Monday, March 15, 2010

Drilling Down Deep Over Offshore Oil.

The Economist has published an excellent article (link below) on the subject of deep-sea drilling. The first offshore oil well completely out of sight of land was drilled in 1947, 17 km off the Louisiana coast in the Gulf of Mexico. The platform of the drilling rig was no larger than a tennis court and was supplemented by several refurbished naval barges remaining from WWII which provided both space for storage and for the crew to sleep. The seabed was a mere 15 feet below, but now far greater depths of water are being fathomed.

In 2008, Shell's 22,000 tonne Perdido spar was towed from Finland where it was constructed to a spot 320 km off the coast of Texas, where it was chained to the seabed 2,400 metres below. This is connected to subsea wells in yet deeper water at 2,900 metres. Such offshore drilling operations are attended with daunting physical challenges. Since a longer and heavier drill string is required in deeper water, the supporting platform must be more heavily engineered. The interlocking sections of pipe are heavy, at around 30 kg/metre.

Since conventional onshore oil is held in countries that are unstable in regard to their political amenability to the West, offshore drilling is forcing private companies to look for oil further afield, in more inhospitable locations, and increasingly in deep water. The pressure of water increases by one atmosphere for each 10 meters of depth, and at almost 3 km, is close to 300 times normal atmospheric pressure, or around 4,500 pounds (2 tonnes) per square inch. The consequent pressure on the seafloor makes it more difficult to pump the oil up to the surface.

Hitting the right spot is a feat in its own right, and Robin Walker, of the oil-services company WesternGeco, uses the following analogy. "Imagine a large offshore oil rig as a matchbox. Next, imagine the matchbox on top of a two-storey building, with the upper floor filled with water and the lower floor filled with rock, sand and, in some cases, salt. Striking an oil reservoir with a drill pipe is then like hitting a coin at the base of the building with a strand of human hair." If the hit is in the wrong place, the costs are enormous, and an industry rule of thumb is that drilling a deepwater "dry hole" - a well with no oil - is around $100 million. BP says it can be as much as $200 million.

To avoid any errors, complex geophysical measurements are necessary including multi-dimensional seismic imaging, where a rough 3-D image is created of the subsurface rocks. Despite the difficulties, a number off major deepwater finds have been identified recently: Tupi, off the coast of Rio de Janieiro is thought to hold 8 billion barrels of oil, but this lies underneath 2,000 metres of water, 3,000 metres of sand and rock and a 2,000 metre layer of salt. Other "ultra-deepwater" discoveries, defined as those under 1,500 metres or more of water, have been made off the coasts of Angola, Sierra Leone and Nigeria, along with several more in the Gulf of Mexico.

Obtaining images of commingled salt and rock poses difficulties because the waves emitted from seismic sources travel faster through salt than in rock. When there is a mixture of reflected and refracted waves present, constructing an image of the subsurface from a normal sonar survey is not readily accomplished. Thus, rather than collecting seismic data in two dimensions, using streamers and them using computational processing to get a 3-D image, an actual 3-D data acquisition was done, using hydrophones and multiple seismic sources from three of four vessels moving in parallel, called a "wide-azimuth" survey. The accuracy can be honed further by passing over the same region a number of times from different angles ("multi-azimuth" survey).

The greatest challenge, however, is processing the data. When the surveys indicate there is a high probability that oil is down there, an exploratory well is drilled. This involves pumping a liquid called "mud" through the drill string to remove borehole cuttings and to cool the drill-bit and maintain pressure at the base of the well. As the drill cuts through the rock and sand under the seabed, the pressure of the "mud" in the drill must be kept within defined limits: if it is too low, the pressure of underground fluids and gases ("pore pressure") on the well wall will drive it to collapse, but if it is too high, the mud can accentuate and expand existing fractures in the surrounding rock which causes a loss in circulation as the mud leaks out into the newly formed fissures.

This is a simple overview, but all in all, the discovery and successful production of oil from these reservoirs offshore will depend on continual advances in technology and computation.

Related Reading.
"Plumbing the Depths."


Anonymous said...

Imagine my surprise when reading this in my local (Canadian)newspaper:
about how the USA outpaces Canada in green investment;

but it's less of a mystery if the EPA(USA) supports this US-based group:

Yes, deep sea drilling only works if the price of oil exceeds greatly that of the price for the used-up 'low lying fruit'. But the external costs are increasing ever faster, i.e. the cost of not abandoning 'black gold' as an economic engine.

Anonymous said...

