The Electrical Stability (ES) is one of the vital properties for oil based mud. It shows the voltage of the current flowing in the mud. The ES number represents the mud emulsion stability. The more ES is; the more the emulsion stability is.
Oil based fluid is a non-conductive material. Therefore, the base fluid will not transfer any current. Only the water phase in the mud will conduct the electricity. If the mud has good emulsion, you will have high ES figures. On the other hand, if the emulsion of the mud is bad, you will have low ES value.
The Electrical Stability (ES) is obtained from an electrical stability tester kit (see the image below).
Electrical Stability Meter
There are several factors that can weaken the emulsion, such as oil/water ratio, solid content, pressure, temperature, some types of weighting material, etc.
What the Electrical Stability (ES) will tell us?
If ES is lower than a normal mud specification, it indicates that there is something unusual in the mud such as water or salts, which will make emulsion of the oil based mud in bad shape. Moreover, the ES can be utilized to determine an interface between water and oil based mud while displacing water with oil based fluid.
For good drilling practices, it is required to frequently monitor the ES level and watch for any unusual changes. Changes in the ES can be seen while drilling into green cement or while adding any conductive material as stated earlier. These known factors affecting the ES must be noted in order to prevent any confusion when interpreting the mud’s property.
References
Andy Philips, 2012. So You Want to be a Mud Engineer: An Introduction to Drilling Fluids Technology. Edition. CreateSpace Independent Publishing Platform.
Ryen Caenn, 2011. Composition and Properties of Drilling and Completion Fluids, Sixth Edition. 6 Edition. Gulf Professional Publishing.
What is a good ES value?
ES value above 500 is called good based on my experience.
ES can be very misleading. Monitoring the quantity of water in the HTHP filtrate is a much more definitive test of emulsion strength than ES (Usually, any water in your HTHP filtrate is a bad sign). At least that is the case in diesel based muds. Simply using ES>500 as an indicator that your emulsion is in good shape can be very dangerous and could result in water wet barite.