When most of us switch on the hob, we don’t really think about where our gas came from, other than perhaps a vague awareness that there’s a pipe in the street. But the journey your gas makes is truly epic: it started millions of years ago and may cover thousands of miles.
How oil and gas forms
Oil and gas form in the same way. When tiny marine plants and algae die, they fall to the bottom of the ocean where, along with other sediments, they settle on the seabed. Over millions of years, this sediment builds up and the material lower down is squashed by the weight of the material above it.
Because this is an anaerobic environment with no oxygen, the tiny plants and animals don’t rot. Instead, they turn into a substance called kerogen, which comes from the Greek word for wax. The kerogen is heated, by the pressure from the sediment above it and the earth’s own internal heat below it. When it gets to around 150°C, it starts to change into oil; if the temperature rises further it may become gas.
Here, it is still in pores in the sedimentary rock around it. But if heat and pressure continue to squeeze it, the rock gives it up – rather like a sponge. These hydrocarbons then tend to flow and collect in gaps in the rock structure (or create new openings) where they form large, useful reservoirs.
Drilling a gas well
Once a gas source has been located and deemed worthy of exploration, a well is drilled. In a very basic sense, it’s the same as drilling into a wall at home, although instead of a few inches of brick it will be thousands of metres of rock. This means the drill head needs to lubricated with a special fluid called 'mud' which keeps it cool, carries away the drilling waste and prevents the borehole from filling with fluids such as groundwater.
After drilling, the well is lined with a casing that’s held in place with cement. If the drilling takes place at sea, a platform will need to be put in place first. In shallow water these are relatively simple and are built directly onto the sea floor; but in deep water they’re often complex floating structures which are anchored to the sea bed.
Wells like those in the North Sea may start thousands of metres below the surface and then continue deeper, through thousands of metres of rock until they reach the hydrocarbon reservoir.
Extracting the gas
Once the well is complete, a production facility is constructed to remove the gas. Gas wells demand incredibly robust engineering – they deal with fluids which are corrosive and under very high pressure, and the wells have working lives which are measured in decades.
The extracted gas is usually piped from the wells to refineries on land. It may also be transported as LNG (liquefied natural gas); this is formed by chilling the gas to the point that it becomes a liquid, taking up around 1/600th of the space it would need as a gas, and allowing large quantities to be transported by boats.
The refining and storing process
Refineries are giant chemical plants. Natural gas (methane) is purified and the other components such as propane and butane are separated out for use in other applications.
The gas may then be stored in tanks the size of large buildings or pumped directly into the distribution network. In the UK, this network comprises over 170,000 miles of pipeline.
There are two types of network: one is a kind of spine running the length of the country. This is a high pressure backbone which is linked to gas grids in other countries and supplies large industrial users and power stations. For home use, the gas is pumped from the high pressure network into local, low pressure networks –– connecting to the meter in your home which measures your usage – and ultimately flowing through to your boiler, as well as your cooker and fire, if they run on gas.
The fake smell of gas
For safety’s sake, there’s another crucial element that’s added, and that’s the familiar ‘rotten egg’ smell. Natural gas has no scent, so a non-toxic aroma is added to make it easy to smell a leak.
From ancient sea creatures to the sizzling sausage
And that’s the end of a journey that began hundreds of miles away, thousands of yards deep and millions of years ago. So as that breakfast sausage sizzles in the pan, and the boiler fires up for a bath, give silent thanks to the ancient sea creatures that have helped to make our lives immeasurably better, and to the human endeavour, technology and talent that takes gas from the seabed to your sitting room.
Words: Rhymer Rigby
Rhymer Rigby is a journalist who writes for various publications including the Daily Telegraph and the Financial Times. He is the author of two business books.