Kate Mackenzie Relief wells: Getting the interception right

Whatever the outcome of the latest efforts to contain oil flowing from the Macondo well in the Gulf of Mexico, the problem will ultimately only be fixed by the two relief wells.

BP has said the wells, for which drilling began on May 2 and May 19 respectively, will be completed in July or August. BP’s latest update and this diagram published on Friday (click through for full size), shows the well just shy of 16,000 feet – or about 2,000 feet away from its target depth [Update: to clarify, these distances include the water depth, which is why they are longer than the distances given by Unified Command.]

Regardless of how far the well is from reaching its target depth, intercepting the well look like being the next big unknown. The Ixtoc and Montara relief wells – both in much shallower waters – show how difficult this can be.

The relief well to stem the flow from Mexico’s 1979 Ixtoc oil leak – one of the biggest ever, and one which is being increasingly compared to the Gulf oil spill – took nine months.

Similarly, efforts to stop the Montara oil leak in the Timor Sea off Australia’s northwest coast last year ran into unexpected hitches on four occasions.

Drilling the relief well to the required depth is one thing; that was carried out within the expected time frame in the Montara leak. But actually intercepting the well at the right point took more than three weeks and five separate attempts to hit the right spot, despite coming within about 70cm of the target by the second pass. There were several causes of delay: equipment needed to be changed as harder rock than expected was encountered, enhanced monitoring needed to be carried out, and each unsuccessful effort had to be stopped up with concrete. Actually killing the well only took a few days once the interception was successful.

There’s less information available about the Ixtoc relief wells, but this paper suggests they took well over six months to effectively kill the well. One of the relief wells, Ixtoc-B, which started in mid-July 1979, reached the well in December; while Ixtoc-A, which had commenced drilling a month earlier, did not reach the well until February. The well was not killed until late March 1980, more than nine months after the leak began.

Admiral Thad Allen, the commanding officer in charge of the Gulf of Mexico crisis response, told a media conference call on Thursday that the drilling was ahead of schedule, in terms of reaching its required depth, and that the relief wells could come close to the target within three to four weeks, though he later said this didn’t mean it would be intercepted at that point:

What they are doing, I wanted to go through how they do this.  They’re going to come very close to the well very shortly.  And what they do is they actually come, they actually pass by the well and then turn down and go down about a thousand feet and then go back into it.  That allows them to very accurately locate the well, the well casing and where they actually want to enter.

So they’re going to be very close to the well in the next couple of weeks.  The last thousand feet and then drilling through the casing are what becomes very, very tricky and has to be done very, very carefully and precisely.

He also said:

We anticipate over the next three to four weeks they will close in and be able to tap into the well itself.

But when exactly is the target reached?

POLESON: So they’re close to the well, close to the well is this month.  Actually intercepting it is still August.

ADMIRAL ALLEN: Correct.  And it could be sooner than that because they are ahead of schedule but I don’t think we should (inaudible) till we know that they’ve actually got down there and been able to intercept the well.

Confused? So are we. Time for another Kent Wells video, perhaps; until then, this list of bullet points will have to do.

Update: BP spokesman Robert Wine has clarified the process somewhat. By email:

The first well from the DD3 is 200ft laterally (sideways) from the well. It will “range in” using electro-magentic sensors – detects the presence of the steel casing – drill a bit closer, sense, drill, sense, etc getting closer each time. The intention is to draw close to the leaking
well, and run alongside for a length. This will give best chance of ‘flooding’ the surrounding stratum, the annulus and the well bore with mud, then cementing it.

He adds that the drilling progress will slow as the target is homed in on.

Allen seems to be saying the interception will take place in the next three to four weeks (probably) and killing the well could take another month after that. Montara only took three days to kill once the well was intercepted; and that included a delay when not enough drilling mud was available. However Montara was in much shallower waters (80 metres) than Macondo, and the relief wells were about half as deep as those being drilling for Macondo.

In any case, the timing of both the interception and the kill remain open questions.

The New York Times quoted a couple of sources saying that the Macondo relief well will be easier then previous experience suggests – because the Macondo well is already better surveyed than Montara, and the technology today is much better than was available for the Ixtoc relief wells.

Let’s hope they are correct; other engineers have suggested that “taking into account the differences in technology, Ixtoc was probably as difficult to tackle as Macondo is today”.

BP itself says:

A relief well should be faster to drill than the original well thanks to the knowledge already gained about the geology and pressure in the reservoir. However, 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 and cement pumping operations are effective.

Related links:

Why not drill the relief wells first? - FT Energy Source
How difficult are relief wells? Some comparisons with Montara - FT Energy Source