Friday, February 6, 2009

Why Horizontal Drilling?

There are many reasons why horizontal, or directional wells are valuable and almost indispensable to modern oil and gas production. The following simplified article explains just a few. The BIG question, from a geologist's point of view is "HOW does a person at the surface know where the drill bit is and where it is going?" Remember, the drill bit and hole can be up to tens of thousands of feet below the surface of the Earth and thousands of feet away from the surface location. In order to "steer" the well you must know where it is. In order to know what layers of rock the well has drilled through a geologist must be able to recognize them.

This is where the art and science of interpretation comes to play and I'll attempt to answer these questions in future posts.

Horizontal drilling (source)
We talk about "pools" of oil, but in fact the stuff exists between the grains of porous rock like sandstone. Oil can travel through rock, but so slowly that it would probably lose a race with a whip-tailed paramecium.
So if you're interested in oil, you gotta make house calls. Translated: You gotta drill right into the reservoir. Not only can it be a tiny target, but even if you hit a bulls-eye, the well may be unproductive. Say a vertical drill pierces 5,000 feet of rock into an oil reservoir that's 20 feet thick. Because oil moves slowly, the 20-foot exposure would not tap much oil.
Over time, of course, more oil would seep toward the well, Bergt says. "If you could wait one million years for nature to refill it, that would be great." But drillers can't wait that long, and "In the old days, you'd move 200 feet and drill another well."

Once upon a time, drillers extracted oil with this Swiss cheese routine; new techniques reduce the need for wells. (Image courtesy of the United States Department of Energy.)

Do the math.
In a big field, thatsa lotta holes. Expensive holes.
With horizontal drilling, the entire picture changes, Bergt says. "Instead of drilling 20 wells, you'd drill two or three for the same recovery." On land, the technique also reduces the "footprint," the area damaged by drilling operations. At sea, it allows drilling many wells from a single platform.
Bent pipe solution? Because the pipe that drives oil drills is surprisingly flexible, a horizontal well can snake around to reach isolated pockets or follow a reservoir that meanders across the terrain.
Horizontal drilling has evolved over the past 25 years, and even though it remains more expensive than vertical drilling, greater productivity led to rapid acceptance. Between 3,000 and 4,000 wells are drilled annually with the technology. The record hole is a long-haul monster that wanders almost 7 miles, on the coast of southern England in the Wytch Farm oil field.

This roller cone drill bit was adapted from one used by 19th -century dentist (NOT). Use it to cut hard and/or abrasive rock.
Courtesy University of California-Berkeley petroleum engineering program.

Coil tubing
Traditionally, oil bits were driven by 30-foot sections of steel pipe. Pulling a bit up for sharpening involved hours or days of yanking pipe out of the ground and unscrewing it. In the past five years, drillers have come up with an alternative -- coil tubing.
Packed in giant reels holding 4,000 feet of tubing, the stuff is simply unreeled and lowered into the hole. Instead of rotating the tubing to spin the bit, high pressure drilling mud is sent, as usual, through the tubing. At the other end, however, is a hydraulic motor that rotates in response to mud pressure.

Peter Meenan says coil tubing also lends itself to scavenger operations -- tapping pockets of petroleum that seismic techniques show are near to existing wells. Meenan, who directs the Oil and Gas Institute at the University of Aberdeen, Scotland, says coil-tubing drilling, combined with steerable drill bits, may be used when a new pocket of hydrocarbons is discovered, say, 1,000 feet from a deep well. Rather than drill from the surface, it's possible to start drilling part-way down and veer off to reach the new deposit.

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