Unrecognized gas influx during wireline sampling in an HPHT well
IOGP Well Control Incident Lesson Sharing #24-6
While performing wireline sampling on a HPHT well, a gain was observed in the trip tank. The initial flow check failed to detect an influx and sampling was continued. After observing increasing gains, a flow check was repeated, and the influx was identified. The well was finally stabilized after pulling out the wireline string and running in a clean-out BHA under gaining trends to circulate the influx out from the casing shoe.
The Wells Expert Committee/Well Control Incident Subcommittee believes that this incident description contains sufficient lessons to be shared with the industry. We further encourage the recipients of this mail to share it further within their organization.
What happened?:
After well TD, performed logging runs.
While performing WL run #3 (sample station #1 @ 4900m MD), observed 460lts discrepancy in trip tank.
Performed flow check: well stable.
Performed sample station #2 and noted similar discrepancy (835 lts over 7hrs).
Performed flow check: observed 0.8 bbl/hr increase.
Decision to POOH wireline tool string at controlled speed monitoring trip tank. Observed continuous 1.2bbl/hr gain. Once wireline tool string above BOP, shut-in well and monitor pressure; built and stabilized at 130 psi. 3.4 m3 cumulative gain.
Meanwhile, circulated riser clean; no presence of gas observed. RIH wellbore clean out BHA down to BOP depth, then opened BOP and kept RIH well bore clean out BHA to 9-5/8" shoe, observed same gaining trend (1.2 bbls/hr). Closed annular and circulated well clean through choke.
Noted gas peak at 16% behind MGS and mud properties drop (MW from 1.38sg to 1.35SG, drop in Electrical Stability, and Oil/Water ratio drop from 75/25 to 70/30).
Opened BOP and performed flow check: well stable.
Tripped in to TD at 5797mMDRT. Spaced out and closed annular. Circulated well clean through choke, max gas of 20% and same drop in mud properties (1.38sg -> 1.35sg, drop in ES and OW ratio on lagged depth of top of OH).
Flow check - well stable. Increased MW by 0.04sg to 1.42sg.
What Went Wrong?:
- Narrow Mud Weight Window.
- Lack of hydrostatic margin (possible migration of pumped out fluid during WL operations).
- Suspect sampling being a factor (perform simulations for max sampling volume before underbalance conditions).
Corrective Actions and Recommendations:
Increase hydrostatic safety margin for logging / tripping Event appear on 3rd WL run: Assess hole volume before / after each WL run (easier to determine over a great scale due to cable capacity affecting measurement).
safety alert number: 368
IOGP Well Control Incident Lesson Sharing http://safetyzone.iogp.org/
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