American Oil and Gas Reporter - January 2016 - 83

TECH TRENDS
Frequency (Hz)

FIGURE 1

By monitoring the well after it has
been put on production, the size of an alternative volume-the active production
volume (APV)-can be determined. Seismic emission activity caused by the stress
and fluid perturbations that occur during
production is used to map the APV.
Most microseismic monitoring methods
use microearthquake (MEQ) detection
and location as the basis for determining
SRV. Energy from these short-duration
events is mapped and the SRV is generated
by identifying the high-density zone of
MEQs surrounding the wellbore.
Lower amplitude signals that are continuous for longer durations are not always
detected by these methods. Ambient monitoring captures this continuous background seismicity and also the MEQs
that are too small to generate detectable
arrivals.
Figure 1 shows a comparison of spectrograms for an MEQ and a long-duration-signal (LDS) event. Both spectrograms
are one minute long. The vertical axis is
frequency from 0 to 80 hertz. Warm colors
indicate more energy at a given frequency
and time. The time of high-intensity
activity for the microearthquake event is
only two seconds, while the LDS event is
active for at least 15 seconds.
The LDS event also appears to have a
greater drop-off in strength with frequency,
resulting in a lower level of high-frequency
energy. This long-duration energy dominates the passive data, and as such, ambient technology captures far more seismic-emission energy than does the MEQ
method. Ambient monitoring provides

15,000

45,000

60,000 0

Time (ms)

15,000

45,000

60,000

Time (ms)

The spectrogram for a microearthquake at left compares to that of a long-duration-signal
event at right. Both spectrograms are one minute long. Hot colors indicate more energy
at a given frequency and time.

the most reliable images of the subsurface
and better constrained SRVs.
Physical Model
Seismic energy is emitted as rocks release stored elastic strain energy. This energy is not distributed evenly in the earth's
crust, but is released preferentially on
fracture/fault surfaces and in the damage
zones surrounding these surfaces.
Fracture mechanics predict stress concentrations associated with fractures. Both
field studies and laboratory experiments
show clear evidence for these stress concentrations, recorded in the damage zones
associated with fractures. Damage zones
consist of rock volumes with a high
density of smaller fractures that display
exponentially higher densities with proximity to the main fracture surface.
The brittle crust is in a state of unstable
frictional equilibrium, and therefore, very
small changes in stress (less than 0.01
atmospheres) can cause rock failure. Fail-

FIGURE 2

At left, an active production volume is generated from the fully populated depth volume.
First a threshold is applied in order to remove voxels with low-amplitude background
(center). Then only the high-amplitude locations with direct connection to the wellbore
are retained in the APV (right).

ure occurs preferentially on small, optimally oriented fractures and in the zones
surrounding the fractures where cracktip stress concentrations amplify the stress
magnitudes.
During well treatment, the unstable
equilibrium is disturbed significantly as
additional fluid volumes alter the stress
state around the wellbore and reduce the
normal stress on pre-existing fractures.
During production, more subtle movement
of fluid produces a similar effect. In both
cases, seismic waves are emitted as the
rock releases stored elastic strain energy.
Ambient Seismic Imaging
Our ambient seismic imaging method
uses depth migration applied to both
lower amplitude MEQs and signals that
are continuous for longer durations. The
energy from these low-amplitude events
and LDS is focused to the location of
origin within the earth.
The process is a one-way travel time
prestack depth migration. The signals
travel one way-from the reservoir to the
receiver-as opposed to surface reflection
seismic data, in which the signal travels
over a two-way path from the seismic
source to the reservoir and back to the
receiver.
Field data for ambient seismic imaging
are collected using a surface array or
shallow buried grid. Downhole data can
be processed by this method if there is a
sufficient density of receivers and adequate aperture, but this is infrequently
the case.
A velocity model is built and calibrated.
JANUARY 2016 83

American Oil and Gas Reporter - January 2016

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