American Oil and Gas Reporter - November 2016 - 63

SpecialReport: MWD/LWD Technology

Study Analyzes Marcellus Petrophysics
By Mohamed Elsaig,
Kashy Aminian,
Samuel Ameri
and Mehrdad Zamirian
MORGANTOWN, W.V.-Obtaining
reliable values for a shale formation's
key petrophysical properties-including
permeability and porosity-is necessary
to estimate the original gas in place,
predict the production rates, and optimize
hydraulic fracturing treatments. However,
quantifying petrophysical properties is
challenging because of the complex nature
of shale formations.
Unsteady-state techniques are used
commonly to estimate shale core sample
permeability, since the values typically
are in the nanodarcy range, but measuring
permeability values by these techniques
suffers from a large margin of uncertainty
and reproducibility problems. Furthermore,
unsteady-state measurements cannot be
performed under reservoir stress and temperature conditions.
In an experimental study conducted
by West Virginia University researchers,
porosity and permeability were measured
under reservoir conditions using a Marcellus core plug from a vertical "science
well" drilled for research purposes at
WVU's Marcellus Shale Energy & Environment Laboratory field site in northcentral West Virginia. The site is owned
and operated by Northeast Natural Energy
LLC, and contains several producing horizontal Marcellus Shale wells.
The study used a fully automated Precision Petrophysical Analysis Laboratory
(PPAL) setup designed and constructed
specifically for obtaining accurate and
repeatable measurements of ultralow-permeability petrophysical properties from
core plugs under steady-state conditions.
It has a resolution of 0.000001 standard
cubic centimeters per second for gas flow
rates and 0.01 cubic centimeters for pore
volume measurement.
Automated temperature control ensures
measurements are performed under isothermal conditions, and permeability and
porosity may be obtained under a range
of net stresses by applying confining
pressure on shale core plugs.
Scope Of Study
The permeability of the core plug
from the Marcellus science well was

measured under different gas pressures
at a constant net stress. The absolute permeability then was determined by applying
the appropriate gas slippage correction.
Finally, the plug's porosity and permeability were measured under a range of
net stresses, and the measured values
were found to be sensitive to stress.
The permeability measurement results
exhibited two distinctive behaviors with
respect to the net stress attributed to
natural fracture and matrix properties.
The experimental results were utilized to
determine natural fracture closure stress.
The measurements also revealed that
when an adsorbent gas was used, gas adsorption resulted in the sample's reduced
absolute permeability.
As the source rock-as well as the
reservoir-organic-rich shale formations
store gas in the limited pore space, and a
fraction of the gas in place may be adsorbed
on the organic material. The ultralowpermeability values typical of shale formations are a result of shale pore structures,
which are only a few nanometers in diameter. It is not practical to measure the
permeability of shale samples by conventional steady-state laboratory techniques
because of their extremely low flow rates
and the length of time required to establish
steady-state conditions.
Consequently, unsteady-state laboratory
techniques such as the crushed sample
test method developed by the Gas Research
Institute and pressure-pulse decay are used
frequently to measure the extremely low
permeabilities of shale samples. However,
permeability values measured using these
techniques often suffer from large margins
of uncertainty and non-uniqueness.
Key among the limitations of unsteadystate laboratory techniques is the inability
to apply the gas slippage correction. The
measured permeability of a rock sample
by gas flow decreases as gas pressure increases. At low pressures, the slippage of
gas molecules at the surface of porous
media leads to the higher gas rates. Under
the slip flow regime, the permeability to
a gas is a linear function of the inverse of
gas pressure.
Transition Flow
The flow regime in the shales, which
have average pore sizes less than 10
nanometers, can be described best by
transition flow. Permeability values meas-

ured under transition flow generally are
higher, and absolute permeability must
be determined from the plot of gas
permeability versus the inverse of the
gas pressure squared (double-slippage
correction).
Experimental studies indicate that transition flow is prevalent in shale core
plugs at low pore pressures (below 250
psia), while slip flow is prevalent at
higher pressures (above 1,000 psia).
Analyzing the results of PPAL measurements performed under steady-state
flow conditions does not require complicated
interpretations, as do pulse-decay and GRI
methods. The flow of gas passing through
the core sample is measured with extremely
accurate differential-pressure transducers.
Consequently, permeability measurements can be performed quickly, allowing
permeability to be measured as many
times as needed to produce repeatable
results. The transducers also provide accurate porosity measurements performed
under isothermal conditions and confining
pressure.
This laboratory setup is fully automated
to eliminate any human error, and maintains a stable temperature within the enclosed unit. The flow rate is monitored
continuously throughout the experiment
to determine when the sample is fully
saturated (adsorbed or desorbed) when
adsorbent gases are used for measurements. Therefore, there is no need to correct for gas sorption.
One of PPAL's key advantages is its
capability to measure shale core plug
permeability and porosity under a wide
range of net stresses by adjusting the
confining and pore pressures. Shale is a
naturally fractured formation. The differences in the compressibility of the
natural fractures and the matrix result in
nonlinear permeability and porosity responses relative to stress.
At low-stress conditions, the fractures
and matrix both contribute to permeability.
As stress increases, the more compressible
fractures begin to close, resulting in a
major reduction in total permeability. At
higher-stress conditions, the fractures
would be completely closed and the
matrix would become the only contributor
to total permeability.
When more than one porous system
is present in the rock, the Walsh plot
yields more than one straight line with
NOVEMBER 2016 63

American Oil and Gas Reporter - November 2016

Table of Contents for the Digital Edition of American Oil and Gas Reporter - November 2016