Kick tolerance and the kill sheet, worked through

Kick tolerance is the largest gas influx a well can take, shut in and circulate out without breaking down the casing shoe. It decides whether a section can be drilled at all, and the kill sheet decides how the well is brought back under control when a kick comes. This article works both through on the 8½″ section of a deep gas well onshore Abu Dhabi: the equations, a hand check, the kill sheet for a 15 bbl kick, and what the casing gauge does while the gas comes up.

What kick tolerance measures

When a well kicks and is shut in, the influx sits at the bottom of the hole. The drill pipe is full of mud of known density, so the shut-in drill-pipe pressure (SIDPP) reads how far the formation pressure is above the mud's hydrostatic. The annulus holds the influx as well, and gas is light, so the shut-in casing pressure (SICP) reads higher. Every barrel of gas in the annulus adds to it.

The weak point is usually the last casing shoe. Its leak-off test sets the maximum allowable annular surface pressure (MAASP): the surface pressure that, added to the mud column above the shoe, reaches the pressure the shoe broke at. Kick tolerance asks two questions about the shoe.

• The bottom check. With the gas on bottom, how tall a column brings SICP up to MAASP? The gas sits around the bottom-hole assembly first, where the annulus is narrowest, so a small volume makes a tall column.
• The shoe check. Circulated up, the gas is at its most dangerous with its top at the shoe. How much gas can be there without fracturing it? That volume is carried back to bottom-hole pressure with Boyle's law, so the two checks compare like with like.

The kick tolerance is the smaller of the two, for a chosen kick intensity: how far the formation pressure is assumed to be above the mud weight. It is quoted in barrels or cubic metres of gain at bottom-hole conditions, and compared with what the rig can detect and shut in.

The equations

ρ_LOT, ρ_m

shoe test equivalent density and the mud weight

G_m, G_g

mud and gas gradients; gas at 0.1 psi/ft is the usual planning value

D, D_shoe

true vertical depth of the bit and of the shoe

P_f, P_frac

formation pressure (mud hydrostatic plus SIDPP) and the shoe's fracture pressure

H

gas column height; the volume is the annulus over that height, around the collars first

The kill sheet uses the standard relations. Kill mud = mud weight + SIDPP / (0.0981 × TVD), in SG and bar per metre, rounded up to the next mixing increment. The initial circulating pressure (ICP) is the slow-circulation pressure plus SIDPP. The final circulating pressure (FCP) is the slow-circulation pressure × kill mud / original mud. The drill-pipe pressure steps down from ICP to FCP as the kill mud travels to the bit.

The example well

The example is the 8½″ section of a vertical deep gas well set in typical onshore Abu Dhabi conditions, the same well whose pressures are built from its logs in pore pressure, fracture gradient and the mud-weight window. The section drills 505 m of overpressured Triassic shale, peaking at 1.66 SG on the logs, to a 7″ liner point above the gas reservoir. The 9⅝″ shoe's leak-off was 1.92 SG.

Kick tolerance by hand

At the section TD, 4,145 m, for a kick intensity of 0.06 SG (0.5 ppg), the usual planning value for a development well.

1. MAASP. (1.92 − 1.71) × 0.0981 × 3,640 = 75.0 bar, or 1,088 psi.
2. SIDPP. 0.06 × 0.0981 × 4,145 = 24.4 bar, 354 psi.
3. Gradient difference. Mud at 1.71 SG is 0.740 psi/ft, gas 0.1 psi/ft: 0.642 psi/ft, or 14.5 kPa/m.
4. Gas height. (1,088 − 354) psi ÷ 0.642 psi/ft = 1,143 ft, 348.5 m.
5. Volume. The first 160 m sit around the 6½″ collars in 8½″ hole, 0.0152 m³/m: 2.43 m³. The other 188.5 m are around 5″ pipe, 0.0239 m³/m: 4.51 m³. Total 6.95 m³, 43.7 bbl.
6. Shoe check. The same arithmetic with the gas top at the shoe gives 49.8 bbl after Boyle's correction, so the bottom check governs.

DrillIQ's engine gives 6.946 m³, the same to the litre. The bottom check governs here because the collar annulus is a third smaller than the annulus above it: the gas stands tallest while it is still around the collars.

