Part 1 of this tutorial in the April 2015 issue of TLE, we loaded some logs and used a data framework called Pandas to manage them. We made a lithology-fluid-class (LFC) log and used it to color a crossplot. This month, we take the workflow further with fluid-replacement modeling based on Gassmann's equation. This is just an introduction; see Wang (2001) and Smith et al. (2003) for comprehensive overviews.