Wet trip on a deviated well: strip, pull or circulate?
While pulling out of a deviated development well the crew has pulled 18 stands of 5 in drill pipe from a 12¼ in hole at 12,900 ft MD / 12,500 ft TVD. Mud weight is 12.2 ppg and the trip tank is checked on every stand. After stand 18 the trip tank shows a 4.2 bbl gain with the pumps off, and a 10-minute flow check confirms a slow but steady flow. The well is shut in on the annular preventer and pressures stabilise at SIDPP 350 psi and SICP 480 psi, with a total gain of 4.2 bbl. The slow circulating rate is 30 spm at 480 psi. The 9⅝ in shoe is at 10,000 ft TVD and the last leak-off test gave 15.0 ppg equivalent. There is 480 bbl of 12.2 ppg mud in the active system, the mud engineer needs about 5 hours to weight it up, and company policy requires a 0.2 ppg trip margin.
1. Calculate the kill mud weight, the initial circulating pressure and the final circulating pressure at 30 spm. State the assumptions behind the numbers and say why the SIDPP may not be the true pore pressure in this situation.
Read the worked answer
Kill mud weight = OMW + SIDPP ÷ (0.052 × TVD) = 12.2 + 350 ÷ (0.052 × 12,500) = 12.2 + 0.54 = 12.74 ppg. Adding the 0.2 ppg trip margin gives 12.94 ppg, which is the figure to weight to.
- ICP = SCRP + SIDPP = 480 + 350 = 830 psi at 30 spm.
- FCP = SCRP × (KMW ÷ OMW) = 480 × (12.94 ÷ 12.2) = 509 psi at 30 spm.
Assumptions: the influx is at the bottom, the string is full of 12.2 ppg mud so the SIDPP is hydrostatic plus reservoir pressure, and 12,500 ft is the true vertical depth of the bit. The bit is only about 380 ft off bottom, but if any gas has already migrated above the bit the SIDPP under-reads the reservoir pressure and 12.74 ppg is a minimum, not a maximum. Likewise, if the 4.2 bbl gain is gas that has broken out, the 130 psi SIDPP/SICP split will grow and the 350 psi figure will be stale by the time the mud is ready. Both numbers should be re-recorded after the string is back on bottom and before the kill starts.
2. The string is 18 stands off bottom with the influx below the bit. Decide between stripping back to bottom, pulling to the shoe, and circulating the kick off bottom. Justify the choice and outline how you would execute it.
Read the worked answer
Strip back to bottom. Pulling is the worst option: the well is flowing, the bit is above the influx, and every stand pulled further reduces the hydrostatic and swabs more gas in. Circulating off bottom has limited appeal because the influx is below the bit in a deviated hole, so a conventional kill schedule cannot be trusted and you would still have to trip back into the same live well afterwards.
Execution:
- Record the shut-in pressures and confirm the influx volume each stand stripped.
- Strip through the annular preventer using closed-end displacement, holding the casing pressure on the choke to keep bottom-hole pressure constant as the pipe is introduced.
- Apply volumetric bleed if the pressure rises while the pipe is stationary (gas migration), and note the volume bled against the pressure held.
- Re-check the trip tank against the calculated displacement on every stand; a departure either way means the well is still live.
- On bottom, re-record SIDPP/SICP, recalculate the kill weight with the corrected bottoms-up data, then kill conventionally.
3. Compute the maximum allowable annular surface pressure at the 9⅝ in shoe with the current mud and with the kill mud. What do you do with the well during the 5 hours of weight-up?
Read the worked answer
MAASP = (LOT EMW − mud weight) × 0.052 × shoe TVD.
- With 12.2 ppg: (15.0 − 12.2) × 0.052 × 10,000 = 1,456 psi.
- With 12.94 ppg kill mud: (15.0 − 12.94) × 0.052 × 10,000 = 1,071 psi.
The current SICP of 480 psi is far inside that, so the shoe is not the immediate limit - the migration rate is. Do not simply sit on the well for 5 hours. Keep the well shut in, log SIDPP and SICP every 15–30 minutes, and watch the rate of rise: both pressures climbing by the same amount means gas is migrating, not that reservoir pressure has changed. As the casing pressure approaches the prevailing MAASP, use lubricate-and-bleed - bleed a measured volume of mud from the choke to let the gas expand while keeping bottom-hole pressure constant - rather than a straight bleed. If the pressure rises quickly, start the kill with the mud you have and weight up while circulating instead of waiting for the active system.