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Press, 6 December 1985, Page 10

From

“The Economist”,

London

The known reserves of natural gas in the United States are around 200 trillion cubic feet, with perhaps two or three times that amount not yet charted. These estimates take into account only reserves that are easy to exploit-so-called conventional sources.

Relying on these, America would, without imports and at its current rate of consumption of nearly 20 trillion cubic feet a year, exhaust all its known conventional reserves within a decade or so.

Small wonder, then, that the Department of Energy in Washington is seeking to open up unconventional sources—for example, the Devonian gas shales that lie under 275,000 square miles of Appalachia and other parts of eastern America. Up to 1100 trillion cubic feet of gas may be trapped in these shales.

The catch is that they, like most other unconventional sources, are too expensive to exploit with existing techniques. The term “unconventional” is something of a misnomer. The

first commercial gas well in the United States was drilled into Devonian shale—in 1821, near Fredonia in New York state. Since then, more than 10,000 shale wells have been drilled in Appalachia, but total production over the past 70 years has amounted to less than 3 trillion cubic feet, or about two months’ supply.

The problem is that the gas is diffused through rock that is dominated by vertical fractures. This means that a vertical well usually passes through a smallnumber of fractures. The result is a small, and unpredictable, amount of gas from each well. Two favourite tricks in the modern gas engineer’s repertoiresetting off underground explosions or pumping in high pressure liquids to open up more fractures—have achieved little success—althouth experiments with slow-bum explosives that give rise to a precalculated pressure wave have been encouraging. The Department of Energy is to investigate a simple idea that

might, it reckons, increase production up to four-fold: horizontal gas wells. Horizontal holes would pass through many more of the vertical fractures and so, perhaps, produce more gas. Though the idea of drilling a horizontal well is not new, the technology that makes it worthwhile on land is. The poser for technologists is to get a drill bit to turn sharply—in as little as 30 feet—after it has already gone straight down for perhaps several thousand feet.

Drilling round corners is difficult as well as expensive, but it can be done. One way of doing it is to have mud-powered motors at the end of the drill that have a built-in bias to kick out in a particular direction. As long as the whole assembly is slowly rotated from the surface the thing will keep on going in a more-or-less straight line. Stop turning it and off it goes to the side. This approach has the advantage that it is not necessary to change to a special drill bit for turning comers. If this solved all the difficulties, horizontal drilling would now be a

commonplace technique on land (something similar is done fron offshore oil platforms), but technologists also need to know exactly where the drill bit is goingessential information if* a hole hundreds of feet long is to be confined to a thin layer of rock. New sophisticated electronic instrument packages that go down the hole with the drill bit may be the answer.

The Department of Energy is impressed enough with the idea to have given B. D. M. Corporation of Virginia $1.2 million to drill an experimental horizontal well in Devonian shale. The plan then is to make new fractures intersecting the well by pumping in highpressure fluids at several different points, thereby creating (or so the department hopes) a network. Computer simulations predict a two- to four-fold increase in gas flow—which is usually more than enough to change an uneconomic well into an economic one.

If the technique proves successful, it might be applied to other heterogeneous gas sources where horizontal movement of gas is difficult: gas trapped in coal de-

posits, for instance. The department is investigating several other unconventional sources of gas apart from Devonian shales in the, hope of stimulating small energy companies to risk drilling. . \ Another is the “tight sand” deposits found in the western United States—sandstone that is much less permeable that the usual sort. In these, .small lens-shaped deposits of gas called “lenticular formations” might contain 240 trillion cubic feet of gas. Here, hydraulic fracturing with a mixture of a thickening agent, water, and sand is proving successful, although much remains to be done before exploitation becomes routine. A large part of the effort is devoted to developing improved diagnostic techniques that can determine the location and shape of individual lenses and map the growth of fractures.

The work is being conducted jointly by America’s Department of Energy and the Gas Research Institute, an organisation paid for by industry. Copyright, “The Economist,”