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.
How do you know that?
Let’s do the math. We have total volume in the system (downhole volume and surface volume) of 700 bbl and we have total of 80 bbl of 15.00 pgg of slug.
The following formula is used to determine the final mud weight in the system.
(V1 x D1) + (V2 x D2) = VF x DF
V1 = volume of fluid 1 (bbl, gal, etc.)
D1 = density of fluid 1 (ppg,lb/ft3, etc.)
V2 = volume of fluid 2 (bbl, gal, etc.)
D2 = density of fluid 2 (ppg,lb/ft3, etc.)
VF = volume of final fluid mix
DF = density of final fluid mix
Note: You can read more details here => https://www.drillingformulas.com/mixing-fluids-of-different-densities-without-pit-space-limitation/
V1 = 80 bbl (Total volume of slug in the well)
D1 = 15.0 ppg (density of slug)
V2 = 700 bbl (Total system volume)
D2 = 13.0 ppg (density of drilling mud)
VF = V1 + V2 = 80 + 700 = 780
(80 x 15) + (700 x 13) = 780 x DF
DF = 13.2 ppg
From the equation above, it tells us that we have too much slug in the well which can increase 0.2 ppg of total mud weight. The increment of mud density will directly affect the equivalent circulating density. This is the reason why we have losses while circulating.
Conclusion: Always ensure that you don’t have a lot of slug in the system or you need to prepare to dilute to maintain the mud weight.