You sometimes need to reduce your mud weight in hole. The method to reduce mud weight is dilution. The concept of dilution is to add light weight fluid into heavy fluid and when they mix together whole drilling fluid weight will decrease. The light weight fluid frequently used are fresh water, sea water, base oil, diesel, synthetic oil, etc which are normally your based fluid. You can calculate how much light weight fluid in barrel required reducing original mud weight to new mud weight with this following formula:
Mud weight reduction by diluting with light weight fluid
Light weight fluid volume in bbl = V1 x (W1 – W2) ÷ (W2 – Dw)
Waar;
V1 = Strating volume in bbl
W1 = Start drilling fluid weight in ppg
W2 = Final drilling fluid weight in ppg
Dw = Density of light weight fluid in ppg
Voorbeeld: Determine the number of barrels of base oil 7.2 ppg (required to reduce 200 BBL (V1) of 13.8 ppg (W1) to 10.0 ppg (W2):
Light weight fluid volume in bbl = V1 x (W1 – W2) ÷ (W2 – Dw)
Light weight fluid volume in bbl = 200 x (13.8 – 10) ÷ (10 - 7.2)
Light weight fluid volume in bbl = 271.4 BBL
You need to mix 271.4 bbl of base oil 7.2 ppg into 200 bbl of 13.8 ppg in order to achieve final mud weight of 10.0 PPG
Please find the Excel Sheet to calculate mud dilution.
Ref boek: 
Formulas and Calculations for Drilling, Produksie en Workover, Tweede uitgawe
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