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:

Oilfield Unit

Hydrostatic pressure decrease = (Mud gradient ÷ Annular Capacity) × pit volume gainWhere:

Hydrostatic pressure decrease in psi

Mud gradient in psi/ft

Annular Capacity in bbl/ft

pit volume gain in bbl

**Example**

Mud weight = 13.0 ppg

BHA diameter = 6 inch

Hole diameter = 8.5 inch

Pit volume gain = 15 bbl

Mud gradient = 0.052 × mud weight = 0.052 × 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) × 15 = 288 psi.

Metric Unit

Hydrostatic pressure decrease = (Mud gradient ÷ Annular Capacity) × pit volume gainWhere:

Hydrostatic pressure decrease in KPa

Mud gradient in KPa/m

Annular Capacity in m

^{3}/mpit volume gain in m

^{3}/m

**Example**

Mud weight = 1440 kg/m^{3}

BHA diameter = 152 mm

Hole diameter = 216 mm

Pit volume gain = 2.4 m^{3}

Mud gradient = 0.00981 × mud weight = 0.00981 × 1440 = 14.126 KPa/m

Annular capacity = (216^{2} – 152^{2}) ÷ 1,273,240= 0.0185 m^{3}/m

Hydrostatic pressure decrease = (14.126 ÷ 0.0185) × 2.4 = 1833 KPa

**Note: **if you would like to understand about mud gradient and annular capacity, please check out the following links:

**Download Free Excel Spread Sheet **

**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.

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.