The Incident

The G4 Well was drilled to a total depth of ~ 5900m MD; with a 3 Casing x 1 Liner x 1 Tubing Policy as shown in Table 4.1.

In January 2011, the 7” Liner collapsed and in February 2011, the G4 Well was isolated from the main producing reservoir (The Fulmar Reservoir) with a deep-set plug.

In the past, the G4 well had sustained pressure in the ‘A’ Annulus (Tubing x Casing 3) due to ingression of Gas (Believed to be from the Chalk Interval). However, the ‘A’ Annulus is optimally rated to withstand the formation pressure within the Chalk.

However, on 25th Feb 2012, the 10-3/4” x 9-7/8” Production Casing failed well below its rated design pressure. Post failure, the ‘A’ Annulus pressure dropped whereas the ‘B’ Annulus pressure started to rise. Following the failure of the production casing, a few hours later, the 13-3/8” casing failed as well thereby allowing communication from the ‘B’ Annulus to the ‘C’ Annulus.

There was no hydrocarbon release observed during the above events. However, the threat of an uncontrolled release of gas was imminent. Therefore, it was decided to intervene on G4 and kill the well by pumping heavy fluids into the gas-flooded annuli.

The Intervention of the G4 Well began on 5th March 2012. The Rowan Viking Jackup Rig was contracted for this purpose.

• During the initial stages of intervention, the high pressures trapped within the annuli required that they be bled off repeatedly in order to maintain pressures within limits while circulating the kill fluid.
• As part of the well intervention operations, there were multiple activities underway with respect to increasing number of perforations and to cut the tubing string.

The two factors mentioned above caused an inability to continuously circulate kill mud as required to re-establish control of the well. The resulting intermittent breaks in circulation further increased the intensity of the gas influx into the well.

Eventually, it was no longer possible to bleed off the ‘B’ and ‘C’ annuli without exceeding the operating limitations. An eventual surge in the ‘C’ annulus pressure led to the failure of the 20”casing.

On 25th March 2012, gas flowed from the Chalk interval through the failed casing strings and connected annuli into the outermost ‘D’ annuli before breaching surface.

Note: The 30” conductor (Pile Driven) does not form part of the Well Barrier system for pressure containment. The flow paths through the ‘D’ annulus are open and allow clear access to the atmosphere.

Immediately after the gas release, the platform’s safety measures functioned as required and the Elgin WHP was efficiently shutdown and depressurized. In an orderly fashion, the platform as well as the adjacent drilling rig were evacuated.

Two hours after the initial release, 108 members of the crew were evacuated. 19 members of the core crew were left behind on the platform to monitor the situation and were evacuated 12 hours after the release.

Immediately following the final evacuation, the Elgin Platform was ‘Shutdown and Electrically De-Energized’.

Root Causes

The investigation carried out the appointed team revealed the following root causes to the G4 gas release:

• Failure of the Production Casing below it Operating Limits.
  When the 10-3/4 x 9-7/8 production casing failed unexpectedly well within its design limitation, it resulted in communication with the ‘B’ annulus. The resulting pressure buildup in the ‘B’ led to the failure of the 13-3/8” intermediate casing and opened communication with the ‘C’ annulus. This was followed by the failure of the 20” surface casing.

  The 30” conductor, not begin a pressure containment barrier allowed access for gas into the atmosphere through the open wellhead ports.

• Flow Activity from the Chalk Interval.
  Initially, it was believed that the Chalk interval possessed a very low Poro-Perm relationship. Therefore, the Chalk was supposed to have an inefficient flow potential. However, the events in the G4 well were a result of significant flow of gas from the Chalk.

  Therefore, there is a possibility that the pore geometry of the chalk had been altered due to geological reasons such as compaction due to overburden, internal fracturing of the chalk, etc.

• Ineffective Well Intervention Program.
  The Well Intervention program was unable to kill the G4 well with heavy kill mud prior to the failure of the outermost casing barrier – The 20” Surface Casing.

Aftermath

Following the successful evacuation of all members aboard the platform and drilling rig, preparatory work was underway to kill the G4 well and halt the release of hydrocarbon gas.

Two different strategies were developed and implemented simultaneously to stop the hydrocarbon release into the atmosphere as quickly as possible:

• Relief Well to Intersect the G4 Well and Kill it with Heavy Kill Mud.
• Carry Out a Top Kill by Pumping Heavy Kill Mud directly from the G4 Well.

Way Forward

Following the Elgin G4 Incident, a thorough well review was carried out and critical Well Integrity criteria developed for all wells in the Elgin field.

• A new Well Integrity Operating Philosophy was introduced to define the recommended practices to be followed when a well failure situation should arise.
• A full technical review of all wells in the Elgin field was carried out and evaluated with respect to the most recent conditions of the sub-surface formations.
• Well Annuli monitoring and reporting systems were established to function in accordance with the Well Integrity Operating Philosophy.

Additionally, the policies and procedures followed in the Elgin field for Well Control and Well Integrity underwent several modifications and new additions to prevent a recurrence of similar incidents.