American Oil and Gas Reporter - July 2015 - 105

SpecialReport: Seismic Acquisition & Reservoir Modeling
activation to the right side is incremental
and spaced over the total pump time.
LDS Signals
Long-duration signals are continuous
seismic waveforms originating in the
reservoir. While they last for seconds or
minutes, they are episodic, sporadic and
pulsating in nature. In contrast, microearthquake signals, by definition, have
very short durations and higher amplitudes
than LDS waveforms. During pumping,
there is a large fluid flow and a large
pressure change at the frac stage location.
The fluid flow and differential stress
cause seismic signals from the frac stage
location in the form of LDS waveforms.
This signal has been studied in detail for
two stages of a frac treatment project.
For each of the two stages, LDS signals
were tracked and demonstrated to exist
for 60 percent of the minutes in the 100
minutes of pump time.
LDSs appear and disappear on time
scales ranging from seconds to many minutes. Figure 4 shows an example of an
LDS. Only a few seconds of trace data
were recorded, illustrating the importance
of graphical presentation for recognizing
LDS waveforms. Without sorting, the
LDS signals would not be visible.
To demonstrate the signal-to-noise
(S/N) ratio versus frequency for the LDS,
a time window of many seconds was
used to compute the LDS amplitude spectrum. The same length of time, but from
a different part of the trace data with no
LDS, was used to compute the amplitude
spectrum of the noise.
The two computations were overlaid
in the inset image in Figure 4, with the
LDS signal amplitude spectra in red and
background noise in black. This illustrates
that the frequency range of positive S/N
ratio for LDS is from 15 to 55 hertz, or
the same frequency band that is recorded
for the dominant frequency range of surface reflection data.

well that will interact with one or more
producing wells.
The left panel in Figure 5 shows a
top-down view of the total reservoir volume that was activated during the frac
treatment of well A in an unconventional
gas reservoir. Many months after the
well started producing, the two images
to the right were computed from quiettime passive data recorded before the

frac treatment of well B.
The middle panel shows the fractures
active during the production of well A,
while the right-hand panel shows the
rock volume producing around well A
before pumping the well B treatment.
The fractures shown in the middle panel
predict which stages in the well B treatment will cause pressure hits in well A
(fracture imaging shows well B fractures

FIGURE 4
LDS Signal Trace Display (with LDS Signal/Background Noise Inset)
Traces with Filtering and Sorting

20

30

40

50

60

0.012
0.011

LDS Spectrum

0.010
0.009
0.008
0.007
0.006
0.005

Noise Spectrum

0.004
0.003
0.002
0.001

FIGURE 5
Predicted Frac Performance
Pumping Time Recording
Well A
Activated Fracture Volume
During Frac Pumping

Quiet Time Recording Before Well B Frac
Months After Well A Frac
Fracture Images before Frac
Treatment of Well B

Activated Volume while
Producing

Predicting Frac Performance
Passive data recorded during quiet
times are used to image the seismic
signals from the reservoir. These signals
show the fractures that are active because
of production in the adjacent wells not
undergoing treatment. One of the images
computed from this quiet-time passive
data is the image of the larger-area fractures. Mapping the locations where these
fractures cross a new wellbore predicts
the stages of the treatment in the new
JULY 2015 105
American Oil and Gas Reporter - July 2015

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