With no end to record-high energy prices in sight, those in the business of supplying our country with fuel are looking with increased enthusiasm to a variety of “unconventional” sources for oil and natural gas. Once deemed too expensive to produce economically, these reservoirs of nonstandard hydrocarbon energy are moving closer to center stage and taking a role in the transition to more renewable resources. Leading the way among these sources in the U.S. are the natural gas shales, which occur beneath the surface of large swaths of the country.
One of the busiest of these “shale plays” today is the Barnett Shale right here in Texas, a vast layer of sedimentary rock that stretches from Dallas to Fort Worth, beneath 21 counties. The shale originated during the Mississippian Period, about 325 to 350 million years ago. It is now buried beneath layers of sedimentary rock at depths from about 5,000 to 9,000 feet, with a thickness varying from about 50 feet to 1,000 feet. When a geological survey in the early 20th century found rich, organic black shale rock in an outcrop near the Barnett Stream, geologists named it the Barnett Shale. The Barnett Stream was named for John W. Barnett, a late 19th century settler.
The Barnett Shale has huge amounts of natural gas locked up inside it—as much as 250 trillion cubic feet. That amount could fill a cube almost 12 miles on each edge, enough to provide all the natural gas needed in the U.S. for ten years.
|photo credit: indigoprime|
Why are we just now getting this natural gas out?
In regular natural gas wells, once a driller reaches the gas reservoir, the gas flows naturally to the well through tiny pathways in the porous and permeable reservoir rock. The gas in the Barnett Shale is locked in tinier pores with no pathways to let it flow. If you just drill a well to it, the gas won’t budge.
Up until the last few decades, there was no economical way to get the gas out of the shale. Recently, we have solved that problem with new technology. 3D seismic imaging has made it possible to get accurate pictures of the underground layers so engineers can better plan well pathways to avoid obstacles like faults and water zones. Horizontal drilling lets us avoid sensitive areas on the surface like parks or schools, and put more of the well into the shale by going sideways rather than straight down. Finally, better hydraulic fracture methods allow us to inject high-pressure water into the rock to make millions of tiny cracks in the shale so the gas flows to the well. These procedures have made the Barnett Shale a very popular place for drilling.
The success of the Barnett Shale has renewed interest in other major shale formations in North America that were previously too tricky to drill and produce, including the Haynesville Shale on the Texas borders with Arkansas and Louisiana, the Marcellus and New Albany Shales in the Northeast, and numerous others. Some projections indicate that the Marcellus Shale may end up being more than twice the size of the Barnett. And then there are the Canadian gas shales, which have been estimated as high as one quadrillion cubic feet, a hundred times Canada’s current existing reserves. In comparison, the major Alaskan reservoirs of Prudhoe Bay and Point Thomson contain combined estimated total reserves of 35 trillion cubic feet.
|photo credit: sludgegulper|
Of course, everyone knows that statistics can always be manipulated to suit the purposes of the statistics spewer, and the real numbers of petroleum reserves are a constant source of agitated discussion by legions of petro-pundits, and no one can be too sure of exactly how much gas might be out there, but we do know—however you measure it—that there are significant amounts of recoverable gas locked up in shale, and if we use those reserves wisely, in combination with other unconventional fossil fuels and the right mix of renewable resources, we just might come out smelling like a rose—or possibly, due to the sulfur compounds called mercaptans added to consumer-ready natural gas to give it a detectable odor, like a rotten egg.