A deep look at oil and gas wells

A petroleum well at ground level isn’t that impressive. It is usually only two meters high with some control equipment and pipes around it.

But what is below the ground? How deep is it?

Generally a well is made up of multiple strings of steel pipe, cemented in place. These are all pressure-tested to ensure fluid isolation, and check the integrity of the subsurface system.

For comparison purposes, a water bore for use in agriculture, or to meet home needs, is typically around 20 to 200 meters deep, with a single pipe to hold the rock and soil from collapsing into the hole.

Petroleum wells target rocks laid down in the dinosaur times. We are talking Jurassic-type rocks when large reptiles strode our planet. To get to these rocks, we have to drill very deep.

In Australia, wells looking for these, and older rocks are typically 2000 to 4000 meters deep. The drilling rigs are holding kilometers of pipe beneath their drill floor, and turning the pipe kilometers away. Wells up to 5000 meters deep are fairly standard for the petroleum industry. To handle the drilling process, the drilling rig may have to be able to hold 400 tonnes of pipe while turning it with 1500 horsepower motors.

Some of the deepest wells in the world are found in Sakhalin in the Russian Far East. Well Z-44 is 12,376 meter deep, which is the equivalent to 15 times the height of the world tallest skyscraper the Burj Khalifa in Dubai. Imagine the next time you fly in a jet that there is a pipe hanging beneath the plane which reaches all the way to the ground – that is the length of the pipe, and is what a drilling rig is dealing with beneath the Earth’s surface.

The shaft of a well does not have to be vertical. New technology enables drillers to steer the pipe and drill at high angles, even horizontally to reach petroleum targets. In Australia, it is not unusual for a directional well to reach 3000 or more meters horizontally. This minimises the need to move offshore rigs around, and optimises the site so it has minimum impact on the local area.

The world record horizontal well according to the Guinness Book of records, is in Qatar, and reaches more than 10.9km from the well site.

The new coal seam gas fields in Queensland are quite shallow by petroleum standards. They are usually 400 to 1000 meters deep. One-tenth of the depths associated with other petroleum wells, but still hundreds of meters deeper than an average water well.

So the rather modest petroleum wellhead, is really much more than a tip of an iceberg – it reaches into a Jurassic world kilometers beneath our feet.

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