By Robert M. Koerner
The Marcellus Formation, also known as the Marcellus Subgroup of the Hamilton Group, Marcellus Member of the Romney Formation, or simply the Marcellus Shale, is a unit of marine sedimentary rock found in eastern North America.
Named for a distinctive outcropping near the village of Marcellus, N.Y., it extends throughout much of the Appalachian Basin (see map–right). The shale contains a massive supply of untapped natural gas reserves, and its proximity to the high-demand markets along the East Coast of the United States makes it an attractive target for energy development.
In April 2009, the U.S. Department of Energy estimated the Marcellus contains 262 trillion cubic feet of recoverable gas.(1) State University of New York geology professor Gary Lash has calculated that more than 500 trillion cubic feet (14,000km3) of natural gas may be contained in the Marcellus black shale beds that lie between New York state and West Virginia(2).
In 2008, Terry Engelder, a Pennsylvania State University geosciences professor, estimated the amount of natural gas in the Marcellus at 363 trillion cubic feet of recoverable resource, which would be enough to supply U.S. consumption for at least 14 years.(3) If the entire formation contained gas, Engelder said the formation could contain 4,359 trillion cubic feet. Assuming a 30% recovery rate, this would lead to a 1,307 trillion cubic foot resource.(4) That would be a 40-year supply for the entire U.S.!
As The Economist reports,(7) the situation of gas recovery from shale rock is a worldwide phenomenon.
For example, across Europe a stealthy land-grab is under way. Exxon Mobil is drilling in Germany’s Lower Saxony. ConocoPhillips has joined 3 Legs Resources, a small firm based on the Isle of Man, to explore a large tract of land in Poland. Austria’s OMV is testing geologic formations near Vienna. Shell is targeting Sweden.
A host of smaller firms is fanning out across other countries, including France. They are all looking for natural gas trapped in shale as a resource that has transformed the market for gas in America and may have a big impact on Europe, too.
The extent of such “unconventional” gas reserves in Europe is unknown. The International Energy Agency (IEA), which monitors the energy business for rich countries, recently estimated it at 35 trillion cubic meters—far less than in North America or Russia, but about six times the continent’s conventional reserves.
That would be enough, the IEA calculates, to displace 40 years of gas imports at current levels. Almost half of it is thought to be in shale; the rest comes from coalbed methane and tight gas. The German Research Centre for Geosciences is in the midst of a more-detailed assessment, backed by oil firms.
The Marcellus shale formation is extremely thick in its central locations, e.g., about 900ft (270m) in Pennsylvania, and it “pinches” out in the west by the Cincinnati Arch and in the north by Canada.(5) Unfortunately, a major factor in the recovery of gas from the formation is that it is deep. It is, for example, 5000ft (1500m) in most locations and drilling must go through the upper formations to reach the gas reserves.
To capture the gas, two drilling technologies are used. One is horizontal drilling, with a vertical well reoriented to the horizontal so that it penetrates a maximum number of vertical rock fractures and extends a maximum distance within the gas-bearing rock (see sketch–left).
The second method is “hydrofracting” (or hydraulic fracturing). With this technique, water is pumped into the vertical portion of the well (which is solid casing) and then it continues into the perforated horizontal portion to produce a pressure that is high enough to fracture the surrounding rock. This water contains sand, called a “propant,” which prevents the fractured shale from collapsing back to its original tight formation.
The result is a highly fractured stratum penetrated by a long length of perforated well bore held open by the sand. Upon release of the water pressure a backflush of contaminated wastewater comes to the surface along with, and followed by, the natural gas. It goes without saying that gas royalties to property owners, state agencies, and other related parties is an active, and usually quite contentious, situation.(6)
Disposal and contamination issues
At least 4,000 new oil and gas wells were drilled in Pennsylvania in 2008, more than in any other state except Texas. This intense activity has forced state regulators to confront a problem that has been overlooked as gas drilling accelerates nationwide: How will the industry dispose of the enormous amount of wastewater it produces?
Oil and gas wells generate about nine million gallons of wastewater per day in Pennsylvania, according to industry estimates used by the Department of Environmental Protection (DEP). By 2011, that figure is expected to rise to at least 19 million gallons. That’s more than all of the state’s combined waterways can safety absorb, DEP officials say.
Much of the wastewater is the byproduct of the drilling process called hydraulic fracturing (or “fracking”), which pumps at least a million gallons of water per well into the targeted formation to break the layers of rock and release the gas. When the water is depressurized and brought back to the surface, it can contain natural toxins accumulated during drilling, including cadmium and benzene.(6) It can also contain small amounts of chemicals added to enhance drilling.
That said, DEP officials say one of the most worrisome contaminants in the wastewater is a gritty substance called total dissolved solids (TDS), a mixture of salt and other minerals found underground. Drilling wastewater contains so much TDS that is can be five times as salty as sea water. It is generally referred to as “brine.” Drilling companies currently dispose of this brine in municipal sewage plants, which then discharge it into rivers and streams.
The U. S. Environmental Protection Agency warns against this practice because sewage plants are not designed to remove TDS or any other chemicals the water may contain. Of even more concern, TDS can disrupt the plants’ treatment of ordinary sewage by killing microorganisms that are needed to treat human waste.(5)
Geosynthetic-lined surface impoundments
At minimum, geomembrane—and possibly geosynthetic clay liner barrier—systems can provide for long-term detention of the waste brine.
As seen in the aerial photograph above, a relatively small “self-contained” enterprise consists of the drilling operation and ancillary equipment (sometimes including temporary housing) along with a surface impoundment. While each surface impoundment is quite small (typically about 2.5 acres or 1.0 ha), when multiplied by the number of wells there is a very large market for geosynthetics in this type of application.
When an experienced designer considers, however, that geomembranes used to line such surface impoundments are often accompanied by “whales” or “hippos,” the situation often requires a drainage system located beneath the liner itself. Thus geotextiles, drainage materials and composites, geopipe, and other related products could also be involved.
Lastly, the issue of liner longevity for such an application is a vexing issue for all involved, certainly the local regulatory agency. At the heart of the issue is the eventual disposal of the contaminated brine. This is a current question for all state agencies having such gas-bearing geologic formations.
Bob Koerner, Ph.D., P.E., NAE, is director of the Geosynthetic Institute in Folsom, Pa., and is a member of Geosynthetics magazine’s Editorial Advisory Committee. GSI has conducted presentations for various groups regarding the design of liner systems as described here. GSI: +1 610 522 8440, www.geosynthetic-institute.org
1U.S. Department of Energy (April 2009): Modern shale gas development in the United States: A primer, p. 17, PDF file, downloaded June 11, 2009.
2Bertola, David (2009-02-08). “Researchers: Shale holds vast supply of natural gas.” Business First of Buffalo.
3Esch, Mary (2008-11-04). “Estimated gas yield from Marcellus shale goes up”. Philly.com.
4“Got gas, lots,” Pittsburgh Tribune-Review, Nov. 5, 2008. www.pittsburghlive.com/x/pittsburghtrib/opinion/archive/s_596812.htm.
5Harper, J. A. (2008), “The Marcellus shale, an old ‘new’ gas reservoir in Pennsylvania,” Pennsylvania Geology, Vol. 38, No. 1, Pennsylvania Bureau of Topographic and Geologic Survey.
6Sapien, J. (Oct. 4, 2009), “What can be done with wastewater?” Pittsburgh Post-Gazette, www.postgazette.com/pg/09277/1002919-113.stm.
7The Economist (2009), “Bubbling Under: The hunt for shale gas in Europe,” December 3, 2009, p. 75.