Well control during exploration drilling

While drilling a deepwater exploration well, geological uncertainties led to a well control event after the objective pay target was encountered earlier than expected.

The last controlling shoe was set with 14 in (356 mm) casing at 14,451 ft (4,404.7 m) in salt formation. A formation integrity test achieved 13.7 pounds per gallon equivalent (1,642 kg/m3 equivalent).

The hole section started by drilling and underreaming 12-1/4 in x 14-3/4 in (311 mm x 375 mm) hole with 12.0 pounds per gallon (1,438 kg/m3) synthetic-based mud. A problematic zone was encountered at 14,684-14,746 ft measured depth (4,475.7-4,494.6 m).

The zone, almost immediately below the salt exit, caused general instability, including ballooning, splintered shales and tar. The bottom hole assembly became temporarily stuck at 14,736 ft (4,491.5 m).

Drilling continued after changing the bottom hole assembly and using managed pressure drilling to keep the equivalent circulating density at 12.9 pounds per gallon equivalent (1,546 kg/m3 equivalent). Mud weight was increased to 12.3 pounds per gallon (1,474 kg/m3) at 16,340 ft (4,980.4 m).

Low resistivity objective pay was drilled from 16,432-16,490 ft (5,008.5-5,026.2 m) and 16,548-16,560 ft (5,043.8-5,047.5 m). While controlled drilling for geological identification at 16,664 ft (5,079.2 m), an 11 bbl (1.7 m3) gain was detected and the well was shut in.

While attempting to circulate the well using the driller's method, complete lost returns were experienced and the bottom hole assembly was determined to be packed off.

The downhole circulation bypass sub circulating port was opened and 13.3 pounds per gallon (1,594 kg/m3) mud was bullheaded down the drillpipe. The interpretation was that a shallower rubble zone had broken down, creating a risk of cross-flow from the deeper kick zone.

The pipe was freed. The well kill took eight days, including de-ballooning by bleeding back 353 bbl (56.1 m3) synthetic-based mud lost during well control operations.

The well was controlled for tripping the bottom hole assembly out of the hole, and an 11-7/8 in (302 mm) liner was run using managed pressure drilling to hold surface backpressure and maintain wellbore stability.

There was a lack of understanding of the actual lower fracture gradient in the problematic zone.

A short pressure seal in the kick zone did not allow the pressure increase trend to be identified below logging while drilling tools.

Shallower than planned geological objective penetration caused misinterpretation of both the geology and pressure profile.

The decision was made to keep drilling to total depth and bypass the contingency liner because of the shallower than expected geological objective penetration.

The bottom hole assembly downhole circulation bypass tool supported well control in the early stages, but complicated the final well kill because the circulation dart was in place and there was a possibility of re-opening dart plugging or jamming inside the bottom hole assembly.

Achieve a better understanding of the limitation of drill margin practice in exploration wells with significant uncertainty on the pore pressure to fracture gradient profile.

Improve communications with the palaeontologist and operations geologist to enhance understanding of geological horizons.

Execute contingencies when geological control is challenged or no longer attainable.

Closely observe wellbore behaviour immediately below the salt formation.

Enhance risk assessment in reference to bottom hole assembly configuration and the net benefit of certain added tools, such as a diverter.

Consider using pressure while drilling tools to improve the accuracy of pore pressure to fracture gradient modelling.

Improve understanding of human factors when conducting risk assessments and critical decision making, particularly, but not limited to, biases and group think phenomenon.

How can something like this happen here, for example, on our site?

What safety measures, including procedures, controls or barriers, do we have in place to mitigate the risk of well control events during exploration drilling?

How do we know our geological interpretation, pore pressure and fracture gradient assumptions remain valid as drilling conditions change?

When geological control is challenged or no longer attainable, how do we decide whether to execute contingencies rather than continue drilling?

How do we recognise and manage human factors, including biases and group think, during risk assessments and critical decisions?