American Oil and Gas Reporter - November 2016 - 65

SpecialReport: MWD/LWD Technology
FIGURE 3
Permeability and Net Stress Walsh Plot
1.0

0.75
(k/k0) 1⁄3

altered by the direction of the pressure
change (increasing versus decreasing), suggesting the measurements were not impacted
by gas adsorption. This was expected since
helium is a nonadsorbent gas.
Furthermore, it was clear that the measured permeability values were impacted
by gas pressure. Therefore, it was necessary
to evaluate the absolute permeability of
the sample by applying the gas correction.
Figures 1A and 1B illustrate the application of gas-slippage (Klinkenberg)
and double-slippage corrections to the
measured permeability values at different
gas pressures using helium. As Figure
1A illustrates, applying the gas-slippage
correction resulted in a negative absolute
permeability value. However, applying
the double-slippage correction provided
a plausible absolute permeability value
(124 nanodarcies). Therefore, it can be
concluded that transition flow was prevalent under the gas pressure range utilized
during these experiments.
Using nitrogen at a constant net stress,
the measured permeability values were
altered by the direction of the pressure
change (increasing versus decreasing),
suggesting the measurements were impacted by nitrogen adsorption. Similarly,
the absolute permeability was determined
for both sets of measurements (adsorption
and desorption) by applying the doubleslippage correction.
The absolute permeability values agree
with each other (114 nD). However, it is
important to note that the absolute permeability obtained from nitrogen measurements is lower than the absolute permeability obtained from helium measurements.
This difference can be attributed to rock
expansion (swelling) caused by nitrogen
adsorption, which reduces the pore radius
and leads to lower permeability.
The results of the next set of experiments were used to determine the impact
of stress on absolute permeability. The
absolute permeability of the core plug at
each stress level was determined by applying the double-slippage correction to
measured gas permeability values at different pore pressures at that stress level.
Figures 2A and 2B illustrate the impact
of net stress on porosity and absolute
permeability. The responses of both porosity and absolute permeability to stress
are nonlinear, confirming the presence
of fractures in the core plug.
Figure 3 illustrates the Walsh plot for
same data in Figure 2B. Two straight lines
appear to be present on Figure 3. The first

0.50

Fracture Closure
Stress = 4,770 psi

0.25
0.0

0.5

line (blue) reflects the stress range where
permeability is dominated by fractures.
The second line (red) represents the stress
range when fractures are completely closed
and the matrix is the only permeability
contributor. The fracture closure stress,
determined from the point where two
straight lines intersect, is 4,770 psi.
❒
MOHAMED ELSAIG is a graduate
student in petroleum and natural gas
engineering in the Benjamin Statler
College of Engineering & Mineral Resources at West Virginia University. His
research work is related to measuring
petrophysical properties of organic-rich
shales. He is expected to receive his
M.S. degree in December.
KASHY AMINIAN is professor of petroleum and natural gas engineering in
the Benjamin Statler College of Engineering & Mineral Resources at West
Virginia University. He began his career
as a reservoir engineer at MichCon.
After completing a Ph.D. from the University of Michigan, he joined West Virginia University as an assistant professor
in 1983 and was promoted to the rank
of professor in 1992. Aminian's research
interests include unconventional natural
gas development, reservoir characterization, and natural gas production and
storage.
SAMUEL AMERI is a professor and
chairman of the petroleum and natural

1.0
In (P/P0)

1.5

2.0

Editor's Note: For additional details
on the Marcellus study referenced in this
article, see SPE 18402, a technical paper
prepared for presentation at the 2016 Society of Petroleum Engineers Eastern Regional Meeting, held Sept. 13-15 in Canton, Oh. The preceding article was adapted
from the paper.
gas engineering department in the Benjamin Statler College of Engineering
& Mineral Resources at West Virginia
University. Ameri's in-depth experience
in natural gas and oil extraction includes
more than three decades of work in
both industry and academia. His research
interests include unconventional natural
gas development, formation evaluation,
and reservoir engineering. Ameri holds
a B.S. and an M.S. in petroleum and
natural gas engineering from West Virginia University.
MEHRDAD ZAMIRIAN is a teaching
assistant professor of petroleum and
natural gas engineering in the Benjamin
M. Statler College of Engineering &
Mineral Resources at West Virginia University. He became a teaching assistant
professor at WVU after completing a
Ph.D. in 2015, focusing on the petrophysical properties of organic-rich
shales. Zamirian teaches multiple courses, including formation evaluation, property evaluation, and reservoir engineering laboratories. He holds a B.S. and
an M.S in petroleum engineering.
NOVEMBER 2016 65

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