Flow while running casing, before the string is landed
The 9⅝ in casing is being run in a 12¼ in hole to 12,000 ft TVD. Mud weight is 14.2 ppg, the 13⅜ in shoe is at 8,000 ft with an LOT of 16.0 ppg equivalent, and the stack is a 13⅝ in 10K with 5 in pipe rams, 9⅝ in rams, a 5–9⅝ in variable bore ram and an annular. A float shoe with auto-fill is fitted and the casing has stayed full. At 11,200 ft, still 800 ft off bottom and hanging in the rotary slips, the crew reports the annulus flowing; the pit gains 15 bbl in 6 minutes. There is no casing head or circulating head installed, the casing cannot be broken out of the slips without the elevators, and the cement unit is already rigged up for the job. The well was static before casing running and the mud has not been changed.
1. State your immediate actions, including whether you close the BOP and which element you close, and what you cannot yet read.
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Stop running immediately and keep the casing in the slips so it cannot be dropped, then close the variable bore ram on the 9⅝ in casing body. The VBR is the correct element: the 9⅝ in fixed rams are for a different weight/wall combination than may be in the hole, and the annular is a sealing device that will not hold pressure reliably against an 800 ft string hanging on it, and it will not let you strip later without damage. Do not close the pipe rams - there is no drill pipe at the rams.
Having closed in:
- Read and record the casing pressure on the annulus side. You cannot read a drillpipe pressure, because the auto-fill float prevents any pressure reading through the casing string, and the string is not on bottom anyway.
- Check for flow through the choke line and confirm the choke manifold is lined up.
- Do not attempt to break out or lift the casing; any movement with the rams closed will damage the seal or the casing.
- Record the total gain and keep the trip tank isolated and monitored.
The absence of a readable SIDPP is the key limitation: until a circulating head is fitted you are working from casing pressure and gains alone, so the numbers carry more uncertainty than a normal shut-in.
2. The casing pressure stabilises at 640 psi. Calculate the kill mud weight and the pressure limits, and state the assumptions that worry you.
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With the casing full of 14.2 ppg mud, total depth 12,000 ft TVD:
KMW = 14.2 + 640 ÷ (0.052 × 12,000) = 14.2 + 1.03 = 15.23 ppg.
Shoe check with 14.2 ppg: shoe pressure = 640 + 14.2 × 0.052 × 8,000 = 640 + 5,907 = 6,547 psi, against an LOT pressure of 16.0 × 0.052 × 8,000 = 6,656 psi - only 109 psi of margin. MAASP at 14.2 ppg = (16.0 − 14.2) × 0.052 × 8,000 = 749 psi; with 15.23 ppg kill mud it falls to (16.0 − 15.23) × 416 = 320 psi.
Assumptions that worry me:
- The 640 psi assumes the annulus is full of 14.2 ppg mud to surface. If the casing was not kept full, the true pressure at the shoe is higher and the computed kill weight is low.
- The 800 ft of open hole below the casing is where the influx is; using 12,000 ft TVD is correct only if the influx is at the bottom.
- The margin at the shoe is thin, so the kill must be run at a reduced rate and any gas expansion watched closely; a 100 psi surge at the shoe breaks it down.
3. What must be done before you can circulate a kill, and how would you run the kill given the string is unlanded?
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You cannot circulate until there is a pressure-tight way into the casing. The sequence is:
- Land and secure the casing - pick up, set the casing in the wellhead or hang-off, and make up the casing head so the annulus is sealed below the rams and the string can take pressure. Until this is done, the only barrier is the VBR and you cannot pump.
- Install a circulating head or cementing head on top of the casing, pressure-test the connection and the lines, and line up to the choke manifold with a recorded slow circulating rate through the casing at a low rate.
- With circulation established, take a proper slow circulating pressure, then start the kill with 15.23 ppg mud using the wait-and-weight method.
The constraints to manage throughout:
- The 320 psi MAASP with kill mud is small; hold the shoe pressure at or below the LOT and let the choke schedule absorb the rest.
- Kill at a reduced rate, because annular friction in a 12¼ in hole with 9⅝ in casing is low but the margin is lower.
- If the shoe limit is reached before the gas is out, stop, close in, and reassess - do not break the shoe to chase the schedule.
- Keep a full record of volumes, pressures, times and the exact point at which each step was completed; if the well has to be handed over or the stack closed for hours, the next crew needs the state of the well, not a summary.