NETL Studying Upward Migration of Fracture Fluid
The State Journal
13 June 2012
By Pam Kasey
The cracks created by hydraulic fracturing at a site in
southwest Pennsylvania stayed more than a mile below aquifers,
according to first results from a two-part study under way by the
National Energy Technology Laboratory.
The research is taking place at a Pennsylvania site that a
researcher said is particularly interesting, because the three
horizontal wells drilled into the Marcellus shale, about 8,000
feet down, are overlain by a producing gas well in the Upper
"This lets us look at zonal isolation between these two gas fields
that are approximately 4,000 feet apart in depth," said NETL
geologist Richard Hammack. "We have a unique opportunity to see if
any fluids or gas from the Marcellus formation are making it to
this upper producing gas field."
Researchers are gathering data in two stages.
The first, just completed, is a microseismic study conducted in
conjunction with hydraulic fracturing.
By showing in three dimensions where rock is breaking, the
microseismic data map an "envelope" of the impact of fracturing
and help to answer ongoing questions about whether fracturing
could enable fluids or gas to flow upward to drinking water
aquifers, which are typically within hundreds of feet of the
"The (highest activity) we've seen is 1,500 feet up from the
Marcellus," Hammack said in summary of a preliminary look at
the data, which the agency only finished gathering earlier this
That's still more than a mile below drinking water aquifers.
These data are in line with data accumulated earlier from hundreds
of fracturing stages in the Barnett and Marcellus shales.
By contrast, modeling done by hydrogeologist Tom Myers concluded
earlier this year found that hydraulic fracturing could enable
fluids to travel to the surface.
The current NETL study adds to a picture of improbability,
although no study can ever prove it impossible.
"Geology is specific to any particular site," Hammack said. "If
there were more of these studies done in different areas, then you
could start to put together the big picture."
In a second phase of the research, the microseismic data will be
complemented by longer-term monitoring of the producing Upper
Devonian well that overlies these Marcellus wells.
"We're looking for indications of communication between the two
zones and we're using mobile lines of evidence to do that:
pressure differences as well as manmade and natural tracers,"
Pressure in a Marcellus well is typically 1,000 pounds per square
inch or more, while it's closer to 100 psi in the Devonian, he
said. A first indication of communication between the two zones
might be a pressure pulse in the Devonian, along with an increase
in gas production.
The natural tracer is strontium isotope ratios, which Hammack said
differ significantly between the Marcellus and Upper Devonian
"So if fluids from the Marcellus get into the produced water in
these upper Devonian fields, we're going to see a dramatic shift
in the strontium isotope ratios," he said. "Same with the gas —
we'll be monitoring both."
The agency also injected a manmade fluorocarbon tracer in the
Marcellus; detection of that in the Devonian gas will be
conclusive of communication.
That Upper Devonian monitoring, Hammack guessed, may continue for