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AE. Figure 6a shows the breakdown pressures of
the specimen against a logarithmic scale of frac-

turing fl uid viscosity. This fi gure clearly shows that
breakdown pressure increases with viscosity. The

tendency is consistent with the experimental re-
sults by Morita et al. [1996] using water-based and

oil-based muds and, in addition, the numerical si-
mulation by Bunger et al. [2010].

AE source distribution
Although the AE sources with SC-CO2 injection

previously shown in Figure 5 are distributed along

the cracks as expected from the observed surface
cracks, they were more widely distributed rather

than lying along a fl at plane; this tendency was lar-
ger than those with injections of the other fracturing

fl uids. To present the differences quantitatively, the
maximum likelihood fl at plane for the AE distribu-

tion was estimated by minimizing the sum of squa-
res of the distances from a source to the fl at plane.

After that, the average distance, Lav, from a source
to the maximum likelihood plane was obtained. For

the AE distributions previously shown in Figure 5,
the Lav values were 12.06 mm. We obtained the

values of Lav for the all specimens including the
other fracturing fl ud. Figure 6b shows the results.

This fi gure clearly indicates that Lav decreases with Figure 5. AE source distribution and cracks visible on the specimen surface. Dark and light

lines show cracks that appeared in different planes facing each other. (Ishida et al. 2016)

28
(a) Figure 5 3
(c)
(MPa) Viscous Oil SC-CO
(MPa) 24
28 2
(a) 2.5 3 (c)
20 Viscous Oil SC-CO 2 Water
24
2.5
Breakdown pressure
Fractal Dimension 2 Water
Viscous Oil
20
Breakdown pressure 16 L-CO Water L-CO 2
2 Fractal Dimension 2
1.5 Viscous Oil
16
12 Water L-CO 2
L-CO 2 1.5
SC-CO
8
12 2 1
0.01 0.1 1 10 100 1000
0.01 0.1 1 10 100 1000 Viscosity (mPa・s)

SC-CO
8 Viscosity (mPa・s) 1
2

0.01 0.1 1 10 100 1000 0.01 0.1 1 10 100 1000
Viscosity (mPa・s)
Viscosity (mPa・s)

14
(b) (d)

14
12 (b) L-CO 2 (d)
Lav (mm) 10 SC-CO 2 2 L-CO 2 Water
12
Viscous Oil
SC-CO
Lav (mm) 10 Viscous Oil
Water
8
6 8
0.01 0.1 1 10 100 1000
Viscosity (mPa・s)
6
0.01 0.1 1 10 100 1000
Viscosity (mPa・s)

Figure 6. Effect of fracturing fl uid viscosity on (a) breakdown pressure, (b) average distance Lav from a source to the estimated most likely plane, (c) fractal

dimension, (d) the compression ratio to the total number of the polarities in P wave fi rst motion. (Ishida et al. 2016)
Figure 6

Figure 6
3 Impiantistica Italiana - Settembre-Ottobre 2020 55 55

4
3 2
4