American Oil and Gas Reporter - January 2015 - 138

Three Poisson's ratios were calculated
for each sample: one for each pair of
radial LVDTs (two pairs), and one for
the average of all four radial LVDTs.
Figure 2 shows Poisson's ratios versus
stress at 140 (left) and 160 (right) degrees.
As can be observed, the Poisson's ratios
calculated from the two perpendicular
pairs of LVDTs are quite different. Total
Poisson's ratios in all samples are between
15 and 25 percent. These uneven values
are caused by natural fractures. This is

also proved by the way in which the
specimens failed.
After the UCS tests, it was observed
that all specimens failed through a vertical
plane clearly marked from the top of the
sample, continuing all the way to the
bottom, thereby explaining the existence
of the natural fractures, Figure 3 shows
the samples after running UCS tests at
140 (left) and 160 (right) degrees.
Table 1 summarizes the values of
UCS, Young's modulus and Poisson's
ratio for all five samples. The results of
the experiments show temperature has a
detrimental effect on rock compressive
strength and makes the rock weaker, consequently increasing the likelihood of
wellbore stability problems. By increasing
temperature 40 degrees, UCS was decreased up to 18 percent. However, it
also is evident that temperature does not
have a considerable effect on either
Young's modulus or Poisson's ratio in
Eagle Ford shale oil rock.
Because of natural fracturing, Poisson's
ratio is not the same in various directions.

TABLE 1
Measured Parameters
Rock
Sample
140˚ F
150˚ F
160˚ F
170˚ F
180˚ F

UCS
(psi)
8,000
7,600
7,300
7,000
6,600

E (psi)

v
6

0.25

0.15

1.25 x 10
1.40 x 10
1.30 x 10
1.40 x 10
1.30 x 10

The Eagle Ford core samples failed
through a vertical plane from the top of
the sample all the way to the bottom,
which is quite different from how sandstone samples fail.
We recommend running experiments
with different drilling fluids, including
oil-based muds, to investigate the effect
of the various drilling fluids on rock
UCS. We also recommend running triaxial
tests to assess the effect of temperature
on rock triaxial compressive strength,
and investigating the effects of temperature
fluctuations during drilling on rock compressive strength.
Ì
Editor's Note: The authors acknowledge Chesapeake Energy Corp. for providing core samples and data to run the
experiments, and Halliburton for technical
support in conducting the experiments.
They also thank Texas Tech University
Unit Manager Shannon Hutchison and
Senior Technician Joe McInerney for assistance in preparing the Eagle Ford core
samples.

FIGURE 2
Poisson's Ratio versus Stress at 140 (Left) and 160 (Right) Degrees
0.50

0.50
Poisson's Ratio in Direction 1

Average Poisson's Ratio

Poisson's Ratio in Direction 1

Average Poisson's Ratio

0.45
0.40

Poisson's Ratio in Direction 2

0.40

Poisson's Ratio in Direction 2

Poisson's Ratio

0.35
0.30
0.25
0.20

0.30

0.20

0.15
0.10

0.10
0.05

0
0

1,000

2,000

3,000

4,000
5,000
Stress (psi)

6,000

7,000

8,000

9,000

1,000

2,000

3,000
4,000
Stress (psi)

FIGURE 3
140 (Left) and 160 (Right) Degree Rock Samples after Running UCS Tests

138 THE AMERICAN OIL & GAS REPORTER

5,000

6,000

7,000

American Oil and Gas Reporter - January 2015

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