American Oil and Gas Reporter - February 2015 - 71

SpecialReport: Unconventional Resource Science
signals than for vertically traveling signals.
In our experience, a practical limit for detecting signals with sufficient confidence
is when the downhole array is within 3,000
feet of the fracture location. These distance
considerations are relevant to both P&S
picking and downhole imaging.

errors and changes in velocity during stimulation when the wave path between the
event and the receiver array includes the
reservoir volume undergoing stimulation?
* Does P&S picking experience difficulties in automated arrival picking
when arrivals are weak, nonexistent, or
contaminated by other arrivals or noise
conditions? These difficulties lead to a
high number of false positive events in a
P&S picking-based point set. The challenge of weak arrivals increases with
event distance from the downhole array.
* For events very close to the array,
is it difficult to nearly impossible to properly differentiate P&S arrivals because
of significant P- versus S-mode wavelet
overlap?
* Monitoring with a 1-D array includes an inherent 180-degree uncertainty
in determining event direction as a result
of wave field sampling limitations.

Method Comparison
P&S picking is a relatively simple,
straightforward, and computationally
cheap method for processing a downhole
microseismic point set. Proximal event
locations, generally less than 3,000 feet
from the downhole array, possess excellent
detectability and low location uncertainty.
However, the final point set generally is
influenced by the analyst who is picking
event arrivals, and can be quite subjective.
Additional points to consider for P&S
picking include:
* Is there high sensitivity to velocity

Surface
Array

FIGURE 3
Map and Depth Views of Increased Event Location Uncertainty
With Distance for Surface (Top)
And Downhole (Bottom) Microseismic Arrays
-400
-200
0
200
400

Map View

Downhole
Array

picking effort becomes formidable and
completely destroys any hope for realtime results. Furthermore, the same dataset
will end with different picks at the hands
of different processors, often leading to
conflicting interpretations and a lack of
confidence in the entire process.
The full-waveform migration approach
uses the power of the stack to overcome
the noise and stabilize travel time estimates. In the case of downhole, fullwave imaging, the location estimate is
driven by the independent, co-location
of both the P- and S-wave fields. Figure
2 shows a representation for the downhole
recording of a full waveform-migrated
P- and S-wave imaged event. The event
location is where the energy of the migrated images intersects.
A primary concern when monitoring
a frac process with a downhole array is
the distance of the monitoring array from
the treatment location. Two issues are at
play here. One is the geometry of the
array. As the array gets farther away from
the location of the frac events, the uncertainty in image location grows because
there is not much travel time difference
between when the signals arrive at the
various geophones in the array. At extreme
distances, the array acts as if it is a single
phone and the uncertainty in location is
large.
The map and depth views in Figure 3
illustrate how event location uncertainty
increases with distance for both surface
(top) and downhole (bottom) arrays. A
good rule of thumb is that events located
farther away than three times the length
of the array are most likely poor.
The second issue is signal attenuation.
The earth tends to absorb seismic signals.
This is worse for horizontally traveling

1,000

2,000

3,000

4,000

5,000

Map View

400
200
0
-200
-400

Depth View

9,600
9,800
10,000
10,200
10,400
0

1,000

2,000

3,000

9,600

4,000

5,000

Depth View

9,800
10,000
10,200
10,400
0

1,000

2,000

3,000

4,000

5,000

1,000

2,000

3,000

4,000

5,000

FIGURE 4
Full-Waveform Downhole Imaged Point Set for Permian Basin Case Study
(Left = Map View; Right = Depth View Looking East)

FIGURE 2
Full Waveform-Migrated P- and
S-Wave Imaged Event Location

S Image

P Image
Event Location

FEBRUARY 2015 71

American Oil and Gas Reporter - February 2015

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