Foundation, Concrete and Earthquake Engineering

Mud Density and Its Importances for The Drilling Operation

Mud weight or mud density is one of the important drilling fluid properties because it balances and controls formation pressure. Moreover, it also helps wellbore stability. Weight of drilling mud is measured and reported in pounds per gallon (PPG), pound per cubic feet (lb/cu.ft), or grams per milliliter (b/ml).

Mud density 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 mud sample in the cup, any gas in fluid phase is compressed to very small volume so the mud weight measurement is more accurate.

What will be happened if there is insufficient drilling fluid density?
1. Well control - The well will be in an under balance condition so any formation fluids - gas, oil, and water- will enter into the wellbore.

2. Wellbore collapse (wellbore instability) - the wellbore will possibly become unstable, if the hydrostatic pressure provided by a mud column is below formation pressure.

What will be happened if the mud weight is too high?

1. Lost circulation - If the hydrostatic pressure from mud column exceeds formation strength, it will cause formation to break. Once the formation is broken, the drilling fluids will lose into the induced formation fractures.

2. Decrease in rate of penetration - The more density you have while drilling, the less ROP will be. Practically, while drilling, low mud weight is used at the beginning and weight will be increased as the well is drilled deeper in order to optimize ROP.

3. Stuck pipe - Since there are differences between the formation pressure and the hydrostatic pressure, there will be a lot of chances that a drill string will get differentially stuck across permeable rocks.

4. Formation damage - The more mud weight 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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