Mud weight or mud density is a weight of mud per unit volume. It is one of the most important drilling fluid properties because it controls formation pressure and it also helps wellbore stability. Mud weight is measured and reported in pounds per gallon (PPG), pounds per cubic feet (lb/ft3), or grams per milliliter (b/ml).
Mud weight is normally measured by a conventional mud balance; however, if you have some air inside a fluid phase, reading from the conventional mud balance will give you an inaccurate number. Therefore, the most accurate method to measure the mud weight is with a pressurized mud balance.
The pressurized mud balance looks like the convention one, but it has a pressurized sample cup. When you press a mud sample into the cup, any gas in a fluid phase is compressed to a very small volume so the mud weight measurement is more accurate.
What will happen if there is insufficient drilling fluid density?
The well will be in an underbalanced condition so any formation of fluids – gas, oil, and water- will enter into the wellbore.
Wellbore collapse (wellbore instability)
Mud weight will provide pressure to hole back formation. If mud weight is too small, wellbore may collapse.
What will happen if the mud weight is too high?
If the hydrostatic pressure from mud column exceeds formation strength, it will cause a formation to break. Once the formation is broken, drilling fluids will be lost into the induced formation fractures.
Decrease in rate of penetration
Heavier mud weight will result in slower ROP because of hold down effect. Practically, while drilling, low mud weight is used at the beginning and mud weight will be increased, as the well is drilled deeper in order to optimize ROP and mitigate well control.
Deferentially Stuck Pipe
Since there are differences between formation pressure and hydrostatic pressure, there will be a lot of chances that a drill string will get deferentially stuck across permeable rocks.
The more mud weight that is in the well, the more mud filtration invades into porous formations. The invaded mud will cause damage to formation rocks.
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