Geoscience Reference
In-Depth Information
probability density functions are in reasonable agree-
ment with the true model parameters represented by
the solid vertical bars (Figure 5.13).
recoveries of oil reservoirs could benefit from the use
of electromagnetic sensors coupled to three-component
geophones
to monitor
this
type of
seismoelectric
activity.
An impressive example of the occurrence of electro-
magnetic signals associated with a hydraulic fracturing
experiment in a well has been documented by Ushijima
et al. (1999) (see Figure 5.14). The electrical potential
changes recorded at the ground surface were analyzed
by Ushijima et al. (1999) in terms of a dipolar source
associated with the plane of fracture and consistent
with the fracture localization using seismic activity (see
Figure 5.14d). In the case where the injected fluid has a
pore water conductivity that is different from the conduc-
tivity of the groundwater, there are two contributions to
the source current density given by (e.g., Martínez-Pagán
et al., 2010; Revil et al., 2011; Bolève et al., 2011; Ikard
et al., 2012)
5.2.6 Discussion
The previous sections we presented a proof-of-concept
exercise. However, the model used earlier is not intended
to be directly applicable to real earth problems. The first
limitation of our approach lies in the use of the Biot
theory itself, which needs to be modified to account for
the true quality factor of earth materials, especially for
shales (Carcione, 2007). Modeling the quality factor of
porous media is presently an area of fertile research
(e.g., Gelinsky et al., 1998; Müller & Rothert, 2006),
and the inversion of the intrinsic attenuation coefficient
can be set up as an additional inverse problem in the full
waveform inversion of the seismic data (e.g., Ravaut
et al., 2004). Regarding 2D/3D effects, they can be incor-
porated in the present approach (actually our code is
3D + time), but the forward problem has rapidly become
computationally expensive in 3D. This prohibits, at this
point, the use of an MCMC sampler to solve the inverse
problem in real situations. That said, the code can be par-
allelized, and the MCMC sampler strategy can be com-
bined with the deterministic approach discussed earlier.
Regarding the signal-to-noise ratio (SNR), the level of
electrical noise is usually higher at the ground surface
than at depth, and therefore, the electrical field should
be preferentially monitored in shallowwells. The optimi-
zation of the SNR for an electrical monitoring sensor
array is a subject that has been broadly discussed in elec-
troencephalography (EEG) (Grech et al., 2008) but not in
geophysics.
Additionally, electromagnetic disturbances have been
observed to be associated with earthquakes, seismovol-
canic activity, steam injection in hot dry rock reservoirs,
detonation of explosive charges in boreholes, and
underground nuclear explosions (e.g., Ushijima et al.,
1999; Yoshida, 2001; Gaffet et al., 2003; Byrdina
et al., 2003; Yoshida & Ogawa, 2004; Soloviev &
Sweeney, 2005; Honkura et al., 2009). Multiphase flow
in porous media, and more precisely the occurrence of
Haines jumps at pore throats, is also responsible for tiny
seismic and electrical field bursts as observed by Haas
and Revil (2009). Therefore, the monitoring of shallow
earthquakes along tectonic faults and active volcanoes,
the hydraulic fracturing in geothermal fields and hydro-
carbon reservoirs, and the secondary and enhanced
k
b
T
Fe
J
S
=
Q
0
V
w
−
2
t
+
−
1
∇
σ
f
5 34
where
k
b
represents the Boltzmann constant,
T
is the abso-
lute temperature,
F
denotes the formation factor,
e
stands
for the elementary charge,
t
(+)
symbolizes the microscopic
Hittorf number of the cations (0.38 for NaCl), and
σ
f
signifies the conductivity of the water. The first term on
the right in Equation (5.34) is the electrokinetic contri-
bution investigated earlier, and the second term in
Equation (5.34) represents the diffusion current associ-
ated with the gradient of the conductivity of the water.
In the case study reported by Ushijima et al. (1999), the
self-potential was inverted assuming that the source is
dipolar and the location of the source was found to be
coincident with the fracture plane, illuminated by the
localized acoustic emissions (AE). Therefore, the experi-
ment reported by Ushijima et al. (1999) shows that
recordable electrical signals can be measured remotely
in association with hydraulic fracturing experiments.
The computed signals can be compared also to the level
of the background electrical noise at the seafloor and
onshore. At 1 Hz, the level of background noise on the sea-
floor is on the order of 10
−
12
Vm
−
1
(Chave et al., 1991).
Instrumental noise caused by the nonpolarizable Ag
AgCl
electrodes in contact with seawater is about 10
−
24
V
2
m
−
2
Hz
−
1
at frequencies above 1 Hz (Webbs et al., 1985). There-
fore, the background noise level at the seafloor is well
below the magnitudes required to detect the signals
described in this chapter.
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