drilling · well control
Formation Pressure from Shut-In Drillpipe Pressure
Pf = 0.00980665 × MW × TVD + SIDPP
click formula to derive ↑
Inputs
lb/gal
ft
psi
Description
Calculates actual formation (pore) pressure from a stabilized shut-in drillpipe pressure reading after a kick — the standard first step in well control kick analysis, used to determine the kill mud weight needed to regain balance.
Variables
| Symbol | Unit | Description |
|---|---|---|
| Pf | psi | Formation Pressure |
| Pf_EMW | lb/gal | Formation Pressure (EMW) |
| MW | lb/gal | Mud weight in the hole at the time the well was shut in, before any weight-up. |
| TVD | ft | True vertical depth of the influx (kick) zone — commonly taken as bit depth at the time of the kick. |
| SIDPP | psi | Stabilized drillpipe pressure read after shutting in the well following a kick — read only once the gauge has stopped rising or falling. |
Assumptions
- SIDPP has fully stabilized before being read — a still-rising or falling gauge indicates the well has not yet reached true shut-in equilibrium
- The drillpipe is full of the original (pre-kick) mud with no significant gas migration into the drillstring
- TVD used is the depth of the influx zone (commonly assumed at or near bit depth at the time of the kick), not surface or measured depth
Limitations
- A U-tubing effect or plugged drillstring can produce a SIDPP reading that does not reflect true formation pressure — cross-check against shut-in casing pressure (SICP) for consistency
- Significant gas migration up the drillstring before shut-in raises the apparent SIDPP independent of actual formation pressure — read SIDPP as early as practical after shut-in
- Assumes a single-phase mud column above the kick zone in the drillstring, which is the basis for using SIDPP (rather than SICP) here
Use Cases
- → Post-kick well control response: Determine the actual formation pressure immediately after shutting in a well following a kick, as the first step in planning a kill.
- → Pore pressure calibration: Use a confirmed kick as a direct pressure data point to calibrate or validate pre-drill pore pressure predictions (e.g. from the Eaton method) for the remaining well.
- → Mud weight program revision: Feed the calculated formation EMW into the mud weight program to ensure the next hole section is drilled with adequate overbalance.
Related Calculations
Region Notes
Permian Basin
Wolfcamp/Bone Spring kicks are most often modest gas kicks from mildly overpressured intervals — SIDPP values of 200-800 psi are typical when caught early; values significantly above this should prompt a check for a deeper, more pressured zone than planned.
Haynesville
Severely overpressured Haynesville gas kicks can produce SIDPP in the thousands of psi — verify gauge readings and well control equipment ratings before proceeding with standard kill procedures.
Global
Always read SIDPP (and SICP) only after both gauges have stabilized for several minutes — premature readings are the most common source of an incorrect kill mud weight.
References
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