In water based mud, you can not only estimate the maximum surface casing pressure, but you are also be able to determine the maximum pit gain due to gas influx.
The following formula demonstrates how to figure out the maximum pit gain from gas influx in water based mud system.
Maximum Pit Gain in bbl
P is formation pressure in psi.
V is original pit gain in bbl.
C is annular capacity in bbl/ft.
Kill Weight Mud in ppg
Let’s take a look at this following example in order to get more understanding regarding this topic.
Drill well with water based mud.
Pit gain = 20 bbl
Initial shut in casing pressure = 600 psi
Initial shut in drill pipe pressure = 500 psi
Current mud weight = 12.5 ppg
Hole depth = 6,000’MD/4,800’TVD
Hole diameter = 12-1/4 inch
Drill pipe = 5 inch
According to the data, you need to figure out the Kill Mud Weight , formation pressure, and annular capacity.
Kill Weight Mud = current mud weight + (shut in drill pipe pressure ÷ (0.052 x TVD))
Kill Weight Mud = 12.5 + (500 ÷ (0.052 x 4800)) = 14.5 ppg
Note: Please read the following link about kill weight mud -> well control formulas
Formation pressure = surface pressure + hydrostatic pressure
Formation pressure = 500 + (0.052 x 12.5 x 4800) = 3620 psi
Note: Please read the following link about formation pressure -> Formation Pressure from Kick Analysis
Determine annular capacity: Annular capacity = (12.252 – 52) ÷ 1029.4 = 0.1215 bbl/ft
Note: if you would like to understand annular capacity, please check out the following links -> Annular Capacity
Once you get all parameters required, you can add all of them into the equation like this.
Maximum Pit Gain = 98.5 bbl