## Why Slug Does Not Work?

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.

What is slug mud? How much volume and weight of slug mud should be?

Barrels of slug required for desired length of dry pipe

## What is slug mud? How much volume and weight of slug mud should be?

Slug Mud: It is heavy mud which is used to push lighter mud weight down before pulling drill pipe out of hole. Slug is used when pipe became wet while pulling out of hole.

Normally, 1.5 to 2 PPG over current mud weight is a rule of thumb to decide how much weight of slug should be. For example, current mud weight is 10 PPG. Slug weight should be about 11.5 to 12 PPG.

Normally, slug is pumped to push mud down approximate 200 ft (+/2 stands) and slug volume can be calculated by applying a concept of U-tube (see a figure below)

Volume of slug can be calculated by this following equation:

This equation expresses that the higher slug volume, the deeper of dry in drill pipe is met. As per the above equation, length of dry pipe can be substituted by 200 ft.

In normal practice, slug volume pumped to clean drill pipe is around 15-25 bbl depending on drillpipe size. Moreover, it also depends on situations because sometime mud in annulus side may be heavier than measured MW due to cutting, drilling solid contaminated in mud, hence more slug volume is needed.

Ref books:

Lapeyrouse, N.J., 2002. Formulas and calculations for drilling, production and workover, Boston: Gulf Professional publishing.

Bourgoyne, A.J.T., Chenevert , M.E. & Millheim, K.K., 1986. SPE Textbook Series, Volume 2: Applied Drilling Engineering, Society of Petroleum Engineers.

Mitchell, R.F., Miska, S. & Aadny, B.S., 2011. Fundamentals of drilling engineering, Richardson, TX: Society of Petroleum Engineers.