**What is slug**? Slug: 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. This article will demonstrate you how to calculate how many barrel of volume slug required for desired light of dry pipe.

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**. **Generally, slug is pumped to push mud down approximate 200 ft and slug volume can be calculated by applying a concept of U-tube (See Figure below).

**Volume of slug required for required length of dry pipe can be calculated by this following equations:**

**Step 1: **Determine hydrostatic pressure required to give desired drop inside drill pipe:

Hydrostatic Pressure in psi = mud weight in ppg x 0.052 x ft of dry pipe

**Step 2: **Determine difference in pressure gradient between slug weight and mud weight:

Pressure gradient difference in psi/ft = (slug weight in ppg – mud weight in ppg) x 0.052

**Step 3: **Determine length of slug in drill pipe:

Slug length in ft = Hydrostatic Pressure in psi (in step 1) ÷ Pressure gradient difference in psi/ft (step 2)

**Step 4: **Slug volume required in barrels:

Slug volume in barrel = Slug length in ft x drill pipe capacity in bbl/ft

**Example: **Determine the barrels of slug required for the following:

Desired length of dry pipe = 200 ft

Drill pipe capacity = 0.016 bbl/ft

Mud weight = 10.0 ppg

Slug weight = 11.5 ppg

**Step 1 Hydrostatic pressure required:**

Hydrostatic Prssure in psi = 10.0 ppg x 0.052 x 200 ft

Hydrostatic Prssure in psi = 104 psi

**Step 2 differences in pressure gradient between slug weight and mud weight:**

Pressure gradient difference in psi/ft = (11.5 ppg – 10.0 ppg) x 0.052

Pressure gradient difference in psi/ft = 0.078 psi/ft

**Step 3 length of slug in drill pipe:**

Slug length in ft = 104psi ÷ 0.078

Slug length in ft = 1,333 ft

**Step 4 Slug volume required in barrels:**

Slug volume required = 1333 ft x 0.016 bbl/ft

Slug volume required = 21.3 bbl

**Ref book: ** **Formulas and Calculations for Drilling, Production and Workover, Second Edition**

For what reason is slug mud used to produce dry pipe?

To keep drill pipe dry while pulling out of hole.

Can the slug keep the pipe dry if there is a mud motor in the BHA?

Yes. If you pump enough volume and weight, the slug will keep the pipe dry even though you have mud motor in the drill string.

thank you. how different is the calculation of slug volume if there is a PDM?

No different in calculation even though you have the slug in the PDM.

Which are the steps to pumping a slug? (the number of strokes)

You can follow the steps below for pumping slug.

1. Pump slug as per your volume required.

2. Pump volume of mud equal to surface line to chase up the slug

3. Let slug fall

4. Flow check to ensure that slug stops fall into the drill string.

how can know % of return when drilling ( mud i mean )

% return is tracked by flow show.

Hey, nice explanation, but at step 2, u used mud 10.5ppg instead of 10ppg.

Ty

Rafael,

Thanks for this. I correct my typo mistake already.

Regards,

Shyne.