Drilling guides › How to fill in a kill sheet (wait and weight method)
How to fill in a kill sheet (wait and weight method)
After a kick is shut in, the kill sheet tells the crew what mud weight will balance the formation and what drill pipe pressure to hold while that mud is pumped down the string. This guide follows the wait and weight (engineer's) method; the same numbers are used for the second circulation of the driller's method.
The idea behind every line of the sheet is the same: keep the bottomhole pressure constant, a little above the formation pressure, from the moment the pumps start until the well is full of kill mud. The drill pipe gauge is the tool for that, because the string holds clean mud of known weight, so its pressure tells you what is happening at the bottom without the guesswork of an unknown influx in the annulus.
What you need before the kick
- Slow circulating rate (SCR) pressures at one or two kill pump rates, recorded each tour and after mud weight or BHA changes.
- Well data: TVD and MD of the bit, mud weight, string and annulus volumes, pump output, and the MAASP from the last leak-off test.
Most of the sheet can, and should, be filled in before anything goes wrong. A pre-recorded sheet means that after a shut-in the crew only has to add three readings and do four short sums.
What you read after shutting in
Stabilised SIDPP (shut-in drill pipe pressure), SICP (shut-in casing pressure) and the pit gain.
Pressures take a few minutes to build up as the formation around the well recovers. Record them every minute until they level off. If a float valve is in the string, the drill pipe gauge reads nothing until the float is opened: pump very slowly with the well shut in, watch for the moment the casing pressure starts to rise, and read the drill pipe pressure at that point.
The four formulas
OMW is the original mud weight. KMW is normally rounded up to the next 0.1 ppg that the mud system can mix, and that rounded value is used for FCP.
Why the formulas work
- KMW: with the well shut in, formation pressure equals the hydrostatic of the mud in the string plus SIDPP. Turning SIDPP into a mud weight at the bit TVD gives the extra density needed to balance it with no surface pressure.
- ICP: to circulate, the pump has to overcome friction (the SCR pressure at the kill rate) and still hold SIDPP on the well. Adding them keeps the bottomhole pressure where it was during the shut-in.
- FCP: once kill mud reaches the bit, SIDPP is no longer needed because the heavier column balances the formation. Friction is roughly proportional to mud density, so the SCR pressure is scaled by KMW ÷ OMW.
Worked example
| TVD / MD of bit | 9,500 ft / 10,200 ft |
| Original mud weight | 10.5 ppg |
| SCR pressure at 30 spm | 450 psi |
| SIDPP / SICP | 520 psi / 750 psi |
| Drill string volume | 182 bbl |
| Pump output | 0.1018 bbl/stk |
At 30 spm, kill mud takes about 60 minutes to reach the bit.
The drill pipe pressure schedule
While kill mud fills the string, the drill pipe pressure is brought down in a straight line from ICP to FCP. Past the bit, hold FCP until kill mud returns at surface.
| Strokes pumped | Drill pipe pressure (psi) |
|---|---|
| 0 | 970 (ICP) |
| 300 | 891 |
| 600 | 811 |
| 900 | 732 |
| 1,200 | 653 |
| 1,500 | 573 |
| 1,788 (bit) | 497 (FCP) |
Hold the pump at the kill rate and adjust the choke to follow the schedule. A choke change shows on the drill pipe gauge after a delay of roughly one second per 1,000 ft of well depth.
The straight line assumes the vertical height filled by kill mud grows evenly with strokes. In a deviated or horizontal well it does not: the build section and a long tangent add measured length with little vertical height. Many company sheets therefore split the string into sections and work out the pressure at the end of each one. Following a straight line in such a well can let the bottomhole pressure fall below the formation pressure part of the way down.
Second worked example: deeper well, heavier mud
TVD 12,000 ft, OMW 12.2 ppg, SIDPP 380 psi, SCR 600 psi at 30 spm, string volume 240 bbl, pump output 0.0920 bbl/stk.
Rounding the kill weight up from 12.81 to 12.9 ppg adds about 57 psi of overbalance at the bit (0.09 × 0.052 × 12,000). That is a sensible margin; rounding up by a whole ppg "to be safe" would add over 600 psi and could break down the shoe.
Reading the numbers
- SICP higher than SIDPP usually means a light influx (gas or oil) in the annulus.
- If SICP approaches the MAASP, the shoe is at risk. Discuss the options with the toolpusher and company man before going further.
- Recalculate FCP if the kill rate changes: use the SCR pressure for the new rate.
- Both pressures creeping up after they have stabilised can mean gas migrating up the annulus. Bleed small, measured volumes from the choke to hold the drill pipe pressure steady, and record each bleed.
Common mistakes
- Using MD instead of TVD in the KMW formula, which gives a kill weight that is too light.
- Using an SCR taken before a mud weight or BHA change.
- Reading the pressures before they stabilise, or after gas migration has pushed them up.
- Forgetting the surface line volume in the strokes to bit.
- Bringing the pump up to the kill rate without holding the casing pressure constant at the same time.
Good practice
- Take SCRs every tour at the kill rates written on the sheet, and on floating rigs through the choke line as well as up the riser.
- Keep the kill sheet pre-filled in the doghouse, and update it whenever the mud, the string or the shoe changes.
- Bring the pump up to kill rate while holding casing pressure constant, then switch to the drill pipe schedule.