American Oil and Gas Reporter - November 2016 - 64

SpecialReport: MWD/LWD Technology
different slopes. Therefore, the Walsh
plot can be used to detect the presence of
different media and determine the closure
stress at which more compressible fractures
close completely.

measure the petrophysical properties of
the plug, which was obtained at a depth of
7,547 feet from the Marcellus science well.
First, the core plug's porosity and permeability were measured using helium.
The gas (pore) pressure was increased
stepwise and then decreased stepwise to
evaluate the impact of gas adsorption.
During this set of measurements, the confining pressure was adjusted to maintain
a constant net stress.
Second, porosity and permeability
measurements under constant net stress
were repeated with nitrogen. To ensure
that measurements were not influenced
by gas adsorption, permeability was evaluated after the gas flow rate stabilized.
When gas pressure increases, the gas
leaving the core plug is less than the incoming gas because of adsorption. As a
result, the outgoing gas rate increases as
adsorption diminishes. Consequently, the
outgoing flow rate stabilizes (reaches a

Objectives And Methodology
The objectives of the WVU study were:
* To investigate the impact of the
gas (pore) pressure on the measured permeability of the Marcellus core plug;
* Determine the plug's absolute permeability by applying the appropriate
gas correction (slippage or double-slippage);
* Investigate the impact type of the
gas used for permeability measurements;
and
* Assess the impact of stress on measured porosity and permeability values,
and determine fracture closure stress.
Three sets of the experiments were performed during the study using PPAL to
FIGURE 1A

Study Results
Using helium at a constant net stress,
the measured permeability values were not

FIGURE 1B

Absolute Permeability Evaluation
By Gas-Slippage Correction

Absolute Permeability Evaluation
By Double-Slippage Correction

500

500

400

400
Permeability (nD)

Permeability (nD)

maximum) when the sample is saturated
with gas at the pore pressure.
The process is reversed when gas
pressure is decreased. The outgoing gas
rate decreases with diminishing desorption. Similarly, the outgoing flow rate
stabilizes (reaches a minimum) when
the sample is saturated with gas at the
pore pressure. Continuous flow rate
monitoring throughout the experiment
allowed permeability to be determined
when the sample was fully saturated at
each gas pressure.
The plug's porosity and permeability
were measured at seven net stress values
from 1,300 to 7,000 psig. At each net stress
value, permeability was measured at four
gas (pore) pressures ranging from 200 to
500 psig to evaluate absolute permeability.

300
200
100
0

200
100

-19nD

-100
0.000

300

0.001

0.002

0.003

0.004

124nD

0
0.0E+00

0.005

1.0E-05

2.0E-05
2

-1

3.0E-05

-2

1/P (psia )

1/P (psi )

FIGURE 2A

FIGURE 2B

Impact of Net Stress on Porosity

Impact of Net Stress on Absolute Permeability

2.3

1,200

2.2

1,000
Permeability (nD)

Porosity (%)

2.1
2.0
1.9
1.8

800
600
400

1.7
200

1.6

0

1.5
0

500

1,000

1,500

2,000 2,500
Net Stress (psi)

3,000

64 THE AMERICAN OIL & GAS REPORTER

3,500

4,000

4,500

0

2,000

4,000
Net Stress (psi)

6,000

8,000
American Oil and Gas Reporter - November 2016

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