Article: Cool New Tools Look into the Earth
Getting oil out of the Earth is a complicated and risky business. Success depends on evaluating many different kinds of information, some obtained from chipping a piece of rock off a mountain ridge, some from peering through an electron microscope or sifting through the slurry coming up from a well, and some obtained by geophysical exploration techniques.
Over the last few decades, new technologies have transformed the field of oil exploration, enabling geologists to:
• choose an optimal location and drill fewer wells. This results in fewer dry holes, cuts exploration costs, and reduces environmental impact.
• identify as-yet-untapped oil reserves in frontier areas.
• learn more about the reservoir rock as drilling proceeds (e.g., how readily the rock conducts fluid) and tailor extraction methods accordingly.
• track the status of a hydrocarbon reservoir across all stages of its life in order to extract the maximum amount.
Lab Work Traces the Geologic History of the Rocks and the Region
Some of the most valuable information for scientists interested in "reading the rocks" comes from the microfossils of tiny living organisms left inside shales and sandstones. These help determine the age of the rock, and when geologists compare samples from different regions, they can infer a history of the area. Index fossils--organisms that lived on Earth only briefly--are especially useful as time markers that can be tracked from basin to basin.
Some microfossils help geologists track the thermal history of the surrounding rock: whether it's been heated enough to create hydrocarbons. Vitrinite represents the remains of woody plant material and is abundant in many shales. It becomes more reflective as it is buried, compacted, and heated. Vitrinite reflectivity is directly proportional to temperature, so it's widely used to measure whether sedimentary rocks that contain organic carbon reached temperatures high enough to produce hydrocarbons. Conodonts are small tooth-shaped microfossils related to early jawless fishes. They grew by acquiring thin layers of a transparent mineral called apatite interspersed with thin layers of organic matter. When sediments containing conodonts are converted to rock, the organic matter within the microfossil changes from yellow to brown to black to gray to crystal clear, depending on how hot the sediments got during the process. Analyzing this color spectrum is another way to measure whether sedimentary rocks have been heated enough to create hydrocarbons.
Other information--such as the absolute age of rock layers, established through radiometric age-dating (measuring the rate at which certain elements naturally decay), and the relative age of rocks, established by examining position and the fossil content--also contributes to a detailed geologic portrait that is invaluable to geologists and petroleum engineers. Evaluating both the relative and absolute ages of various features in the rock allows geologists to assess when structural traps formed relative to the formation and migration of hydrocarbons in the region. This is extremely important in evaluating a potential oil reserve, because a trapping mechanism that developed relatively late is far less likely to be as effective as one that developed before a significant amount of hydrocarbons accumulated.
Exploration Begins with Sound Waves
Just as ultrasound uses acoustic waves to generate a picture of a fetus inside the womb, seismic exploration tracks the echoes made by acoustic waves as they travel into the subsurface and reflect back to the surface. Conducting a 3D seismic survey involves laying down straight lines of sensitive microphones across the ground in a tight grid, typically 100 feet apart. Specially designed vibrating trucks, which have replaced the dynamite charges used until about 20 years ago, generate sound waves of different frequencies. The microphones pick up the reflected signals, which are recorded on magnetic tape and sent to data processors. These in turn generate a near-continuous cube of dataa three-dimensional portrait of the inner Earthrendering maps and cross sections obsolete. As former exploration geophysicist and current research scientist at the American Museum of Natural History, Charlie Mandeville, points out, "This is an immense asset in an area of complex structural geology."
Drills on the Cutting Edge
Computer-generated simulations of Earth's crust are full of clues about where to drill-but they're only clues. As Jennifer Burton, site geologist with Anadarko Petroleum puts it, "The well bore is the ground truth."
Not long ago, the drill consisted of fairly rigid 4 1/2-inch steel pipes, connected to a bit, that were rotated from the surface. Coiled-tube drilling has changed all that. The tube itself is made of flexible steel stored on huge spools and capable of bending in a big arc. Once in the subsurface, the pipe stays stationary as "mud" (once actual mud, and now a synthetic mixture) is pumped down the drill hole at high pressure. This mud powers a turbine that rotates a bit at the end of the tubing.
