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Unconventional Natural Gas Reservoir in Pennsylvania Poised to Dramatically Increase US Production


By Dr. Terry Engelder
January 17, 2008


Natural gas distributed throughout the Marcellus black shale in northern
Appalachia could conservatively boost proven U.S. reserves by trillions of cubic
feet if gas production companies employ horizontal drilling techniques,
according to a Penn State and State University of New York, Fredonia, team.
"The value of this science could increment the net worth of U.S. energy
resources by a trillion dollars, plus or minus billions," says Terry Engelder,
professor of geosciences, at Penn State.

The Marcellus shale runs from the southern tier of New York, through the western
portion of Pennsylvania into the eastern half of Ohio and through West Virginia.
In Pennsylvania, the formation extends from the Appalachian plateau into the
western valley and ridge. This area has produced natural gas for years, but the
Marcellus shale, a deep layer of rock, is officially identified as holding a
relatively small amount of proven or potential reserves. However, many gas
production companies are now interested in the Marcellus.

Engelder, working with Gary Lash, professor of geoscience, SUNY Fredonia, has
conservatively estimated that the Marcellus shale contains 168 trillion cubic
feet of natural gas in place and optimistically suggests that the amounts could
be as high as 516 trillion cubic feet.

"Conservatively, we generally only consider 10 percent of gas in place as a
potential resource," says Engelder. "The key, of course, is that the Marcellus
is more easily produced by horizontal drilling across fractures, and until
recently, gas production companies seemed unaware of the presence of the natural
fractures necessary for magnifying the success of horizontal drilling in the
Marcellus."

The U.S. currently produces roughly 30 trillion cubic feet of gas a year, and
these numbers are dropping. According to Engelder, the technology exists to
recover 50 trillion cubic feet of gas from the Marcellus, thus keeping the U.S.
production up. If this recovery is realized, the Marcellus reservoir would be
considered a Super Giant gas field.

Engelder, who has studied this area of the U.S. for most of his career and began
looking into fractures under a National Science Foundation grant 25 years ago,
has identified and mapped natural fractures in the Marcellus shale. He and Lash
will present some of their recent work at the 2008 American Association of
Petroleum Geologists Annual Convention and Exhibition this spring.

The researchers look at the patterns of fractures in the shale and determine
which are important for gas production. Fractures that correlate with the
folding of the ridge and valley system are less common in black shale. However,
because of their orientation, the fractures that formed prior to the folding
will release gas if the wells cross the fracture zones.

These fractures, referred to as J1 fractures by Engelder and Lash, run as slices
from the northeast to the southwest in the Marcellus shale and are fairly close
together. While a vertical well may cross one of these fractures and other less
productive fractures, a horizontally drilled well aimed to the north northwest
will cross a series of very productive J1 fractures.

"It takes $800,000 to drill a vertical well in the Marcellus, but it takes $3
million to drill a horizontal well," says Engelder.

Companies that drill gas wells need to be certain that horizontal drilling will
produce the gas they expect and the work by Engelder and Lash suggests that it will.

"We know that the Marcellus shale appears as an outcrop near Batavia, N.Y., east
of Buffalo," says Engelder. "And we can see the fractures in the Marcellus in
the exposed sections of the ridge and valley areas to the southeast. Because we
see them going through the folded areas, we know they were there before the
folding. If it happened earlier, then we know they have to be in the intervening
basin as well."

The natural fractures in the Marcellus shale are the key to recovering large
amounts of gas. As heavily organic sediments were laid down 365 million years
ago, the black shale of the Marcellus formed. As the organic material decayed
and degraded, methane and other components of natural gas formed and dispersed
through the pores in the rock. About 300 million years ago, the pressure of the
gas caused fractures to form in the shale. It was not until 280 million years
ago that the eastern portion of Pennsylvania was pushed into the folding of the
ridge and valley province that makes up that area. Gas that occurs in pockets
underground is considered a conventional reservoir; gas that is distributed
throughout the rock, like the Marcellus, is called an unconventional reservoir.
The Penn State-Fredonia approach is not restricted to production of the
Marcellus shale, but can be applied to any gas-bearing shale with this type of
fracture. Because the approach begins with a vertical well and then drills
horizontally in the direction that will crosscut the productive fractures, old
vertical wells can be reused.

"We can go back to wells that are already drilled and played out, and then drill
horizontal from there," says Engelder. "Reusing old wells has both economic and
environmental value."

 

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