Geoscience Reference
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time histogram shown in Figure 5.20a. The AE hit counts
peak very close to and during the pressure changes. This
indicates some sort of breakage is occurring during these
times. The sequence is highly temporally correlated with
pressure and voltage changes and indicates some sort of
breakage followed by periods of low AE activity. The AE
hit counts close to Event E1p peak at 108 hits; the E2p
count peaks at 477 hits; the E3p event peaks at 270 hits;
the E4p hit count peaks at 532 hits; and the hit count at
E5p is complex having three peaks, with a maximum
count above 680 hits. The hits are based on exceeding
an AE threshold level on each channel in the AE detec-
tion system. Only a few of the hits contain enough SNR
and channel-to-channel correlation without overlap to
allow hit localization. If hits are localized, then they turn
into AE events. Figure 5.20a also shows the temporal cor-
relation of the located AE events. Figure 5.20b shows the
trend removed pressure change data along with the
event correlations, and Figure 5.20c shows the voltage
response and pressure change event correlations.
Figure 5.20 shows that the observed bursts in the elec-
trical field are directly related to pressure changes that
were measured in the injection system and AE hits.
The pressure data indicate that there were some sharp
changes in the flow regime inside the system connected
to Hole 9 and that the detected fluid flows were only
occurring inside the block (the occurrence of electrical
data shows that the fluid that moved was in contact with
porous media; no electrokinetic phenomena would occur
outside the block and directly in the hole). The large
number of temporally correlated AE hits indicates
that something was breaking at the times of the pressure
and voltage changes.
Note that the pressure data shows only small pressure
fluctuations in the early phases of the seal degradation
around Hole 9 while the pressure continued to build.
The drops in pressure and correlated increases in voltage
indicate that fluid is moving in the system. The drops in
pressure indicate that the fluid flow rate through the fail-
ing seal was momentarily higher than the controlled,
constant fluid flow rate that the systemwas programmed
to deliver. This higher fluid flow rate is thought to be
caused by a small increase in the system fluid volume
due to an increase in annulus volume, and as a result,
the pressure momentarily decreases in the entire fluid
injection system until the pathway associated with the
momentarily higher flow rate closes. This flow rate
decrease allows the pressure buildup, from the constant
flow rate injection into the block, to resume until another
portion of the seal fails, generating another increase in
system volume with an associated pressure drop and
electrical impulse. This repeats until the seal completely
fails. If pressure measurements were the only observa-
tions of these events, then these fluctuations could not
be directly attributed to a seal failure mechanism, and
an explanation for the pressure changes could not be
established. However, the existence of electrical and AE
data shows that there is a mechanism other than induced
block fracturing going on. The electrical data provides
information related to the fluid flow processes during
the series of events. In general, the electrical data implies
progressive seal failure, and the pressure data confirms
fluid movement. In fact, the electrical data actually pro-
vides more detail of the early development of the seal
failure process. We will show that the electrical field
can be used to localize these events, showing a progres-
sive seal failure was occurring during constant flow
rate injection. Each of the pressure drops shown in
Figure 5.20 indicates that fluid is moving in the system,
resulting in the burst-like electrical behavior described
in the previous section. However, only when the pres-
sure decreases precipitously (E5p in Figure 5.20) and is
sustained can full seal failure be identified from the
pressure data alone.
This set of observations shows a strong correlation
between mechanical effects and electrical responses,
indicating the breakage of material along with the move-
ment of fluid in the system. Each observation by itself is
insufficient to explain the physical processes occurring
within the block; however, the combination of observa-
tions strengthens the understanding of
the physical
changes within the block.
5.3.4 Electrical potential evidence of
seal failure
The persistent spatial voltage distribution shown in
Figures 5.18 and 5.19 indicates the effects of upward fluid
migration somewhere near Hole 9. We believe that this
set of observations provides a leading indicator of pro-
gressive borehole seal failure. Ultimately, the existence
of a failed injection tube seal was further confirmed
through large drops in the fluid pressure measurements
(see previous subsection), and subsequently, fluid leak-
age external to the borehole was later visually confirmed
through the observation of water at the top surface of the
block in the vicinity of Hole 9. The temporal electrical
