Drilling for oil

To locate underground structures that may contain oil, geologists survey the Earth with devices such as gravity meters or magnetometers. With seismographs similar to those used to measure earthquakes, they explore what they cannot see by sending sound waves underground. (How long they take to return helps determine the depth and characteristics of the rock layers through which they travel.) Using computer models, geologists create three-dimensional pictures of sedimentary basins. With CAT scans and magnetic resonance imaging (MRI) – the same technology doctors use to peer inside the human body – they visualize the flow of oil through the rocks.

On land, exploration teams use trucks equipped with vibrators or explode dynamite to produce sound waves that reveal subterranean geological structures. Undersea, air guns send large bubbles of compressed air into the water to generate sound waves that penetrate the rocks of the sea floor; the patterns of the waves reflected back provide clues to geology below the sea

Oil exploration initially meant drilling near spots where oil seeped naturally to the surface, or making haphazard guesses about where to drill, usually with disappointing results. Even with modern technology, the search for oil is fraught with uncertainty. The odds are against striking oil in a new location. And even when oil is found, there is rarely enough to make production commercially viable.

To locate underground structures that may contain oil, geologists survey the Earth with devices such as gravity meters or magnetometers. With seismographs similar to those used to measure earthquakes, they explore what they cannot see by sending sound waves underground. (How long they take to return helps determine the depth and characteristics of the rock layers through which they travel.) Using computer models, geologists create three-dimensional pictures of sedimentary basins. With CAT scans and magnetic resonance imaging (MRI) – the same technology doctors use to peer inside the human body – they visualize the flow of oil through the rocks.

On land, exploration teams use trucks equipped with vibrators or explode dynamite to produce sound waves that reveal subterranean geological structures. Undersea, air guns send large bubbles of compressed air into the water to generate sound waves that penetrate the rocks of the sea floor; the patterns of the waves reflected back provide clues to geology below the sea.

Oil and natural gas are often discovered in dome-shaped structures called "anticlines." But oil does not reside in underground lakes as is commonly supposed. Instead, it is trapped in rocks with holes like sponges. Natural gas is dissolved in the oil or separates and rests atop the oil as a separate layer.

The porous rocks holding oil and natural gas may be far below the surface. So extracting them requires drilling a well. A rotary tool with a tough diamond bit drills through the rock. Drilling "mud"-a fluid composed of clay, water, and chemicals – keeps the bit cool. The drilling mud is pumped down with the bit and returns through the space between the drilling string and the borehole, carrying rock fragments with it.

Depending on the hardness of the rock, drilling proceeds at a rate anywhere from 30 to 60 meters an hour. Today, a typical U.S. oil well is more than half a kilometer deep.
In the days of the "gushers," the pressure of natural gas released when the drill reached its target sometimes forced petroleum to explode to the surface, polluting the environment and wasting precious oil. To prevent this, drills now have instruments that measure well pressure and special valves called "blow-out preventers."

But most of the oil trapped underground needs to be pumped out. To extract as much as possible, carbon dioxide, other gases, water, or chemicals are injected to maintain well pressure. An assembly of pipes and valves called a "Christmas tree" controls the flow at the wellhead.

Roughly a third of the world's oil comes from offshore wells, mainly in the North Sea, the Arabian Gulf, and the Gulf of Mexico. Offshore wells are usually drilled from fixed platforms, which may be as tall as skyscrapers. Wells can also be drilled from semi-submersible structures or tankers. The distance between the water's surface and the ocean floor, where the well starts, can be a kilometer or more.