by Hai Shunxi

Created on: December 29, 2009   Last Updated: August 04, 2010

The fuel of the twenty-first century is natural gas, at least in the United States. U S oil production has dropped steadily for decades, and “clean” coal is more promise than reality. Meanwhile, huge natural gas reserves have been identified, leading to surging importance of this clean-burning, easily-transported fuel. Many newly-identified gas reserves are, however, “unconventional reserves.” This means that exploiting them requires producers to use new technology, apply old technology in new ways, or both to locate and extract the gas.

Oil and gas companies have produced natural gas from conventional reservoirs for decades. In these reservoirs, natural gas is trapped deep underground in rock layers that are both porous and permeable. “Porous” means that there are spaces between grains of the rock where fluids like gas and oil can be stored. “Permeable” means that the spaces are connected so that fluids can move. In order for the gas to remain in a reservoir rock, they also need a “seal” to act like the cork of a bottle to keep the fluids in place. Many such seals are shale, a type of rock that isn’t permeable. In conventional reservoirs, oil and gas companies drill through sealing shale layers to the permeable layers beneath them and produce natural gas.

Geologists have long known that some shale layers have lots of pores filled with gas, but ignored them because their low permeability means that gas can't reach a well. In the 1990s, companies began using a drilling technique called horizontal drilling. A well drilled horizontally starts out vertical, but begins to bend high above the target zone and keeps curving until it runs sideways in the reservoir layer. The horizontal portion of a well can run for thousands of feet from where it began on the surface. It wasn’t long until someone realized that drilling thousands of feet sideways in a gas-rich shale layer would allow more gas to flow to the well than drilling a vertical hole straight through from top to bottom, a distance of a few tens of feet. That’s the “new technology” part.

Even a horizontal well thousands of feet long doesn’t produce much gas unless permeability can be improved; a process the industry calls “stimulation.” For this step, companies use existing technology called hydraulic fracturing; or as it’s known in the industry, “fracing” or “fracking.”

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