Trip Tank Anomaly: Kick With the Bit Off Bottom
While pulling out of the hole on a trip at 9,800 ft TD (vertical) with 12.8 ppg mud, the crew has pulled 15 stands of 93 ft, putting the bit at 8,405 ft. The trip tank shows 8 bbl of fill taken against an expected 0.9 bbl per stand. No fill at all was taken on the last six stands. The driller stops and lines up for a flow check: after 12 minutes the trip tank has gained 14 bbl and the flow line is still running. The well is shut in on the annular; gauges read SIDPP 180 psi, SICP 410 psi, pit gain 22 bbl. The 9-5/8 in shoe is at 7,000 ft TVD with a 15.0 ppg leak-off. The string is open-ended at the bit with no float, the choke manifold is lined up, and the pumps are available. The toolpusher wants a decision: strip back to bottom, or kill the well with the bit off bottom?
1. Interpret the trip sheet. Compare expected and actual fill and explain which observation proves the well is flowing.
Read the worked answer
Fill bookkeeping
- Expected fill = 15 stands x 0.9 bbl = 13.5 bbl
- Actual fill = 8 bbl
- Deficit = 5.5 bbl, all of it concentrated in the last six stands where zero fill was taken
A 5.5 bbl deficit on its own is not conclusive - it can come from drainage, a wet string or a slightly oversized hole - but zero fill over six consecutive stands is abnormal and should already have triggered a flow check.
What proves inflow
The flow check. Pumps are off, the well is static, and the flow line is still running with 14 bbl gained in 12 minutes. With no pump and no pipe movement there is no legitimate reason for returns. That, not the fill figures, is the primary indicator, and it is confirmed by the 22 bbl total gain at shut-in. The correct reading is that the well was swabbed while pulling, and the influx entered while the bit was still on bottom, so the kick is now sitting below a bit that is 1,395 ft off bottom.
2. The derrickhand proposes a kill mud weight of 12.8 + 180 / (0.052 x 8,405) = 13.2 ppg. Explain why this figure cannot be trusted for TD.
Read the worked answer
SIDPP is measured at surface with the drillpipe full of 12.8 ppg mud down to the bit at 8,405 ft.
- Drillpipe hydrostatic to the bit = 12.8 x 0.052 x 8,405 = 5,594 psi
- Pressure at 8,405 ft = 5,594 + 180 = 5,774 psi
That 5,774 psi is the pressure at the bit depth, not at TD. Below the bit the annulus holds mud plus the influx, whose density and height are unknown. The 13.2 ppg figure therefore balances the well at 8,405 ft only; if the influx is gas, the true pore pressure at 9,800 ft is higher and 13.2 ppg leaves the well underbalanced at the sand. Pumping it would produce a second influx during the kill, with the bit off bottom and no way to circulate the influx out efficiently. The number is a minimum at the bit depth, not a kill weight for the reservoir.
3. Recommend an action plan, including the casing pressure limit that governs it.
Read the worked answer
Limit first
MAASP = (15.0 - 12.8) x 0.052 x 7,000 = 2.2 x 0.052 x 7,000 = 801 psi. SICP is 410 psi, so there is roughly 390 psi of headroom at the shoe.
Recommended plan
Strip back to bottom rather than kill off bottom. Reasons: the true kill weight cannot be established until a valid SIDPP is read at TD, and the well is stable enough (low pressures, 390 psi of margin) to allow it.
While stripping, hold bottom hole pressure constant. As gas migrates and the pipe goes in, casing pressure rises; either let it rise in controlled increments and bleed mud back to the shut-in value, or use the volumetric method, tracking the mud bled against the pressure removed. Keep the casing pressure below 801 psi at all times. Record every stand and every bleed. Once on bottom, take a fresh SIDPP and SICP, recalculate the kill weight against TD, and kill the well with the driller's method. If casing pressure reaches the MAASP limit before you are on bottom, stop and reassess.