DrillX
Well control · field units
Kill sheet calculations, step by step
A kill sheet turns four shut-in readings into a pump schedule. From the shut-in drill pipe pressure (SIDPP), true vertical depth (TVD), original mud weight and the slow circulating rate pressure, you calculate kill mud weight, initial circulating pressure (ICP), final circulating pressure (FCP) and the drill pipe pressure step-down while kill mud travels to the bit. This page sets out each formula in oilfield units (psi, ppg, ft, bbl) with one worked example, and explains how the Driller's method and the Wait & Weight method use the same numbers differently.
Reference material only. Well-control operations must follow your company's approved procedures, the rig's own kill sheet, and the training standards of IADC WellSharp or IWCF. DrillX calculators are advisory.
1. The readings you start with
- SIDPP: shut-in drill pipe pressure, read once pressures stabilise after shutting the well in.
- SICP: shut-in casing pressure. It is normally higher than SIDPP because the influx in the annulus is lighter than mud.
- TVD: true vertical depth of the bit (use TVD, not measured depth, for every pressure calculation).
- OMW: original mud weight in the hole, in ppg.
- SCR pressure: slow circulating rate pressure, recorded before the kick at a reduced pump rate (for example 30 strokes per minute).
- Drill string volume and pump output: to convert volume into strokes from surface to bit (see drill pipe capacity and displacement).
2. The core formulas
Hydrostatic pressure
HP (psi) = 0.052 × MW (ppg) × TVD (ft)
Formation pressure
FP (psi) = SIDPP + 0.052 × OMW × TVD
Kill mud weight
KMW (ppg) = OMW + SIDPP ÷ (0.052 × TVD)
Initial circulating pressure
ICP (psi) = SCR pressure + SIDPP
Final circulating pressure
FCP (psi) = SCR pressure × KMW ÷ OMW
Strokes surface to bit
Strokes = drill string volume (bbl) ÷ pump output (bbl/stroke)
Pressure step-down
psi per 100 strokes = (ICP − FCP) ÷ (strokes to bit ÷ 100)
MAASP
MAASP (psi) = (LOT EMW − MW in hole) × 0.052 × shoe TVD
The constant 0.052 converts pounds per gallon to a pressure gradient in psi per foot. Kill mud weight is usually rounded up to the next 0.1 ppg; some operators add a trip or safety margin on top, which is a company-policy decision and is not part of the basic formula.
3. Worked example
Assume a vertical well shut in after a kick, with these readings:
| Input | Value |
|---|---|
| Bit TVD | 10,000 ft |
| Original mud weight (OMW) | 10.0 ppg |
| SIDPP | 520 psi |
| SCR pressure at 30 spm | 600 psi |
| Drill string volume | 140 bbl |
| Pump output | 0.100 bbl/stroke |
| Leak-off test EMW at shoe | 14.0 ppg |
| Casing shoe TVD | 5,000 ft |
| Result | Working | Answer |
|---|---|---|
| Formation pressure | 520 + 0.052 × 10.0 × 10,000 | 5,720 psi |
| Kill mud weight | 10.0 + 520 ÷ (0.052 × 10,000) | 11.0 ppg |
| Check: KMW hydrostatic | 0.052 × 11.0 × 10,000 | 5,720 psi (balances FP) |
| ICP | 600 + 520 | 1,120 psi |
| FCP | 600 × 11.0 ÷ 10.0 | 660 psi |
| Strokes to bit | 140 ÷ 0.100 | 1,400 strokes |
| Step-down | (1,120 − 660) ÷ 14 | ≈ 33 psi per 100 strokes |
| MAASP, original mud | (14.0 − 10.0) × 0.052 × 5,000 | 1,040 psi |
| MAASP, kill mud | (14.0 − 11.0) × 0.052 × 5,000 | 780 psi |
Notice that MAASP falls once heavier mud fills the annulus above the shoe. A kill sheet should show both values so the crew knows the casing pressure limit at each stage.
