Adding bbl of drilling fluid can help control low gravity solid (LGS) in mud system. However, this is different from the way to control LGS by adding base fluid as base oil or water because mud that is added into system has some Low Gravity Solid (LGS). Hence, when we calculate it, we need to account for Low Gravity Solid (LGS) of new mud into the calculation as well. This post will demonstrate you how to determine barrels of drilling fluid required to achive the desired Low Gravity Solid (LGS).
Formula, used to calculate dilution of mud system, is listed below;
Vwm = Vm x (Fct – Fcop) ÷ (Fcop – Fca)
Where; Vwm = barrels of dilution water or base fluid
Vm = total barrels of mud in circulating system
Fct = percent low gravity solids in system
Fcop = percent total low gravity solids desired
Fca = percent low gravity solids bentonite and/or chemicals added in mud
Example: Determine how much barrels of oil base mud to diluate total 2000 bbl of mud in system from total LGS = 7 % to desired LGS of 3.5 %. The oil base mud has 2% of bentonite slurry.
Vwm = Vm x (Fct – Fcop) ÷ (Fcop – Fca)
Vwm = 2000 x (7 – 3.5) ÷ (3.5-2)
Vwm = 4667 bbl
In order to dilute total of 2000 bbl of the original mud with 7% LGS down to 3.5% LGS, 4667 bbl of mud that has 2% bentonite is requied to add into the system.
Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition
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