Well control · field units

Kill mud weight and kick tolerance, calculated

Kill mud weight is the mud weight needed to balance formation pressure after a kick. In field units, KMW (ppg) = OMW + SIDPP ÷ (0.052 × TVD), where OMW is the original mud weight, SIDPP the stabilised shut-in drill pipe pressure and TVD the true vertical depth of the bit; round the answer up to the next 0.1 ppg. The same shut-in readings give the kill pump pressures: ICP = SCR pressure + SIDPP and FCP = SCR pressure × KMW ÷ OMW. Kick tolerance works the other way round: it is the largest influx, in barrels, that can be shut in and circulated out without breaking down the casing shoe. This page gives the formulas, a deviated-well worked example, a kill mud weight lookup table and a simple volume-based kick tolerance calculation.

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 kill mud weight formulas

SIDPP is used, not SICP, because the drill string is full of mud of a known weight: SIDPP plus the hydrostatic pressure of that mud is the formation pressure. The annulus holds the influx, whose density and height are uncertain. The constant 0.052 converts ppg into psi per foot, and 19.23 is simply 1 ÷ 0.052, the form used in the SPE Petroleum Engineering Handbook. Use the true vertical depth of the bit, never measured depth. The barite formula is the standard field weight-up equation for 4.2 SG barite (35 ppg) from Lapeyrouse; the barite also adds volume, about 100 × (KMW − OMW) ÷ (35 − KMW) bbl per 100 bbl of mud.

2. Worked example: deviated well

A deviated well is shut in after a kick. The readings are:

Had measured depth been used by mistake, the increase would have been 450 ÷ (0.052 × 14,200) = 0.61 ppg, giving 11.81 and a rounded 11.9 ppg: too light to kill the well. Some kill sheets calculate FCP with the unrounded KMW (750 × 11.93 ÷ 11.2 ≈ 799 psi); follow the convention on your rig's sheet. For the step-down schedule, strokes to bit and the Driller's and Wait & Weight methods, see kill sheet calculations, step by step.

3. Kill mud weight lookup table

The table gives the mud weight increase, SIDPP ÷ (0.052 × TVD), in ppg. Add it to the original mud weight and round up to the next 0.1 ppg.

Example: 500 psi SIDPP at 10,000 ft TVD with 10.4 ppg mud gives 10.4 + 0.96 = 11.36, so KMW is 11.4 ppg. The same SIDPP needs a much larger increase in a shallow well, which is why a shallow kick can push the kill mud weight close to the shoe's limit.

4. What kick tolerance means

The IADC Lexicon, quoting NORSOK D-010, defines kick tolerance as the maximum influx to equal MAASP, with MAASP based on the weakest zone in the open hole, normally assumed to be the casing shoe. In practice it is quoted as a volume, in barrels, for a stated kick intensity: how far formation pressure is assumed to exceed the current mud weight, in ppg. A kick tolerance of 70 bbl at 0.5 ppg means a 0.5 ppg underbalanced kick of up to 70 bbl can be shut in and circulated out without fracturing the shoe; a larger one cannot.

Kick tolerance links three things on the same well: the shoe strength from the leak-off test, the mud weight, and the pit-gain detection the rig can achieve. If the kick tolerance is smaller than the volume the crew can reliably detect and shut in, the next section needs a lower mud weight, a deeper or stronger shoe, or better kick detection. There is no single industry standard for the calculation; a 2010 Drilling Contractor article noted that operators and contractors apply the concept inconsistently.

5. A simple volume-based kick tolerance method

The common single-bubble method assumes the influx stays as one gas bubble at constant temperature, ignores gas solubility and migration, and assumes it is circulated out at constant bottom-hole pressure. Four steps:

Kick tolerance is the smaller of V1 and V2. Here SIDPP = 0.052 × kick intensity × TVD, Gm is the mud gradient (0.052 × MW), Gi the influx gradient in psi/ft (0.1 psi/ft is a common gas assumption), and MAASP = (LOT EMW − MW) × 0.052 × shoe TVD. Annular capacities come from (Dh² − Dp²) ÷ 1029.4; see annular capacity.

Worked example

A useful check: with a 1,981-ft gas column whose top is at the shoe, the shoe pressure is 5,460 − 0.1 × 1,981 − 0.52 × 2,019 = 4,212 psi, exactly the fracture pressure. Kick tolerance is very sensitive to kick intensity. With the same well, a 1.0 ppg design kick gives Hmax ≈ 1,362 ft and a kick tolerance of about 46 bbl; a swabbed kick with no underbalance gives about 97 bbl (how swabbing happens, and the trip margin that guards against it, is covered in swab and surge pressure). Raising the mud weight also cuts kick tolerance, because it reduces MAASP.

The Drilling Contractor authors pointed out two traps in this simple method. When the BHA is longer than Hmax, placing the full Hmax at the bottom overstates V2; they recommend taking the height at the top of the collars instead. And deducting choke line friction from MAASP can cut the result sharply in deepwater. Operators' manuals also add margins for choke operator error. Treat the single-bubble result as a planning figure, not a guarantee.

6. Mistakes to avoid

• Using measured depth instead of TVD in the KMW formula or for the shoe.
• Reading SIDPP before it has stabilised, or with a float in the string that has not been opened.
• Rounding KMW down, or rounding LOT EMW up. Both errors are on the unsafe side.
• Forgetting that MAASP, and therefore kick tolerance, falls once kill mud or heavier mud is in the hole; see MAASP and leak-off test.
• Ignoring circulating friction. While pumping, annular friction acts on the shoe; see ECD.
• Quoting a kick tolerance without the kick intensity and influx gradient it was calculated for.

7. In DrillX

DrillX includes a Driller's and Wait & Weight kill sheet, with initial and final circulating pressure, alongside hydrostatic pressure, ECD, MAASP from leak-off test and annular capacity calculators. Values update as you type, in field units, and in English or Arabic. Kick tolerance is worked by hand above; it is not listed as a DrillX calculator. DrillX is in final testing; email info@drillx.app for a demo or early access, or see what DrillX does.

Sources and further reading

• IADC Lexicon, Kick tolerance: definition quoted from NORSOK D-010, Well integrity in drilling and well operations, Rev. 3 (2004).
• H. Santos, E. Catak and S. Valluri, Concepts, misconceptions about kick tolerance, Drilling Contractor (November/December 2010), from the IADC Well Control Middle East 2010 conference: the Hmax, Vshoe, V1 and V2 method and its pitfalls.
• N. Adams, Well Control: Procedures and Principles, SPE Petroleum Engineering Handbook, Vol. II, Chapter 4 (via PetroWiki): kill-weight mud calculation with the 19.23 constant.
• A. T. Bourgoyne Jr. et al., Applied Drilling Engineering, SPE Textbook Series Vol. 2 (1986): hydrostatic pressure, kick analysis and kill mud density.
• N. J. Lapeyrouse, Formulas and Calculations for Drilling, Production and Workover, 2nd ed. (Gulf Professional Publishing, 2002): Chapter 3 (drilling fluids, weight-up) and Chapter 4 (pressure control, kill sheets, kick analysis).
• DrillingManual, Kick tolerance calculation, definition and formula: example kick tolerance policy (0.5 ppg development, 1.0 ppg exploration, more than 25 bbl), citing the BP well control manual.
• IADC WellSharp and IWCF well-control training and certification.

Reference summary only. Worked examples are illustrative arithmetic. Kick intensity, influx gradient, safety margins and minimum kick tolerance are set by your operator's well-control policy.