In the United States, most experts believe that the fuel of the twenty-first century will be natural gas. U. S. oil production has dropped steadily for decades, and clean coal is less reality than a promise. Huge natural gas reserves have been identified, however, leading to increased visibility of a clean-burning, easily-transported fuel. Many of the new gas reserves are, however, termed “unconventional.” This means that companies must use new technology, apply old technology in new ways, or both to locate and produce the gas.
Oil and gas companies have produced natural gas for decades. In conventional reservoirs, gas is trapped deep underground in rock layers that are both porous and permeable. Porous means there are spaces within the rock to store natural gas or oil. Permeable means those spaces are connected so that oil and gas can flow. Another layer, a seal, acts like a bottle's stopper to keep gas and oil trapped in place. Many seals are shale, a rock that isn’t very permeable. In conventional reservoirs, companies drill through seals to the more permeable layers beneath them and produce oil or gas.
Geologists have long known that shale often has lots of pores filled with gas, but ignored them because their low permeability kept that gas from flowing into wells. In the 1990s, companies began using the technique of horizontal drilling. A well drilled horizontally starts as a vertical well, but begins curving high above the reservoir and keeps bending until it ends up going sideways. The horizontal portion of a well often reaches thousands of feet from where it enters the reservoir. It wasn’t long until someone realized that drilling sideways in a gas-rich shale layer would allow more gas to enter the well than poking a vertical hole straight from top to bottom. Horizontal drilling is the “new technology” part.
Even a horizontal well thousands of feet long can’t produce much gas unless shale's permeability can be increased by stimulation. To do this, companies use an existing technology called hydraulic fracturing, or fracking for short. Hydraulic fracturing is fairly simple: huge volumes of water and dissolved chemicals are pumped into the well under high pressure. Water can’t shrink, so the pressure cracks, or fractures, the rock around the well. Fracking also injects sand grains to prop the cracks open even when pressure is released. A frack job increases the permeability in a reservoir, allowing a well to empty pore spaces over a large area. This is a "new application of old technology."
Experts predict that three-quarters of gas wells drilled in the next decade will use horizontal drilling fracturing. Many frack jobs inject several million gallons of fluids. Once the well begins to produce gas, these fluids flow to the surface and must then be disposed of – often by re-injecting into a different subsurface layer.
Widespread use of this combination of technologies is allowing development of shale gas reservoirs across the U. S. Well-known gas reservoirs include the Marcellus (New York), New Albany (Ohio River Valley), Barnett (north Texas), Bakken (North Dakota), and Woodford (Oklahoma) Shales. The Marcellus and Barnett plays are especially active because both are relatively shallow and are found near large potential markets.
These two plays, however, are also under investigation by environmental agencies because of complaints by residents that their drinking water is being contaminated. Operators in both areas have been sued by homeowners and water companies who claim that fracturing fluids are leaking into shallow groundwater aquifers. Since federal law exempts hydraulic fracturing fluids from EPA (Environmental Protection Agency) regulation, plaintiffs claim that the companies do not reveal what chemicals are present in them.
What chemicals are used varies from well to well and from company to company (two of the biggest players in the fracturing business are Baker-Hughes and Halliburton). Though the chemicals are known to regulatory agencies, the "recipes" are trade secrets. Substances like toluene and methanol – both of which are carcinogenic – are used in very low concentrations in most fracking jobs. If a job calls for one million gallons of fluid, however; even a concentration of less than two per cent (a number cited by the industry) would leave 20,000 gallons of chemicals in the subsurface.
The oil and gas industry has mounted a campaign to fend off lawsuits, which begins by assuring the public that fracture technology has been in use for decades without such problems. This is, in fact, true: fracture technology has been used for decades without ill effects. Remember, however, that in the past fracturing was most common in reservoirs miles below the surface. Development of shale gas is most active in shallower deposits, where drilling is cheaper. Shale gas fracturing jobs thus take place closer to the surface; plus fracturing often extends along thousands of feet of horizontal well instead of around the twelve-inch diameter of a vertical well. The biggest difference may be that in conventional exploration, a reservoir is fractured and its seal remains intact. In a shale gas play, the seal is what is being fractured. Under these very different conditions, prior experience may not be a valid predictor.
Industry arguments are, in general, defensive: fracking has been safe in the past, chemically-treated water is carefully disposed of, and the chemicals are judged safe by local environmental agencies (even if the U. S. EPA does not regulate them). Still, independent laboratory tests have shown groundwater contamination near some shale gas developments. In these cases, contamination and drilling appear to be linked in time and space; so studies by regulatory agencies are ongoing.
The industry also campaigns on the basis of what they call “energy independence” as well as “homeland security,” because some estimates of the volume of shale gas in the U. S. place it at a ninety-year supply. Although the industry’s chauvinism may remind cynics of Samuel Johnson’s musings that “patriotism is the last refuge of a scoundrel,” the need to develop this valuable resource cannot be denied. Even with that need in mind, it is vital that regulators do not lose sight of the fact that clean water is even more essential. A balance must be reached, and the first step in reaching balance is careful study of the effects of hydraulic fracturing in shale gas development. If that study shows that fracking can, and has, contaminated groundwater supplies; then the next reasonable step would be to develop chemical mixtures to reduce harmful effects of contamination. No doubt, Halliburton and Baker-Hughes are already working on such new recipes.
For more information:
Does Fracking Cause Earthquakes?
American Petroleum Institute
U. S. Environmental Protection Agency
New York Times
Thursday, March 25, 2010
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