The mud is dense enough to float small rock fragments, called cuttings, to the surface, which technicians analyze for yet more information about the rock's oil-bearing potential. Sensors studding the drill surface also transmit a steady stream of data about rock type, thickness, temperature, porosity, water saturation (indicating what percentage of oil remains in the rock), and other characteristics. Not only is far more information available, it's available in real time, which is crucial during horizontal drilling because it enables geoscientists to follow thin reservoir layers that contain oil and/or gas.
Horizontal drilling is the other major breakthrough. Unlike the near-vertical paths of earlier conventional drilling, coiled-tube technology makes it possible to drill virtually horizontally, and to travel four or fives miles out from the borehole. That means that a drill pad doesn't have to be placed in an environmentally sensitive area like a wetland or migration route in order for the oil beneath the area to be extracted. Several boreholes can extend in different directions from a single drill pad, and old wells can provide access for horizontal drills. The payoff is big. According to the Department of Energy, "Because of both economic and environmental restrictions, development of the North Slope would have halted several years ago without the advent of horizontal wells."
An Evolving Picture of the Rock Below
Oil exploration really gets fine-tuned when seismic and drilling information are combined over the life of a reservoir. At the cutting edge is 4D seismic (the fourth dimension is time), in which a time-lapse picture is built out of data re-recorded at intervals and plotted by computer onto a 3D model. Combined with direct samples acquired during drilling, this data shows petrochemical engineers and production geologists how characteristics such as fluid saturation, temperature, and fluid movements change over time. This in turn guides secondary and tertiary recovery efforts, because, as Mandeville explains, "What you flood the reservoir with, to force the oil and gas out, depends on what type of cements and framework grains are present." Further, 4D seismic data allows exploration geologists and geophysicists to better recognize the characteristic seismic "signatures" of hydrocarbon-bearing rock formations versus those that hold water, an insight they can apply to future explorations.
The Limits of High-Tech Tools: a Limited Oil Supply
According to figures from the U.S. Department of Energy, at the current rate of consumption--approximately 77 million barrels a day, from a global reserve of approximately 1,000 billion barrels--only 36 years' worth of crude oil remain within the Earth's crust.
Some energy experts believe that advances in exploration and drilling technologies will simply empty existing reserves faster. They foresee a peak in oil production around 2010, followed by a steep decline by mid-century.
Others argue that these advances, paired with higher prices, will greatly increase the amount of oil that can be extracted economically from reserves, as has indeed happened in recent decades. They tend to assume that production rates will stay virtually steady until the supply runs out. This optimistic position is exemplified in a March 2002 report from the Department of Energy titled "Oil Resources in the Twenty-First Century: What Shortage?" The report anticipates that non-conventional resources--defined as those that "cannot be produced economically at today's prices and technology"--"will act as a buffer against prolonged periods of high oil prices well into the middle of this century, and perhaps well beyond." No one disputes that oil reserves are finite and only a few decades separate the two sides of the debate.
As Mark Myers, director of the Alaska Division of Oil and Gas, puts it, "Development in ANWR would have significant effects and advantages for the United States. But it's not going to solve the problem. We need multiple tiers to improve our energy situation. We need better energy efficiency. We need alternate fuels technology."
More About This Resource...
Our innovative Science Bulletins are an online and exhibition program that offers the public a window into the excitement of scientific discovery. This essay was published in December 2002 as part of the Reading the Rocks: The Search for Oil in ANWR Earth Feature.
- It begins by explaining that getting oil out of the Earth is a complicated and risky business.
- It then details how new technologies have transformed the field of oil exploration over the past few decades.
- It concludes with a look at some of the limits of these high-tech tools.
Supplement a study of earth science with a classroom activity drawn from this Science Bulletin essay.
- Have students read the essay (either online or a printed copy).
- Working individually or in small groups, have them investigate one of the new technologies discussed in the essay and report their additional findings to the class.
SubtopicTools and Methods