There are some well control formulas that you need to know for wait and weight method.
1. Kill Weight Mud
KWM = OMW + [SIDPP ÷ (0.052 x TVD)]
Where;
KWM is kill weight mud in ppg.
OMW is original mud weight in ppg.
SIDPP is shut in drill pipe pressure in psi.
TVD is true vertical depth of the well in ft.
Note: Read more about it>> Kill Weight Mud
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.
Where
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
References
Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.
Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.
Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.
When a well is shut in due to well control operation, the casing pressure will increase due to gas migration and gas expansion. In water based mud, you are able to estimate the maximum surface pressure with this following formula.
Where;
Max surface pressure in psi.
P is expected formation pressure in psi.
V is pit volume gain in bbl.
KWM is kill weight mud in ppg.
An is an annular capacity in bbl/ft.
This is drilling formula calculation sheet v1 distributed on 28-Mar-10. I add many essential drilling formulas in to one Excel spread sheet and I would like to share with you for FREE. Please check this following link for download.
When you one the file Drilling-formulas-Calculation-shee-V1.xls, you will see the fist page call index page which shows you all of formulas like this.
Migration is natural behavior of gas. It means that when you shut the well in, gas will migrate from bottom to top of the well.
We can estimate the gas migration rate in a shut-in well with this following equation.
