When you get gas-cut into your drilling mud, it will reduce mud weight causing hydrostatic pressure decrease. The concept of this calculation is to calculate pressure based on pit gain. You can calculate hydrostatic pressure loss with this following equation.
The following equation is used for calculation:
Hydrostatic pressure decrease = (Mud gradient ÷ Annular Capacity) x pit volume gain
Hydrostatic pressure decrease in psi
Mud gradient in psi/ft
Annular Capacity in bbl/ft
pit volume gain in bbl
Note: if you would like to understand about mud gradient and annular capacity, please check out the following links:
Please follow this example for more understanding.
Mud weight = 13.0 ppg
BHA diameter = 6 inch
Hole diameter = 8.5 inch
Pit volume gain = 15 bbl
Mud gradient = 0.052 x mud weight = 0.052 x 13 = 0.676 psi/ft
Annular capacity = (8.52 – 62) ÷ 1029.4 = 0.0352 bbl/ft
Hydrostatic pressure decrease = (0.676 ÷ 0.0352) x 15 = 288 psi.