Getting the oil
What is crude oil
Photo of pouring crude oil
Pouring crude oil

The oil we find underground is called crude oil.

Crude oil is a mixture of hydrocarbons - from almost solid to gaseous. These were produced when tiny plants and animals decayed under layers of sand and mud millions of years ago.

Crude oil has to be changed before it can be used for anything. This happens in an oil refinery.

Crude oil doesn't always look the same – it depends where it comes from. Sometimes it is almost colourless, or it can be thick and black. But crude oil usually looks like thin, brown treacle.

It's not just the appearance of crude oil that changes. Crudes from different sources have different make-ups. Some may have more of the valuable lighter hydrocarbons and some may have more of the heavier hydrocarbons. The compositions of different crudes are measured and published in assays. The refinery uses the information in these assyas to decide which crudes it will buy to make the products that its customers need at any given time.

When crude oil comes out of a well (especially an undersea well), the crude oil is often mixed with gases, water and sand. It forms an emulsion with the water that looks a bit like caramel.

The sand is suspended in the emulsion, adding to the caramel effect. The sand will settle out and the water is removed using de-emulsifying agents. They have to be separated from the crude oil before it can be processed ready for transportation by tanker or pipeline.

The dissolved gases have to be removed at the well. Otherwise, they might come out of solution and cause a build up of pressure in a pipe or a tanker.

The crude oil also contains sulphur. This has to be removed from any fractions that are going to be burnt because it forms sulphur dioxide which contributes to acid rain. So any fractions that go into fuels pass through hydrofiners to remove the sulphur..

Photo of crude from a well
A mixture of crude oil, water and sand about two minutes after it has come out of the Captain field in the Moray Firth.

The caramelly effect is caused by the water and oil joining in an emulsion.