In last month’s column, we started a look at mixed-metal hydroxide (MMH) fluids. That column looked at the different types, proper mixing, and started talking about ways to evaluate it. Refer to that issue (A Primer on Mixed-Metal Fluid Systems, February 2020) for the first part of the discussion that continues here. This column goes over a few more ways to evaluate MMH fluids, and touches on troubleshooting when contractors don’t get the results they want.
Last month, I mentioned that the Fann rheometer is not much use in evaluating a mixed-metal type slurry. This statement does not tell the whole story though. The truth is that for most people (especially in the field), the Fann will not be of much use. Mixed-metal fluids behave strangely in traditional rheometers. It is nearly impossible to get a stable reading in a well-designed M-M fluid because the values at each speed will continue to drop for several minutes until the data tells you that your fluid is much, much thinner than what you see with your own eyes. This “dropping” effect is most pronounced at 3 and 6 rpms, and again at 300 and 600 rpms. I don’t know why V100 and V200 don’t show the same decreases.