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Losses Then a Kick at the Same Depth: Two Pressure Regimes at 12,400 ft

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Losses Then a Kick at the Same Depth: Two Pressure Regimes at 12,400 ft

While drilling a 12 1/4 in hole at 12,400 ft MD/TVD with 14.2 ppg mud you take 40 bbl/hr of losses into a depleted sand at 12,400 ft (pore pressure 12.9 ppg EMW). You reduce mud weight to 13.6 ppg and the losses stop. Two hours later, with pumps off, the well gains 25 bbl in 20 minutes; SIDPP is 420 psi and SICP is 620 psi. The 9 5/8 in shoe is at 9,500 ft TVD with leak-off of 15.5 ppg EMW, and the fracture gradient at the current depth is 14.8 ppg EMW. The rig has 400 bbl of 16.0 ppg weighted mud and an LCM supply on location. The toolpusher recommends weighting straight up to kill weight and circulating the influx out.

1. Calculate the kill mud weight and the resulting bottomhole pressure. Compare it with the depleted sand and with the fracture gradient at 12,400 ft, and state what the comparison proves.

Read the worked answer

Kill mud weight

  • KMW = 13.6 + 420 / (0.052 x 12,400)
  • = 13.6 + 420 / 644.8 = 13.6 + 0.65 = 14.25 ppg

Pressures at 12,400 ft

  • BHP with 14.25 ppg = 14.25 x 0.052 x 12,400 = 9,188 psia
  • Depleted sand pore pressure = 12.9 x 0.052 x 12,400 = 8,320 psia
  • Overbalance on the thief sand = 868 psi
  • Fracture gradient at 12,400 ft = 14.8 x 0.052 x 12,400 = 9,543 psia, so only 355 psi of margin above the kill weight

What this proves. The thief sand was already taking 40 bbl/hr at 14.2 ppg and stopped taking fluid at 13.6 ppg, so its leak-off is somewhere in the 13.6-14.2 ppg band - well below the 14.25 ppg needed to balance the influx. A static kill at kill weight will immediately re-open the depleted sand. The two operating windows do not overlap:

  • Above ~14.25 ppg: kick is controlled, thief zone flows mud away.
  • Below ~13.6 ppg: thief zone is stable, well is underbalanced and flowing.

The toolpusher's plan loses the kill mud to the thief zone, drops the hydrostatic column and re-establishes the influx, this time with an unknown volume of gas already in the hole.

2. What does the shut-in data tell you about the two zones, and what kill sequence would you actually run?

Read the worked answer

Two zones are open at the same depth. For the well to flow at all with 13.6 ppg in the hole, the influx source must have pore pressure at least 13.6 ppg EMW - and the shut-in data put it at 14.25 ppg EMW or above. The thief sand sits at 12.9 ppg EMW. These are two different pressure regimes exposed at 12,400 ft, not one zone behaving inconsistently. That is the diagnosis that drives everything else.

Kill sequence

  1. Hold BHP constant on the choke. Keep enough casing pressure to balance the 14.25 ppg equivalent while working. Do not shut in and let pressure build against the shoe (15.5 ppg EMW gives 9,500 x 0.052 x 15.5 = 7,660 psi; keep the shoe pressure below that).
  2. Seal the thief zone first. Spot an LCM pill (or a sized-salt/crosslinked pill) across 12,400 ft while circulating on choke, then test injectivity. You cannot kill this well until the depleted sand accepts pressure.
  3. Weight up in stages to 14.25 ppg only after losses are controlled, watching the loss rate continuously. If losses restart, stop, spot more LCM, and re-assess.
  4. If LCM will not hold, either set a liner/casing to isolate the thief sand before drilling ahead, or use a barite/DV plug to abandon the interval, or run a controlled dynamic kill with a lower-density fluid at high rate to stop the influx without exceeding the depleted sand leak-off.
  5. Document and circulate the influx out with the choke held, monitoring pit gain against calculated volume at all times.

Blindly weighting up is the one option that fails on both counts - it fractures the thief zone and does not get you to a balanced well.

3. How would you confirm which interval is taking fluid, and what would you change on the next well in this area?

Read the worked answer

Confirmation

  • Run a temperature/spinner survey or a noise log while injecting to locate the actual loss point rather than assuming it is at the bit.
  • Perform a controlled injectivity test: measure loss rate against pressure in steps, which gives an estimate of the depleted sand leak-off pressure instead of a bracket (13.6-14.2 ppg).
  • After the well is static, an inflow test or a short build-up confirms which zone is contributing.

Next well

  • Pre-drill pore pressure prediction one well ahead of the bit is not enough in a stacked, depleted sequence; use the offset data from this well to build a depth-pressure profile with two distinct sands.
  • Consider a casing seat above the depleted sand so the two regimes are never open simultaneously - that is the permanent fix, and it is cheaper than the LCM and the lost time on this well.
  • Evaluate managed pressure drilling or a low-ECD mud with annular pressure management so the equivalent circulating density can be held above the kick zone and below the thief zone; the static window does not exist, but the dynamic window may.
  • Plan the cement job for a depleted zone: stage cementing or a lightweight lead slurry, because the same leak-off that caused the losses will put the cement in the wrong place.