Lessons Learned: Well Control Event During Deepwater Exploration
While drilling a deepwater exploration well, geological uncertainties led to a well control event due to encountering the objective pay target earlier than expected.
MPD operations were employed during the deepwater exploration prior to the well control event. However, complications arose during the well kill operation of the deepwater exploration due to a shallower zone below the shoe having a lower than anticipated frac gradient below that of the last casing shoe’s formation integrity test. Several mitigations were subsequently employed to successfully manage the well kill and several lessons were recorded from the event.
What happened?
The last controlling shoe was set with a 14″ casing at 14,451 ft in salt formation. A successful formation integrity test achieved 13.7 ppge. The hole section started by drilling and underreaming 12-1/4″ x 14-3/4″ hole with 12.0 ppg SBM. A problematic zone was encountered at 14,684 ft MD and extended to 14,746 ft. This zone was almost immediately under the salt exit and caused general instability (ballooning, splintered shales and tar) and BHA temporarily stuck at 14,736 ft. Due to very common instability zones below salt, drilling operations continued after changing the BHA and keeping the ECD at 12.9 ppge using MPD, MW was increased to 12.3 ppg at 16,340 ft.
Drilled low resistivity objective pay from 16,432-16,490 ft and 16,548-16,560 ft. While performing controlled drilling for geological identification at 16,664 ft, 11 bbl gain was detected and the well was shut-in. While attempting to circulate the well (driller’s method, 1st circulation) complete lost returns was experienced and the BHA was determined to be packed off.
Opened downhole circulation bypass sub (diverter) circulating port. Bullhead 13.3 ppg mud down DP. Diagnose well control situation and issues with downhole circulation bypass sub. Interpretation was that shallower formation (rubble zone) had broken down; risk of cross-flow from deeper kick zone. Freed pipe. Spent eight days killing well including “de-ballooning” by bleeding back 353 bbl SBM lost during well control operations. Controlled well for tripping BHA out of the hole and ran 11-7/8″ liner using MPD to hold SBP to maintain wellbore stability.
What went wrong?
- Lack of understanding of actual lower FG in problematic zone.
- Short pressure seal in kick zone did not allow to identify pressure increase trend (below LWD tools).
- Shallower than planned geological objective penetration caused misinterpretation of both geology and pressure profile.
- Election to keep drilling to TD and bypass contingency liner due to shallower than expected geological objective penetration.
- BHA downhole circulation bypass tool supported well control in the early stages but complicated final well kill due to circulation dart in place and possibility of re-opening dart plugging or jamming inside BHA.
Corrective actions & recommendations - Achieve better understanding of the limitation of “drill margin” practice in exploration wells with significant uncertainty on the PP-FG profile.
- Improve communications with palaeontologist and operations geologist to enhance understanding of geological horizons.
- Execute the contingencies when geological control is challenged or no longer attainable.
- Closely observe wellbore “behavior” immediately below salt formation.
- Enhance risk assessment in reference to BHA configuration and the net benefit of certain added tools (e.g. diverter).
- Consider use of pressure while drilling tools to improve accuracy of PP-FG modeling.
- Improve understanding of human factors when conducting risk assessments and critical decision making, particularly but not limited to biases and group think phenomenon.
Content credit: IOGP (International Association of Oil & Gas Producers), “WCI resulting from geological uncertainties in an exploration well,” IOGP WCI Lesson Sharing 25-1.