4. Driller's method vs Wait & Weight
Both are constant bottom-hole pressure methods: the aim is to hold bottom-hole pressure slightly above formation pressure while the influx is circulated out. They use the same ICP, FCP and KMW; the difference is when kill mud goes in.
Driller's method (two circulations)
First circulation: pump the influx out with the original mud, holding drill pipe pressure at ICP at the kill rate. Second circulation: pump kill mud. While it travels from surface to bit, hold casing pressure constant; once kill mud reaches the bit, hold drill pipe pressure at FCP until kill mud returns to surface.
Wait & Weight (one circulation)
Weight up the pits to KMW first, then circulate once. Drill pipe pressure is stepped down from ICP to FCP on the schedule while kill mud travels to the bit (about 33 psi per 100 strokes in the example), then held at FCP until kill mud returns.
The Driller's method can start circulating immediately and needs simpler arithmetic. Wait & Weight takes one circulation instead of two and generally produces lower casing-shoe pressure when the influx reaches the shoe, at the cost of waiting while mud is weighted up. Which method is used is set by the operator's well-control policy, not by the calculator.
5. Mistakes that kill sheets catch
- Using measured depth instead of TVD in a deviated well. Every 0.052 × MW × depth term needs TVD.
- Using an SCR pressure recorded at a different pump rate from the planned kill rate.
- Forgetting to recalculate MAASP after mud weight changes.
- Rounding KMW down. Round up to the next 0.1 ppg.
- Treating a straight-line step-down as exact in a tapered or deviated string; complex wellbores need a step-down calculated section by section.
How DrillX helps
DrillX includes a Driller's and Wait & Weight kill sheet alongside hydrostatic pressure, ECD, annular pressure loss, MAASP from leak-off test, triplex pump output, annular capacity and velocity, and buoyancy and string weight calculators. Values update as you type, in field units, and in English or Arabic. For a kill mud weight lookup table, a deviated-well example and how to calculate kick tolerance, see kill mud weight and kick tolerance. DrillX is in final testing; email info@drillx.app for a demo or early access, or see what DrillX does.
Questions
How do you calculate kill mud weight?
Kill mud weight (ppg) equals the original mud weight plus SIDPP divided by 0.052 times the true vertical depth. For example, with 10.0 ppg mud, 520 psi SIDPP and 10,000 ft TVD, kill mud weight is 10.0 + 520 ÷ 520 = 11.0 ppg.
What is the difference between ICP and FCP?
Initial circulating pressure (ICP) is the slow circulating rate pressure plus SIDPP, and is the drill pipe pressure at the start of the kill. Final circulating pressure (FCP) is the slow circulating rate pressure multiplied by kill mud weight divided by original mud weight, and is held once kill mud reaches the bit.
Why is SICP higher than SIDPP?
Shut-in casing pressure is usually higher than shut-in drill pipe pressure because the annulus contains the influx, which is lighter than drilling mud, so the annulus has less hydrostatic pressure and needs more surface pressure to balance the same formation pressure.
Which is better, the Driller's method or Wait and Weight?
Neither is better in every case. The Driller's method starts circulating immediately and uses two circulations; the Wait and Weight method weights up first and kills the well in one circulation, usually with lower pressure at the casing shoe. The operator's well-control policy decides which is used.
Does MAASP change during a kill?
Yes. MAASP depends on the mud weight above the casing shoe, so it falls when heavier kill mud fills the annulus. In the example, MAASP drops from 1,040 psi with 10.0 ppg mud to 780 psi with 11.0 ppg mud.
Sources and further reading
- International Association of Drilling Contractors, WellSharp well-control training: iadc.org/wellsharp
- International Well Control Forum, well-control certification and standard kill sheets: iwcf.org
- American Petroleum Institute, RP 59, Recommended Practice for Well Control Operations: api.org