Kick tolerance against kick intensity

The tolerance falls as the assumed intensity rises, because every bit of SIDPP comes off the MAASP before the gas adds anything. At zero intensity, a swabbed kick, the section can take 69.0 bbl. At 0.06 SG it can take 43.7 bbl, and it reaches zero at 0.185 SG: a kick that strong could not be shut in on this shoe at all.

A common minimum is 25 bbl: about 15 bbl for the crew to detect the kick and 10 bbl more gained while shutting in. This section meets it up to an intensity of 0.104 SG, about 0.87 ppg. Through the section, at 0.06 SG, the tolerance falls from 47.0 bbl just below the shoe to 43.7 bbl at TD, so the bottom of the section is the case to plan for.

The green line is the reason to test every shoe. Had the leak-off come in at 1.88 SG instead of 1.92, the tolerance at 0.06 SG would drop by a third, to 28.9 bbl, and the 25 bbl minimum would hold only up to 0.069 SG. A 0.04 SG difference in one test moves the section from comfortable to marginal. On the design side, the model fracture gradient at this shoe matched the test within 0.005 SG; at the shallower 18⅝″ shoe it was 0.05 SG optimistic.

The kick

Drilling at 4,060 m, the pits gain 15 bbl and the well is shut in. SIDPP stabilises at 350 psi and SICP at 680 psi. The formation pressure is 1.71 SG plus 350 psi over 4,060 m: 1.771 SG, 10,228 psi. The logs had put the shale at 1.61 SG at this depth, with a P90 of 1.77 SG, so this is the high case arriving, in a permeable stringer inside the shale.

The influx type comes from the difference between the gauges. The 15 bbl sits around the collars, 157 m tall. (680 − 350) psi over 515 ft is 0.641 psi/ft, so the influx gradient is 0.740 − 0.641 = 0.10 psi/ft: gas. At this depth and intensity the engine's kick tolerance is 44.0 bbl, so a 15 bbl gas kick can be circulated out without breaking the shoe.

The example mud is water-based. In oil-based mud, gas dissolves at bottom-hole pressure, the pit gain understates the influx, and the gauge difference reads a heavier fluid than the gas really is. It then comes out of solution near surface, fast.

The kill sheet

1. Kill mud. 1.71 + 350 psi ÷ (0.433 psi/ft per SG × 13,320 ft) = 1.7706 SG, 14.78 ppg, rounded up to the 14.80 ppg mixing increment: 1.7734 SG.
2. ICP. 425 + 350 = 775 psi.
3. FCP. 425 × 1.7734 / 1.71 = 440.8 psi.
4. Strokes. The string holds 36.1 m³: 1,902 strokes, 63 minutes at 30 spm. The annulus holds 96.9 m³, 610 bbl: 5,103 more strokes. One full circulation is 7,005 strokes, just under 4 hours.
5. Barite. Weighting 228 m³ of system from 1.71 to 1.7734 SG takes 104 kg per m³ of mud, 24.8 t or 548 sacks: 137 minutes at 4 sacks a minute.

With Wait & Weight the drill-pipe pressure starts at 775 psi and falls by 17.5 psi per 100 strokes while the kill mud goes down the string, then holds 441 psi until the kill mud is back at surface. On a vertical well with one pipe size that is a straight line. Here the last 300 m of heavy-weight and collars hold only 1.3 m³, so the kill-mud front runs through them in about 67 strokes and the last 29 psi comes off in the final 95 strokes. A straight line drawn in strokes would sit up to 12.6 psi below the engine's schedule near the bit, slightly underbalanced at the moment the kill mud arrives. The Driller's method holds 775 psi for one full circulation on the original mud, then runs the same step-down on a second circulation with kill mud: 14,010 strokes in all.

After the kill, with kill mud above the shoe, MAASP falls to 759 psi. Recalculate it and post it at the choke panel before drilling on.

Driller's or Wait & Weight

The casing gauge tells the rest of the story. At shut-in it reads 680 psi. As circulation starts and the gas leaves the narrow collar annulus, the column shortens and casing pressure drops to about 573 psi. The gas top passes the shoe after 340 strokes. From then on the gas expands as it rises, and the choke has to hold more and more pressure to keep bottom-hole pressure constant: 1,549 psi when the gas reaches surface with the Driller's method, 1,445 psi with Wait & Weight.