From your description, deep ocean drilling is indeed an inspired technology. However, there must be a limit, of funds, of successes, of safety issues, of political issues etc.
You mention a Shell project with Perdidio Spar (?). I remember an affair involving another Shell rig, the Brent Spar. In this affair Shell found themselves dragged into the political/environmental limelight with the Case of the Dismantling of the Brent Spar. Do you recall it? It was about 40 years ago, Shell had a rig which had become obsolete, and they wanted to dispose of it. So they quite reasonably, in the views of people in the trade, decided to tow it out to sea and sink it. There it could decay and in the meantime provide a haven for marine life. They actually started preparing for the tow, when Greenpeace intervened. Brent Spar was a typical rig, about as big as a large office block.They decided that the sinking of the rig would cause massive pollution (it wouldn’t, all pollutants had been safely removed). Much to the astonishment of many people in the oil trade, Shell never mounted any serious defence against Greenpeace’s allegations, which Greenpeace backed up by organising a very effective campaign against buying Shell petrol, especially in Germany, which was big market for Shell, and miles from Scotland and Brent Spar. So in the face of loss of sales revenue, Shell capitulated, and Brent Spar was parked in Scotland and cut up with countless million cubic feet of acetylene.
This is an ironical ending to the story, since much later Greenpeace embraced man-made global warming and became one of the AGW’s most important advocates. Yet it could be argued that by forcing Shell to dismantle Brent Spar instead of sinking it in an environmentally friendly manner, Greenpeace as a group has made a huge contribution to AGW. It will be amusing to see what happens when Shell decide to scrap Perdido. Which course will please the environmentailst best do you think, sink it or acetylene cut it?
Anyway, all of this is by the way. Surely the deep offshore industry must inevitably, evolve into a remote, robotic industry, with the development of a whole host of new and as yet unimagined devices? The big rigs will go away and be replaced by motherships. The oceans will be, on the surface at least, as unchanged as ever, as quiet as a swan, on top, but pedalling furiously beneath the surface.
Rgds Peter Melia

Anonymous said...

Hi Chris, hope the continent is treating you well!
There is an article I noticed in "The straight Dope" and wondered if perhaps you might have missed it. Please don't infringe any copyrights in my name!
Peter Melia

Is peak phosphorus the next global crisis?

Professor Chris Rhodes said...

Hi Peter,

yes, peak phosphorus is a serious threat, as that article you refer to puts well.

In answer to your earlier question, my memories of Brent Spar are hazy, being a relatively young man of 50!

However, I do know the story and yes, I agree with what you say. Probably the main issue is not pollution, but the sheer cost of this type of deep-drilling and when it is done off Brazil say, when miles of rock and salt need to be drilled through, the price of a barrel of "deep-oil" is going to be very high.

So, however much oil is down there it's going to be an expensive commodity and peak oil is really the end of plentiful cheap oil rather than of oil per se.

Not quite sure what you mean about "copyright" but indeed I usually try to avoid infringing all such tights.

Best regards,


Mark said...


Could it be that we were led down the primrose path when it comes to how large oil companies are prepared in dealing with massive oil spills in very deep waters? The way BP has been handing the disaster at the Deep Water Horizon oil platform in the Gulf of Mexico appears that they had no clue in dealing with such incidents. It seems that in dealing with this spill, which some say BP and the US government have under estimated and could be larger than the Exxon Valdez disaster has been a seat at the pants effort at best. Needless to say I am not confident that this well will be capped anytime soon.

Maybe we should suspend new drilling in very deep waters until the technology is in place to deal with such disasters.

I know we need that oil those platforms produce but at what point does the risk outweigh the benefits of deep water drilling?

Professor Chris Rhodes said...


I think what this awful incident indicates is that much of the deep-water and other located oil that adds up to the final one trillion barrel reserve will not be easily got. Not only will the EROEI fall in recovering it but the novelty of the technology required to do so is likely to result in a number of different accidents, at least in the immediate term.

I agree, the prognosis is not good for clean recovery but we can't have it all ways. If we need oil on the scale we use it now, there will be attendant environmental costs, given that the "easy oil" is running short.

The present incident in the Gulf of Mexico does at least forewarn of what may come.


Mark said...


I wonder if president Obama has been reading your blog. During his press conference last Thursday he made a little noticed comment that pretty much all of the easy to get oil has been gotten and that is why BP has to drill so far under the ocean depths to get the stuff. That bit of candor is pretty rare for a politician but I guess it takes a huge disaster for the truth to come out.

Speaking of blogs, when are you going to start posting again. I find your writings an antidote for the happy horses*** press releases that the mainstream media prints verbatim.

Professor Chris Rhodes said...

Hi mark,

maybe he has! Indeed, I take this as a sign that the age of easy oil is coming to an end and it is hard to get oil that the world will need but will test the limits of technology.

I have been incredibly busy lately, both promoting my novel "University Shambles", setting up a new company and travelling around the world giving lectures and consulting.

Thanks for reminding me - I shall be posting more shortly, as the world remains a good source of material!