1. Basic Formulas

Table of Content

1. Pressure Gradient
2. Hydrostatic Pressure
3. Converting Pressure into Mud Weight
4. Specific Gravity
5. Equivalent Circulation Density
6. Maximum Allowable Mud Weight
7. Pump Output
8. Annular Velocity
9. Capacity Formulas
10. Control Drilling
11. Buoyancy Factor
12. Hydrostatic Pressure Decrease POOH
13. Loss of Overbalance Due to Falling Mud Level
14. Formation Temperature
15. Hydraulic Horsepower
16. Drill Pipe, Drill Collar Calculation
17. Pump Pressure, Pump Strokes
18. Cost Per Foot
19. Temperature Conversion Formulas

1.12 Hydrostatic Pressure (HP) Decrease When POOH

When pulling DRY pipe

Step 1
Burrels = number of stands pulled × average length per stand, ft × pipe displacement displaced, bbl/ft
Step 2
$\mathrm{HP\; psi\; decrease}=\frac{\mathrm{barrels\; displaced}}{(\mathrm{casing\; capacity,\; bbl/ft}-\mathrm{pipe\; displacement,\; bbl/ft})}\times 0.052\times \mathrm{mud\; weight,\; ppg}$

Example: Determine the hydrostatic pressure decrease when pulling DRY pipe out of the hole:
Number of stands pulled = 5, Pipe displacement = 0.0075 bbl/ft, Average length per stand = 92 ft, Casing capacity = 0.0773 bbl/ft, and, Mud weight = 11.5 ppg
Step 1
Barrels displaced = 5 stands × 92 ft/std × 0.0075 bbl/ft displaced
Barrels displaced = 3.45 bbl
Step 2
$\mathrm{HP\; psi\; decrease}=\frac{\mathrm{3.45\; bbl}}{(\mathrm{0.0773\; bbl/ft}-\mathrm{0.0075\; bbl/ft})}\times 0.052\times \mathrm{11.5\; ppg}$
$\mathrm{HP\; psi\; decrease}=\frac{\mathrm{3.45\; bbl}}{\mathrm{0.0698\; bbl/ft}}\times 0.052\times \mathrm{11.5\; ppg}$
HP decrease = 29.56 psi

When pulling WET pipe

Step 1
Burrels = number of stands pulled × average length per stand, ft × (pipe displacement, bbl/ft + pipe capacity, bbl/ft)
Step 2
$\mathrm{HP\; psi\; decrease}=\frac{\mathrm{barrels\; displaced}}{\mathrm{casing\; capacity,\; bbl/ft}-(\mathrm{pipe\; displacement,\; bbl/ft}+\mathrm{pipe\; capacity,\; bbl/ft})}\times 0.052\times \mathrm{mud\; weight,\; ppg}$

Example: Determine the hydrostatic pressure decrease when pulling WET pipe out of the hole:
Number of stands pulled = 5, Pipe displacement = 0.0075 bbl/ft, Average length per stand = 92 ft, Pipe capacity = 0.01776 bbl/ft, Mud weight = 11.5 ppg, and, Casing capacity = 0.0773 bbl/ft
Step 1
Barrels displaced = 5 stands × 92 ft/std × (0.0075 bbl/ft + 0.01776 bbl/ft)
Barrels displaced = 11 6196 bbl
Step 2
$\mathrm{HP\; psi\; decrease}=\frac{\mathrm{11.6196\; bbl}}{\mathrm{0.0773\; bbl/ft}-(\mathrm{0.0075\; bbl/ft}+\mathrm{0.01776\; bbl/ft})}\times 0.052\times \mathrm{11.5\; ppg}$
$\mathrm{HP\; psi\; decrease}=\frac{\mathrm{11.6196\; bbl}}{\mathrm{0.05204\; bbl/ft}}\times 0.052\times \mathrm{11.5\; ppg}$
HP decrease = 133.52 psi

⬆ Table of Content