Both peaks are well above the 1,088 psi MAASP, and that is correct. MAASP protects the shoe, and once the gas is above it the shoe sees bottom-hole pressure less the mud column below it, which no longer changes much. A choke operator who opens the choke because the gauge passed MAASP at stroke 3,800 lets more gas in. The checks that matter later in the kill are the casing and surface equipment ratings, and the gas handling: the 15 bbl influx reaches surface as 87 bbl with the Driller's method and 92 bbl with Wait & Weight, at lower pressure.

The shoe sees its highest pressure at shut-in, 9,536 psi against a fracture pressure of 9,944 psi, a margin of 407 psi. Wait & Weight does not help it on this well, because the kill mud reaches the bit at 1,902 strokes and the gas passed the shoe at 340. Its benefits are the 104 psi lower peak at surface and one circulation instead of two. Its cost is the time the well stays shut in while 548 sacks of barite are mixed.

While the well is shut in

Gas does not wait for the kill mud. Shut in and left alone, it migrates up the annulus without expanding, carrying the formation pressure with it. In the engine's unbled migration, the shoe breaks down once the gas top has risen 199 m, at a casing pressure of 1,182 psi. That is less than 200 m of the 4,060 m to surface. At the 1,000 ft per hour (300 m/h) often used as a rule of thumb for migration in water-based mud, it takes about 40 minutes.

So the 2¼ hours of barite mixing for Wait & Weight has to be covered. Either bleed volumetrically, letting casing pressure rise by a safety margin and then bleeding small, measured volumes of mud at constant casing pressure, or start the Driller's first circulation on the original mud at once and weight up while circulating. On this section, with a 407 psi shoe margin at shut-in, the Driller's method is the safer start.

A checklist for the rig team

• Take a leak-off or FIT at every shoe, and recalculate MAASP whenever the mud weight changes.
• Know the section's kick tolerance at its TD before drilling out the shoe, and the intensity at which it falls below the minimum.
• Take slow-circulation rates at the start of each tour, after mud weight changes and after BHA changes.
• Fill in the pre-recorded kill sheet for the current depth: string and annulus strokes, ICP, FCP and the step-down per 100 strokes.
• Read SIDPP and SICP once they stabilise, and check the influx gradient against the pit gain.
• Watch for migration while shut in: rising SIDPP and SICP together mean bleed volumetrically or start circulating.
• Use MAASP to judge the shoe while the gas is below it; after that, watch equipment ratings and the gas separator.
• In oil-based mud, treat the pit gain and the gauge difference as lower bounds on the gas.

How DrillIQ handles it

DrillIQ's Well Control module keeps a section sheet for every hole section with the shoe test, MAASP, slow-circulation rates and the kick tolerance from both checks on the well's own string, hole and survey, and a kick-tolerance profile through the section. Its kill sheet works from the same geometry: strokes by string section, the step-down following the kill-mud front in TVD, Wait & Weight, Driller's, concurrent, volumetric, bullheading and stripping plans, barite for the whole system, choke-line friction for subsea stacks, and a PDF for the rig floor. It simulates the circulation and unbled migration shown above, monitors pit volume, flow and trip fill live, and reads the pressure window published by the Pore Pressure & Geomechanics module.

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References

1. API RP 59. Recommended Practice for Well Control Operations, 2nd ed. American Petroleum Institute, 2006.
2. Watson, D., Brittenham, T. and Moore, P.L. Advanced Well Control. SPE Textbook Series Vol. 10, 2003.
3. Grace, R.D. Blowout and Well Control Handbook. Gulf Professional Publishing, 2003.
4. Bourgoyne, A.T., Millheim, K.K., Chenevert, M.E. and Young, F.S. Applied Drilling Engineering. SPE Textbook Series Vol. 2, 1986.

About this example. The well in this article is a representative scenario set in typical onshore Abu Dhabi, UAE conditions and worked through with DrillIQ's engines. It is not a client's well, and its figures are not field results.