Page 58 - 83

ENGINEERING

viscosity. This suggests that low-viscosity frac- the injection hole. The results indicated that the
turing fluids induce cracks that propagate farther tortuosity and the crack number both tend to in-
away from a flat plane, whereas viscous fluids tend crease with the decrease of fracturing fluid viscosity
to induce cracks along a flat plane. (Ishida et al. 2016). The trends are consistent with
We also obtained the fractal dimension, FD, for those obtained from the AE distribution, where the
the AE source distributions. FD can take decimals average distance, Lav, and the fractal dimensions,
unlike the Euclid dimension. When the AE sources FD, increased with the decrease of fluid viscosity as
distribute on a line, on a plane and three dimensio- shown in Figures 6b and 6c.
nally, FD becomes 1, 2 and 3 respectively. Figure
6c shows that FD of the AE distributions decreases
with viscosity, suggesting that low-viscosity fractu- Advantages of HF using
ring fluids induce cracks that propagate three di- SC-CO and remaining
mensionally, rather than two dimensionally along a 2
flat plane. problems to realize it
The laboratory experiments suggest that HF using
Fracturing mechanism SC-CO2 induces three-dimensionally sinuous
cracks with many secondary branches, which
deduced from P wave first seem to be desirable pathways for shale gas re-
motion polarity of AE covery, heat extraction from hot dry rock and other
projects for energy production, for example, coal-
The differences in the crack features possibly aro- bed methane. In an actual operation in field, pree-
se from a difference in fracture mechanism. The xisting joints and cracks often affect crack exten-
P wave first motion polarity of an AE wave shows sions of HF. To investigate their effects, we have
compression or dilatation without exception. Theo- already conducted small scale field experiments
ry of earthquake mechanics indicates that the ratio using 10 m long well in a tunnel of hot granite. We
of compressions to the total number of AE sensors will report the results in academic journal within a
whose polarities could be distinguished compres- few years, although their preliminary results have
sions or dilatations would be 50% for a pure shear been already published on conference papers (Ishi-
fracture and 100% for a pure tensile fracture (e.g. da et al. 2017 and 2018).
Kasahara 1981 and Walter and Brune 1993). Thus, Besides the results based on our research activities
we compared ratios of the P wave first motion po- summarized above, we can point out the following
larity of approximately 30 AE events that were de- advantages of HF using CO for shale gas recovery.
2
tected by at least seven sensors, allowing P wave (1) HF using CO could realize economically via-
2
arrivals to be determined for a source location. ble CO2 sequestration, because large amounts
Figure 6d shows that the compression ratio in P of CO2 would remain underground distributing
wave first motion tends to increase with viscosity. across many wells even if only a small amount of
Thus, the results suggest that a low-viscosity frac- CO is used in each well injection,
2
turing fluid tends to induce shear dominant fractu- (2) Because CO has higher affinity for shale than
2
res, whereas viscous fluids tend to induce tensile methane (CH ) does [Nuttall et al., 2006], CO ab-
2
4
dominant fractures. sorption should be preferred to CH absorption
4
with a molecular ratio of up to 5:1 [Nuttall et al.,
Crack Features Deduced From 2010]. This effect would result increase of produc-
tion of CH4 from the shale and also chemically trap
Direct Observation CO2 within the shale, as indicated by McKenna
(2012) with introducing our research (Ishida et al.
We also observed crack propagation patterns 2012) on Web site of New Scientist Magazine.
around the injection hole on the thin sections cut (3) The use of CO as a fracturing fluid would elimi-
2
from the specimens. The cracks are tortuous and nate formation damage and residual fracturing fluid
never propagate straight. Fluids with lower visco- [Sinal and Lancaster, 1987; Liao et al., 2009].
sity induce cracks with higher tortuosity, because (4) The use of pure CO without any additives can
2
intermittent and stepwise crack extensions beco- realize HF free from environmental contamination.
me more conspicuous. Examining the correlations (5) In a reservoir getting easier CO rather than wa-
2
between crack patterns and the constituent mi- ter, for example, that in a desert, HF using CO2 sol-
neral grains of the specimens, the cracks induced ves the problem of water shortage, as indicated by
by SC-CO propagate tortuously along the grain Bullis(2013) with introducing our research (Ishida et
2
boundaries of the constituent minerals, whereas a. 2012) on Web site of MIT Technology Review.
the cracks induced by viscous oil extend straightly Remaining problems to realize HF using SC-CO :
2
cutting through many mineral grains. To compare (6) Because we obtained the experimental results
quantitatively, we measured tortuosity defined as only in granite and it is necessary to confirm the
a ratio of a length measured along a crack to the results also in shale, although we partially obtained
length of a straight line from the starting to the en- some results in shale (Bennour et al. 2015).
ding point of a crack. In addition, an average num- (7) Because it is difficult for SC-CO to transport
2
ber of cracks crossed the scanning lines at inter- and inject proppant into induced HF cracks due to
vals of 1 mm on the thin sections, for 24 mm from its low viscosity, we need to solve the problem.
56 56 Impiantistica Italiana - Settembre-Ottobre 2020