“The Rig Continues to Burn and has Partially Collapsed,” Chevron Contracts Transocean to Drill Relief Well

On 16 January 2011, a jack-up drilling rig offshore Nigeria caught fire, forcing a complete evacuation.  2 rig workers are still reported missing, and according to the latest press release from Chevron, “the well continues to burn and the rig has partially collapsed.”

As you’ll notice from the images, this looks somewhat similar to the Deepwater Horizon from 2010… However the situation is fundamentally less dire.

Images courtesy Chevron

At first glance, this situation appears a lot like the Ixtoc I where 3,000,000 barrels of oil was spilled into the Gulf of Mexico in 1979 from a blown out jack-up rig…

It is actually, in many ways, except for the fact this well is spewing natural gas condensate (aka wet gas), and not crude oil.  The gas will continue to flow until the well is killed, or until it bridges off, but it seems unlikely that a catastrophic environmental disaster such as Ixtoc 1 is imminent.  From an environmental standpoint, it may make the most sense to try and keep the gas burning for as long as possible considering the fact natural gas is a far more powerful greenhouse gas than CO2.

How this likely occurred:

From a drilling safety standpoint, jack-up rigs are inherently more dangerous than floating rigs for two main reasons.

First, if the well blows out and catches fire, there’s no way to unlatch the rig from the wellhead and move off station, thus removing the fuel source.  Either the blowout preventer will close and the fire is extinguished, or the rig is going to burn to the waterline.   There’s very little in-between.

Secondly, because the blowout preventer is located roughly 60 feet below the rig floor, and not hundreds (if not thousands) of feet below on the sea floor, if the rig crew is unable to detect a “kick,” or influx of gas into the wellbore, before it reaches the blowout preventer (BOP), then it’s pretty much guaranteed that there will be a flammable geyser of mud, gas, and possibly oil, over the rig.  On a deepwater rig, such as the Deepwater Horizon, a huge steel pipe called a riser connects the floating rig to the blowout preventer on the sea floor.  There are added dangers in this situation as well, however there are effective means of mitigating these risks as well.

Do kicks happen often?

A kick is a well control issue that a driller will contend with dozens, if not hundreds, of times throughout a career in the oilfield.  While drilling, natural gas from the surrounding formation will quite frequently enter the wellbore as the rig crew is drilling down into the earth, and this pressurized flammable gas naturally wants to float to the surface…

So imagine a barrel-sized unit of natural gas enters the wellbore at 10,ooo feet.  According to Boyle’s Law, the volume of gas will double each time the pressure is halved.  So at 5,000 feet, it’s expanded to 2 barrels, as it migrates up to 2,500 feet, it’s at 4 barrels, 1,250 feet – 8 barrels, and so on, until the bubble has expanded to the rig floor.

Left unmitigated, this kick can cause serious disaster on board a drilling rig as witnessed in this case, and as seen in the Deepwater Horizon disaster.

So how do you know when kick is migrating up a well bore?

While drilling a well, a fine, soupy, and dense mud is continuously being pumped down the drill string (which turns the drilling bit), and it returns up the annulus of the wellbore, where it is then filtered via the “shakers”, and then recirculated back down the drill string.  Most of the time, it’s a closed loop process, meaning what is pumped down the well, eventually returns back up the wellbore to get reprocessed again.  The volume, and rate of return of the mud is highly predictable, and is easily calculated.

This scenario changes when a gas bubble enters the well bore.

As bubble migrates up the wellbore, it pushes the drilling mud (and well cuttings) above it up with greater and greater force as it continuously expands.  Drillers can sense this situation in progress when they start seeing an increase in the flow rate of mud returning from the well.   It’s not a fool-proof system however because the expansion of gas is exponential as is rises, and an increase in mud returns is not always an indication of a kick.  Without getting too technical and discussing all the different possible scenarios, the important factor is to understand that at the end of the day, if a kick goes unnoticed, and the rapid gas expansion goes unchecked, a “blowout” is guaranteed to occur.

Did this happen off Nigeria?

We’re still waiting on specific details, but it seems very likely the answer is yes.  Chevron is reporting that the surface well control equipment (the BOP) may have failed in some fashion.  They also report that Transocean’s GSF Baltic rig has been contracted to drill a relief well that will intercept the blown out well and kill it in a similar fashion as the BP’s Macondo well.

The unchecked flow from this well could also bridge off and kill itself naturally, but the best option in this case is certainly to try and stem the flow via the relief well.

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