When oil was struck for the first time in the Forties Field under the North Sea in 1969, it led to the discovery of at least 350 million tons of oil. But how is oil actually won from beneath the sea or the land?
Oil wells are drilled with special cutting tools, known as drill bits, which spin round to chip away at the rock. The steel, or diamond-studded steel drill bit is at the end of a strong steel pipe called the drill string, which is rotated either by a motor at the surface or by a turbine down the hole.
The rock chippings are carried upwards and out of the hole by pumping a material known as ‘mud’ down through the drill string. It is not real mud, but a combination of chemicals and water which brings up the chippings and prevents the drill bit from becoming too hot from friction.
As the hole gets deeper, fresh sections of drill string have to be added – usually in 30ft (9m) lengths. At the top of the drill string is the Kelly, which fits into a rotating table on the floor of the drilling derrick, like a nut fits a spanner. To fit a new section, the drill string is lifted up enough to remove the Kelly, then the new section is attached to the top of the drill string before the Kelly is replaced – allowing drilling to continue.
From time to time – every few hours, or every few days, depending on the rock – the bit itself has to be replaced. Then the entire drill string has to be pulled up, separated into 90ft (27m) ‘stands’, each consisting of three lengths, and stacked vertically on the derrick. When the bit finally emerges and is replaced with a new one, the whole string has to be reassembled and lowered down the hole again. The process, known as a ‘round trip’, can take up to ten hours if the well is already deep.
To prevent the hole caving it, it is lined with casing – heavy steel pipes are lowered in as drilling proceeds and cement is pumped around them to fix them in place. The casting gets progressively narrower as the well deepens. A 15,000ft (4500m) well may have 30in (760mm) diameter casing at the surface, decreasing in steps to 7in (180mm) at the bottom.
If the drill strikes oil, the weight of the mud ensures that the oil cannot escape, but there is an additional safeguard – a special valve called a blow-out preventer is fixed to the top of the casing.
The rate at which a well is drilled depends entirely on the type of rock. It can be as slow as 12in (300mm) an hour in the impervious cap rock, or as fast as 200ft (60m) in soft, sandy rock.
When oil is found, a whole series of production wells has to be drilled to bring it to the surface.
Offshore and in difficult terrain, the first step is to drill a number of wells designed to reach all corners of the oil-bearing rock. This can be done from a single derrick by angling the holes to different parts of the oil field. In a large field, several derricks or drilling platforms may be used, each drilling directionally according to a plan so that the whole area is exploited.
When the production wells have been drilled and lined with casting, a perforating gun is lowered down them to drive explosive charges through the casing and cement and into the rock beyond – allowing the oil to get into the reservoir rock to displace the oil towards the production wells.
Later, electrical or mechanical pumps may be used. But even with the help of such techniques it is seldom possible to extract more than about 30 to 50 per cent of the oil in a field.
Picture Credit : Google