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MISINTERPRETED SWABBED KICK FROM SHALLOW RESERVOIR LEADS TO DELICATE WELL CONTROL SITUATION
Incident Date: --- Time: ---
Type of Activity: Drilling, workover, well services
Type of Injury: ---
9 5/8” casing set and cemented at 767 m MD/TVD with hanger at 469 m.
FIT on 9 5/8” shoe was performed to 1.45 sgEMW.
Drilled 8 ½” hole section from 9 5/8” casing shoe at 767 m to TD at 1594 m. Cored three intervals from 800 –827 m, 834 –889 m and 889 –935 m
Drilled section with 1.17 –1.18 sgWBM.
Gas reading were generally less than 1% during drilling. Max gas during coring was 17%. Several potential hydrocarbon bearing formations exposed in open hole.
Plan forward was to POOH with drilling BHA (BHA length 274 m) and perform WL logging. Back-reamed out of hole and into 9 5/8” casing shoe due to over-pull when pulling without circulation.
Performed flow-check in shoe (767 m MD/TVD) and continued to trip out of hole. Observed rapid increase in trip tank at 531 m.
Shut in well on LAP and evaluate situation. Performed off-bottom kill and circulate out gas.
Analysis of situation before POOH :-
Performed swab calculations. Well was filled with 1.17 sgWBM (1.16 sgused in calculations). Predicted pore pressure in shallow reservoir at 823 m was 1.11 sg. 2 points safety margin included in swab calculations.
The swab calculations show that maximum pulling speed at casing shoe was 120 sec/stand. Limited pulling speed (132 sec/stand) past reservoir.
Pumped 3 m31.46 sg slug and started tripping out of hole from casing shoe. Swab calculations and pulling speed restrictions not discussed prior to continue POOH.
Noticed discrepancy in trip tank volume, but this was attributed to the slug not being in balance. Pulled two more stands and trip tank was still not taking correct volume. Driller discussed with tool pusher and decided to pull one more stand very slowly to verify slug effect.
While breaking connection on the next stand (bit @ 531 m) there was a rapid gain of 5 m3in the trip tank. Shut in well with the lower annular preventer (LAP) on 6 ¾” spiral DC. Secured drill string by making up top drive and lifting string out of slips.
Gain volume = 5.6 m3. SICP = 25.5 bar and SIDPP = 16 bar (after bumping float).
Jar and accelerator part of the BHA above the closed LAP –stripping not an option. Well gas-filled from 658 m to LAP based on gain in active system.
Pressure at shoe = ~1.27 sgEMW.
Choke line pressure increased to 45 bar after bleeding off 800 liter –gas in choke line.
Circulated hydrocarbons out through choke with 460 LPM while keeping constant back-pressure on kill line. Completed five rounds of circulation. Observed erratic returns and large volumes of gas going through the poor-boy degasser.
What Went Wrong?:
Gas filled well with bit ~300 m above most likely gain zone.
Well successfully closed in with annular preventer on spiral drill collar.
At the time the well was closed in it was uncertain whether the blind shear ram could shear and seal drill collar.
Unable to strip in hole due to jar located above upper annular preventer.
Corrective Actions and Recommendations:
Surge / Swab calculations are still valid when tripping in cased hole.
Low pressure margins when dealing with shallow reservoirs.
6 points (1,17 MW vs. Est PP of 1,11 sg) overbalance at 823 m is only 4,8 bar
Small margins when circulating out an influx with a shallow casing shoe.
SICP of 25 bar equals 1,51 sg. –Estimated fracat casing shoe.
SICP of 18 bar equals 1,41 sg. –Max pressure at casing shoe.
Perform flow check and circulate bottoms up if indications of gain.
safety alert number: 281
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