Losses Followed by a Kick: Kill Weight Against a Thief Zone
You are drilling at 13,400 ft MD / 12,600 ft TVD with 14.2 ppg mud at 650 gpm when the pit drops 45 bbl in 12 minutes. The pumps are stopped and within 8 minutes the well is flowing and the trip tank shows 18 bbl. The well is shut in: SIDPP 95 psi, SICP 300 psi, pit gain 26 bbl. The 9-5/8 in shoe is at 9,000 ft TVD with a leak-off test of 15.8 ppg. A loss zone was logged on an earlier well at 12,200 ft TVD with an estimated fracture gradient of 14.6 ppg equivalent, and that is where the losses are going. The 12,600 ft sand is the producing zone. Kill mud can be mixed quickly, and the slow circulating pressure is 400 psi at 30 spm. The company man asks whether to add a 0.3 ppg trip margin to the calculated kill weight.
1. Calculate the kill mud weight and explain what the loss-then-flow sequence tells you about the two zones.
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Kill mud weight
Kill MW = 14.2 + 95 / (0.052 x 12,600) = 14.2 + 95 / 655.2 = 14.2 + 0.145 = 14.35 ppg
Cross-check the pore pressure: 14.2 x 0.052 x 12,600 + 95 = 9,303 + 95 = 9,398 psi, which is 14.34 ppg equivalent. Consistent.
What the sequence means
The mud was only just overbalanced. When the thief zone took 45 bbl, the fluid level in the annulus dropped and the hydrostatic at the sand fell below pore pressure, so the well started to flow. The thief zone lies above the sand and has a lower fracture gradient (14.6 ppg equivalent) than the margin the kill requires. So the same well has a zone that breaks down at about 14.6 ppg and a zone that flows below 14.34 ppg - a window only about 0.25 ppg wide. Any kill plan has to fit inside that window.
2. Why is the proposed 0.3 ppg trip margin a bad idea here? Support the answer with pressures at 12,200 ft.
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Static case
- Fracture pressure at 12,200 ft = 14.6 x 0.052 x 12,200 = 9,262 psi
- Kill mud at 14.35 ppg = 14.35 x 0.052 x 12,200 = 9,104 psi - margin 158 psi
- With 0.3 ppg added (14.65 ppg) = 14.65 x 0.052 x 12,200 = 9,294 psi - above the fracture pressure by 32 psi
Circulating case
It is worse than that, because ECD adds annular friction. At 30 spm the extra friction is worth perhaps 0.15-0.25 ppg at 12,200 ft. So even the correct 14.35 ppg mud is already at or over the 14.6 ppg fracture limit while circulating, and the 14.65 ppg mud would guarantee losses.
The margin would restart the losses, drop the annular hydrostatic again and re-open the sand - the same cycle that caused this kick. The correct approach is to kill with the minimum balance weight (14.35 ppg, or 14.4 ppg at most), keep the pump rate low to hold the ECD down, and control the shoe with the MAASP of (15.8 - 14.2) x 0.052 x 9,000 = 749 psi.
3. What will you monitor during the kill to detect the thief zone taking fluid again, and how will you respond?
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Indicators
- Active pit and trip tank falling while the pump rate and the drillpipe pressure stay constant
- Casing pressure falling away from the schedule while the drillpipe pressure is held on schedule
- A loss of returns at the flow line, or a pump pressure drop with no rate change
- Any fall in the annular pressure at the 12,200 ft level - watch the casing pressure trend against the kill sheet
Response
- Reduce the pump rate step by step to the lowest rate that still carries the cuttings and keeps the choke controllable; lower rate means lower ECD at 12,200 ft.
- Hold the drillpipe pressure on the recalculated schedule for the new rate.
- If losses persist, spot an LCM pill and displace it to 12,200 ft - do not stop the kill with the sand open.
- If the loss zone cannot be sealed and the shoe is at risk, the well cannot be left in this state: pull up and set a cement plug across the thief zone, then re-enter, re-drill and run casing. Keep the well shut in with the casing pressure monitored throughout.