Slug mud is typically pumped into the drill string in order to push the mud inside the drill pipe down so the drill pipe will be clean and ready for pulling out of hole. Since slug is heavier, it will push the lighter mud out of the well. Figure 1 demonstrates how slug displaces current mud out of hole.
Figure 1 – Diagram Shows Volume Gain from Slug
Slug is heavy drilling mud that is used to pump when you want to pull pipe dry. However, excessive volume of slug in the well can create higher mud weight and cause the problem. Today I would like to share my experience regarding slug in the well.
The situation happened on the drilling rig like this.
The well was TD with 13.0 ppg mud and circulation was conducted until the shale shakers. After that we pull 10 stands wet and pumped 40 bbl of 15.0 ppg slug and came out of hole without any problem. Then we run the logging and we got stuck. We picked the fishing gear and grabbed the fish. We pulled out 10 stands wet and hole was taking proper fill. We pumped same amount of slug 40 bbl of 15.0 ppg and came out of hole. The decision was made by town to trip in hole to do the wiper trip.
Tripping was done with caution and break circulation was conducted every 3000 ft to break the gel. Prior to TD 100 ft, we made up top drive and slowly washed down to TD. While we were circulating, the mud weight out was varied from 13.2 ppg to 14.2 ppg. Additionally, we start losing mud while circulating.
Do you know what went wrong?
The two slugs in the well cause us trouble.
Slug is typically used to push mud in the drill string down therefore pipe will dry while pulling out of hole. Dry pipe while pulling out has some advantages as minimizing crew to expose to drilling mud when breaking a connection, reducing time to handle the drill string, etc. Sometimes, even you already pump slug but you still have wet pipe instead of dry pipe. You may wonder why the slug does not work very well.
The following reasons why the slug does not do its job are as follows:
• Slug volume is not enough to slug the pipe. Recommended volume is around 25- 40 bbl.
• You should chase slug by pumping mud at least surface volume from mud pump to a rotary table. Otherwise, you will not get desired slug volume in the drillstring because it is still left in the surface volume.
• Weight of slug is not sufficient. As a normal practice, the slug weight should have at least 2 ppg over your current mud weight.
• There is something inside the drill string so the slug could not push mud in the drill string down.
If you want to learn more about slug, please read the following articles:
What is slug mud? How much volume and weight of slug mud should be?
Barrels of slug required for desired length of dry pipe