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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**

Where:

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:

**Calculate mud gradient**

**Calculate annular capacity**

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 = (8.5^{2} – 6^{2}) ÷ 1029.4 = 0.0352 bbl/ft

Hydrostatic pressure decrease = (0.676 ÷ 0.0352) x 15 = 288 psi.

**Ref book: ** **Formulas and Calculations for Drilling, Production and Workover, Second Edition**

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why not the drill pipe diameter instead of the BHA diameter ? I think DP diameter would be more accurate in this case… any feedback is appreciated.

Typically, gas or any influx will influx into the annulus because most of BHA’s have float valves which will prevent the kick coming inside the drill string.