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.
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 – 10.5 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
Please find the Excel sheet for calculating barrels of slug required for desired length of dry pipe.
Ref book: Formulas and Calculations for drill, production and workover by Norton J. Lapeyrouse
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