Science Interviews


Sun, 4th Jun 2006

OIL : From formation to Extraction

Dr Nicky White, Bullard Laboratories, University of Cambridge

Part of the show Oil, Fuel Cells and Alternative Energy

Chris - Now tell us a bit about the science of oil because it's something we all take for granted. We just go to the filling station and just fill up. How much oil is being burnt off every second around the world? Do you know?

Nicky - That's not a question that's easy to answer. What I can say is how much oil we use annually. We use 30 billion barrels of oil world wide.

Chris - And how much is that in a barrel?

Nicky - There are six barrels of oil in a cubic metre, so a cubic metre is a large box; a metre by a metre by a metre.

Chris - So how much space underground is this that was oil and is now empty?

Nicky - Well that's a complicated question. The best way to answer it is to look att he North Sea. Most people probably know that the North Sea is an important oil province. If you go out into the middle of the North Sea there's about 200 metres of water and underneath that water there are about ten or fifteen kilometres of layered sediment; so dirt basically. If you go down about four kilometres into that dirt, what you find is oil. But it doesn't occur in caverns, it occurs within the pore spaces of sandy rock. A sandy rock would have a porosity of about 30% or so, which isn't very much. Those pore spaces would be filled with oil.

Chris - So how do we know where to go looking?

Nicky - Yes that's the hard bit for the oil industry. The easy picking were all on shore. The main way in which they were found was through oil seeps which occur at the surface. So that makes it very easy indeed. So the classic oil fields in places like Texas, Louisiana and indeed the Middle East were all found in that sort of easy way. And you can drill relatively cheap holes down to the oil.

Chris - And they must have some sort of cap over the top of where the oil is, to stop the oil floating up the surface, in other words trapping it.

Nicky - Yes. There are several ingredients to all this. You need something to make the oil in the first place but you certainly need something to trap it at depth. These sandy horizons which have the oil trapped in their pores usually have an impermeable layer, like an area of limestone or salt for example on top, which stops the oil coming out.

Chris - So what's the difference between oil and coal? They contain many of the same types of chemicals don't they?

Nicky - They do indeed. They're both basically decayed organic matter. So if you decay organic matter in the absence of oxygen, you'll get things like coal and oil. So why are they different? The main difference is that oil is formed from decaying algal matter primarily. This is rather surprising I suppose. Whereas coal is mostly due to the decay of things like trees and organic matter.

Chris - Why should there be that distinction? Is it obvious?

Nicky - The main reason is that when you decay organic matter, depending on whether it's trees, algae or indeed sheep, it'll have a different composition in an organic chemical sense. So when you cook it, which is the primary thing you need to do to release oil or gas, what you produce in the cooking depends on the ingredient you start with to some degree.

Chris - So we've got to the stage where we've produced the oil. I assume that most oil in this world is millions of millions of years old.

Nicky - Yes, so if we take the North Sea, the oil there was made between 60 and 30 million years ago.

Chris - So the end of the dinosaurs to fairly recently really.

Nicky - Yes.

Chris - And why is that the cut off? Was there not enough biomass, plants and living things on Earth to make things like that?

Nicky - It's a little bit of both, but it's primarily because of the cooking time. The layer in the North Sea which is the best source rock, which is the layer of organic matter, is a layer that occurred in Jurassic times. That's about 150 million years ago. That layer got progressively buried by later sediment pouring in on top of it, which made in sink down. As it did so it cooked up. It took all that time to cook and was basically ready between 60 and 30 million years ago.

Chris - So we have a porous sponge of rock which has this stuff stuck in it. How do you get it out? Most of us have this idea that someone sticks a drill bit down and then you see the old Wild West picture where loads of stuff comes gushing out the top of your derrick. How is it achieved?

Nicky - In places like Texas in the old days that was all you had to do, and people drilled more or less at random until they found something, which can be expensive even on shore. The trouble these days is that most oil is found off shore and it's rather hard to find. Drilling holes off shore is extremely expensive. So if you drill a hole in three kilometres of water off West Africa you could pay up to 60 million dollars per hole, so you kind of need to know it's there before you drill the hole. So what do you do? To find out what's down there you have to drill a hole. Well the major technique that is used by the oil industry to locate favourable structures and indeed favourable rocks is called echo-sounding. Ina way we think of it as a downward-looking Hubble, so it's a way in which we can produce spectacular 3D images of the sub-surface from the sea bed or the land surface down to about twenty kilometres depth. We can see that hidden world in fully three dimensions and identify structures. So it's still a risky business, but you can basically reduce the risk quite a bit by collecting all this echo-sounding data first.

Chris - So say you have ticks in all the right boxes and you drill a hole, how do you actually get the stuff out?

Nicky - The oil that occurs in the pore spaces of the buried sandy rocks is under quite a lot of pressure, so in fact it pretty much comes out of its own accord, at least to start with. In the lifetime of producing an oil field, that pressure starts to drop. One of the things you then need to do is encourage the pressure back up. The main way in which you can do that is put explosives down the bore hoels and break up the rocks, which encourages better flow. Or you can drill holes which are called water injectors. You drill extra holes around the side of the oil field and you pump down seawater. It's like pushing oil on a pan-full of water. You can push the oil towards the producing gaps.

Chris - So how much oil in the reserve can we actually get at? When you say that this field is spent, roughly how mush oil is actually left in there?

Nicky - Unfortunately most of the oil is left behind. We can produce between 30 and 40% economically.

Chris - That's terrible.

Nicky - It's appalling. We're getting a bit better at inching those numbers up but it's a great technological feat.

Chris - So how much oil is left in the world? Because when I was at school just over ten years ago, people were saying that by the millennium we'll be really worried about how much oil there is. Now people are saying figures like 300 years of coal, 50 years at least of oil. How much is actually out there still?

Nicky - There's a lot of doom and gloom. The pessimistic line is that if you take liquid hydrocarbons, so oil only and not coal or tar sands or gas, liquid hydrocarbon production will peak at about 30 billion barrels a year in something like 10 to 15 years time and then decrease rapidly over the next 100 years. However there are optimists who believe that even that scenario is too pessimistic.


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