Oil's origin
Crude oil -- properly called petroleum -- includes natural gas, a flammable fluid usually found with oil. Petroleum is the remains of organic material that was deposited millions of years ago. One seep, in Michigan's Upper Peninsula, comes from billion-year-old rocks, although most commercial petroleum was generated from rocks that are between 65 million and 213 million years old.
To distill our discussion, oil starts forming when organic matter accumulates and gets
covered quickly enough to shut off the supply of chemicals that bacteria need to oxidize
the carbon (otherwise, they will burn it and form carbon dioxide).
Microbes, explains Richard Kettler of the department of geological sciences at the university of Nebraska at Lincoln, are ubiquitous. "If there''s something to eat, if there's a chemical reaction they can run at a reasonable temperature and pressure, you can almost guarantee there will be a life form that will be using that reaction." And living, he stresses, means devouring the organic matter that would otherwise become oil.
We should mention that while a few renegade scientists claim that petroleum has an inorganic origin, the conventional wisdom, as presented above is pretty convincing:
We can watch organic sediments accumulating at the bottom of the ocean
Many chemicals in crude oil have a structure characteristic of molecules with organic origins
Hopanes, a group of hydrocarbon molecules found in petroleum, are made by bacteria, Kettler notes
Petroleum may also contain molecules of waxes that land plants use to prevent drying, and many markers of it
biological origin
Finally, there's a linguistic argument: Tack an "e" onto organic crud drifting down to the sea floor, and you got crude!
Hearing the Earth
One of the best ways to learn about conditions underground is to bang on the Earth. Much as family doctors (remember them?) thumped your back to find phlegm in your chest, oil prospectors have long used loud noises to find oil in what's called "seismic" work.
There are two reasons to use seismic soundings: sound sounds different when bounced off hard, non-porous rocks like granite compared to soft, porous, and possibly oil-bearing
rocks like sandstone. "The geophone (earth hearing microphone) can collect vibrations as they bounce off rocks" says Larry Nation, communications director, American Association of Petroleum Geologists.. The data are stuffed into computers that then draw a picture that makes sense to the human eye.
And the second reason? the old-time prospectors made those big noises by setting off TNT - and getting paid for it!
It is our sad duty to report that truck-mounted "thumpers" and underwater gas guns have largely displaced pyrotechnics in seismic searching. At first, a seismic peak inside the Earth produced two-dimensional (2-D) maps. Although helpful, these maps had as little depth as a computer screen. In the 1980s, however, properly located microphones that could ` detect the direction of a sound wave's origin added a third dimension, depth, to the maps (with the help of considerable computerised data massaging).
A decade ago, 3-D seismic started becoming as popular in the oil patch as mammoth mergers are today. In 1989 , only 5 percent of new wells in the Gulf of Mexico used 3-D, compared to 80 percent in 1996. One reflection of that advance was a rise in the success rate for new wells in the Gulf from 19 percent in 1985 to 40 percent in 1994.
One more D?
The latest bells and whistle is 4-D modeling, which adds the dimension of time. Don't be duped: this complicated-sounding method simply compares 3-D maps taken months or years apart. Four-dimensional modeling can answer some practical questions: How fast am I sucking this field dry? Where is the rest of the oil hiding, and how can I get my mitts on it?
As with earlier seismic techniques, 4-D depends on microphones and microprocessors. Once the domain of major oil companies, the drop in computer prices has made it available to smaller ones as well.
Four dimensional modeling is especially helpful for finding isolated pockets of oil, which are increasingly found in many played-out oil fields in the United States and the North Sea. Overall, the technique is part of what Nation calls an effort to "find out what's beneath the surface of the Earth without spending more than the oil is worth".
Measurements while drilling
Because dumbly drilling and hoping to drench yourself in a geyser of oil is, well, dumb, geologists appreciate a constant stream of information from the hole.
