Category Archives: Basic Drilling Formulas

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 gain

Where:

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 = (8.5² – 6²) ÷ 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 gain

Where:

Hydrostatic pressure decrease in KPa

Mud gradient in KPa/m

Annular Capacity in m³/m

pit volume gain in m³/m

Example

Mud weight = 1440 kg/m³

BHA diameter = 152  mm

Hole diameter = 216 mm

Pit volume gain = 2.4 m³

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

Annular capacity = (216² – 152²) ÷ 1,273,240= 0.0185 m³/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:

Calculate mud gradient

Calculate annular capacity

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.