Kate Mackenzie How difficult are relief wells? Some comparisons with Montara

PTTEP Australasia

Source: PTTEP Australasia

Although the LMRP plan is still under way, focus is turning to BP’s effort to drill relief wells to ultimately stop the oil gushing from Deepwater into the Gulf of Mexico.

The relief wells will be critical to finally stopping the flow, whether or not the temporary fixes now under way are successful.

The Obama administration in mid-May seemed to be sceptical that the relief wells were a sure thing, although BP’s America chairman Lamar McKay said the company had a “high level of confidence” that it would be successful.

The question is: how long will they actually take to work?

The FT today looks at comparisons between the Gulf of Mexico disaster and the massive blowout of Pemex’s Ixtoc 1 exploration well in Mexican waters in 1979, which became one of the biggest offshore oil leaks ever.

A more recent incident, last year’s Montara oil leak in the Timor Sea, was eventually stopped by a relief well operation after more than 10 weeks. It provides plenty of examples of just how difficult relief well operations can be even today.

BP's relief well diagram guide. Click through for full description  and full-size imageA few caveats: the circumstances of the Montara well leak and relief wells were quite different to BP’s leaking Macondo well. It was in much shallower water (about 75 metres compared to Macondo’s 5km), in a completely different region, and leaking at a much smaller rate. PTTEP, the Thai-based operator of the well, says it was 400 barrels per day; BP’s well is estimated at 12,000 to 19,000 barrels per day. However both are extremely deep below the seabed; Montara was 2.6km, while BP’s are expected to intercept the well at more than 5km below the seabed.

A quick breakdown of the Montara relief well efforts:

- Waiting for the relief well drilling rig (three weeks)
- Drilling the relief well itself (three-and-a-half weeks, as PTTEP had predicted)
- Reaching the site of the leak – (four weeks, requiring five ‘passes’ and some changes of equipment)
- Securing the well by pumping fluid – drilling ‘mud’ and sea water – (three days)

We’ll look at these segments, and other aspects of the operation, in more detail below.

One well or two?

PTTEP, with advice from ALERT Well Control, drilled only one actual well to stop the Montara leak, but once the well was drilled to the approximate site of the leak, it took five passes to locate the 25cm piece of steel casing needed to dump mud down the well and ultimately block the leak with cement.  Each pass involved in a ‘sidewell’, most of which were then filled with cement.

BP is drilling two relief wells at Macondo, the first commenced on May 2 and the second, a ‘contingency’ well, almost a week later.

Drilling equipment

A big delay in stopping the Montara leak was the three-week wait for an appropriate jackup rig, the West Triton, to be moved from Indonesia to drill the relief well. PTTEP said it refused the offer of a more proximate semi-submersible rig because a more stable type of rig was needed for the ‘well kill’ operation.  BP was able to get the first relief well rig into place somewhat more quickly, beginning it 11 days after the explosion.

Drilling through the rock

Hard rock at depths of several kilometres can make drilling slow and unpredictable. Will this slow down the relief wells?

Jerry Milgram, a professor of ocean engineering at MIT, told DiscoveryNews:

Once the rig is in location, the long, difficult process of drilling can begin.

“In this case it’s kind of hard,” said Milgram. “You are drilling through mostly rock. It’s not going to be fast.”

When the same oil-bearing rock is eventually reached by the relief well drill rig, the sealing of the broken well can finally begin.

However, at Montara this only resulted in a few days’ delay. This was the cause of an unexpected delay in the second attempt at drilling to the Montara well leak, leading to a ‘more aggressive’ drilling assembly — (specifically, replacing the rotary steerable drilling assembly with a downhole motor assembly).

As BP points out in its description of how a relief well works, the drilling through rock might be somewhat easier due to its prior knowledge of the area. It can’t hurt, too, that the Gulf of Mexico — the site of the first offshore oil drilling – is among the most heavily-surveyed oil producing regions in the world.

Hitting the right spot

The sheer precision required of relief wells at such depth is likely to be one of the biggest challenges.

BP’s well relief guide points out:

“…drilling a well of this nature presents many technical challenges to ensure that the flowing well is intersected in the right position and that the fluid pumping operations are effective”.

This was also the big challenge at Montara; locating the precise location took almost a month, from October 6 to November 1.

The fluid pumping operations required two attempts but succeeded after about three days.

Sealing up the well

Once the right spot is located, actually sealing up is the next challenge.

In a process reminiscent of BP’s ‘top kill’, though in somewhat different circumstances, PTTEP injected thousands of barrels of dense drilling ‘mud’ into the well to gain control over the leak. Its first attempt failed after using an insufficient volume of mud, but a subsequent attempt, which shot 3,400 barrels and 1,000 barrels of brine into the well, succeeded in controlling the flow.


One thing Montara did not have to contend with was hurricanes; hurricane season began today in the Gulf and the National Oceanic and Atmospheric Administration is predicting it will be ‘active to very active‘, with 14 – 23 named storms and eight to 14 hurricanes.

Other disasters: Fires, and new blowouts from relief wells

On the same day that the well itself was intercepted, the original drilling rig, West Atlas, and the Montara well head platform caught fire, and this was not brought under control until the well itself was ‘killed’ three days later.  PTTEP says that the cause of the fire will be announced by the Commission of Inquiry that is investigating the accident, which is due to report this month.

BP itself identified another risk of the relief wells: that they would suffer a similar blowout to the original Macondo well, thus causing even more oil to leak into the ocean.

Bloomberg reported in mid-May that BP, in a regulatory filing with the Department of the Interior, warned that the relief wells risked a similar blowout to the original well:

The relief wells will pump cement into the leak to seal it. To do that, BP will need to first drill into the same deposit of oil and gas that caused a pressure surge known as a blowout at the original well, igniting an explosion that killed 11 workers and sank a $365 million drilling rig.

In a regulatory filing BP made to drill the relief wells it estimates another blowout could release as much as 240,000 barrels of oil a day into the ocean.

However BP’s chief executive Tony Hayward discounted the chances of that happening:

“The relief wells ultimately will be successful,” Hayward told reporters in Houston. Drilling back-to-back relief wells is a “belt and braces” approach, “and will assure ultimate success,” he said.

Related links:

US casts doubt on BP’s fix for oil leak - FT