The first detailed data on what's underground came from instruments that were lowered into the hole. To use this "well-logging" technology, you would simply pull your entire drill string - which can weigh dozens of tons - out of a hole that may be a mile or more deep. Then you would lower a package of instruments while taking measurements. After withdrawing the instrument package, you would simply re-insert the drill string.
Although a considerable improvement over blind luck, well- logging gobbles hours of expensive drilling time. Far slicker would be piggy-backing the detectors on the drill - if you can figure out how to marry a bunch of sensitive instruments to a giant steel bit that's spinning 100 times a minute in a maelstrom of drilling mud and bucking worse than a rodeo bull.
The first so-called "measurements while drilling" packages lasted only a few hours before the instruments croaked. These days, says David Bergt, a drilling expert from Schlumberger, a giant oil-field services company, an instrument package can last 1,000 hours or more - plenty long enough to drill a monster hole.
One problem solved. But how do the instruments "phone home" from deep underground? Radio waves can't penetrate the Earth, so you can throw the idea of using a cell phone down the hole. Instead, the instruments use a variation on Morse code. They alter pressure in the drilling fluid, and a pressure detector at the surface reads this a binary code.
Bergt says these signals have many purposes, but first among equals is alerting drillers that they have actually reached oil. That was less problematic in the olden days, when petroleum was often pressurized enough to blow out of a hole in a gusher. These days gushers are frowned upon by environmentalists and oil drillers alike: the former for wasting the environment, and the latter for wasting oil.
To prevent gushers, the weight of drilling mud - the lubricant that floats drill cuttings to the surface - creates pressure that counterbalances the upward pressure of petroleum. Using mud, a hole "can drill right through the pay zone and you wouldn't know it" says Bergt.
It's irresistible
Instruments, however, allow you to continuously drill and only stop when you've found oil.
For an exploration well, you can put around a dozen fancy instruments on a drill. But if you're working
in an established field, you are most interested in porosity - so the oil can flow into the well - and the
electrical resistivity of the rock.
Petroleum, limestone and sandstone have very high resistivity when dry - they are insulators. But most underground rock is saturated with salty water, giving it great conductivity, but almost zero resistivity. It's the in-between zone that interests drillers, Bergt says. "Since oil is non-conductive, the overall formation resistivity goes up when oil pushes out of the water, so now we have a way to detect the presence of oil'.
Data from subsurface instruments can also guide a "streerable" drill bit through a narrow plane of oil. Each time the drill leaves the "pay-zone", the bit can be redirected to return to the good black crude.
Which brings us to the subject of drilling horizontal.
More bright ideas
Horizontal drilling
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've got to make house calls. Translated: You've got to 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 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."
Do the maths. In a big field, that's a lot of 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.
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 great 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 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.
Gas-to-liquid
What to do about all the natural gas that accompanies oil in remote regions? Gas, after all, is an incredibly useful material that contains lighter hydrocarbons like methane. The ancient Chinese delivered natural gas through bamboo pipes and burned it for lighting.
But gas is also expensive to ship by ship, and big steel pipelines cost a bundle. Now to the rescue comes a technology pioneered to supply Hitler's Luftwaffe -- converting natural gas to a liquid fuel.
"It's been talked about for so long that some people think it's like turning lead into gold," Nation says. The problem is efficiency: Most processes need high temperatures. To the rescue come catalysts -- chemicals that help other chemical reactions occur without getting consumed in the process.
There are signs that gas-to-liquid could be ready for prime time. A pilot plant has begun operation in Bellingham, Wash. More ambitiously, Chevron and Sasol, a firm in South Africa that once tried to convert coal into oil, are collaborating on a $1-billion plant for Nigeria.
The technology could be used to convert gas that's now burned off at oil wells into a usable fuel. In a larger sense, it could convert isolated, or "stranded," gas into a usable product. "If you found a huge gas field in the South China Sea," says Nation, "it might not justify building a billion-dollar pipeline to transport it. And if it's not economical, the gas is just not there." If gas-to-liquid works, however, a ship housing the conversion machinery could tread water above the wells, feeding tankers that would haul the fuel to market.
A CRUDE STORY
