American Oil and Gas Reporter - July 2015 - 82

SpecialReport: New Products/Technology
FIGURE 4
Proximal In-Zone Cross-Well Events Observed
While Fracturing Wells 4 and 5

Well 6

Well 5

Well 4

Out-Of-Zone Microseismicity
Stimulation of wells 4 and 5 produced
significant energy release in two out-ofzone clusters located about 1,100 feet
above the Onondaga. Microseismicity extended upward 1,900 feet above the Onondaga in the larger cluster and 1,500 feet
above the Onondaga in the smaller cluster.
Out-of-zone energy release was produced by all three wells. Out-of-zone energy released during the 10 treatment
stages along well 4 was 9,515 joules,
while the eight stages along well 5 released
only 2,751 joules of out-of-zone energy.
Altogether, a total of 35,084 joules of energy were released in wells 4, 5 and 6
through out-of-zone microseismicity several hundred feet above stimulation points
in the reservoir. The sensors used to record
microseismic activity produced during
hydraulic fracture treatment of these three
wells were deployed down a vertical well
placed along well 4 to the south.
Out-of-zone energy released in the
82 THE AMERICAN OIL & GAS REPORTER

form of proximal cross-well events within
75 feet of one another provides a measure
of overlapping energy released from separate treatments. Figure 4 shows the proximal in-zone cross-well events observed
while treating wells 4 and 5 (the distance
to in-zone microseismic events along
well 6 from the monitoring well was too
great for events near the stimulation point
to be recorded). The red circle denotes
the position of the vertical monitoring
well. The grid lines are spaced at 500foot intervals.
The total energy released by out-ofzone proximal events induced during hydraulic fracture treatments of wells 5 and
6 was 7,475 joules, or 29 percent of the
total energy released in both treatments
separately. Out-of-zone energy released
by proximal events produced in the treatment of wells 4 and 6 reached 20,437
joules, or 63 percent of the total out-ofzone energy released while treating the
two wells.

These observations reveal a strong
tendency for rupture to occur repeatedly
at the same or nearby out-of-zone locations. This, combined with the fact that
the area was initially ruptured at a distance
of 1.0-1.5 kilometers from the frac point,
suggests that residual stress in this area
may be close to its yield strength and
that the additional stress produced during
hydraulic fracture treatment easily fractured or reopened fractures in the out-ofzone region.
We also note that, through time, outof-zone rupture produced during the single
treatment stage monitored in well 6 developed upward from 900 to 1,700 feet
vertically above the top of the Marcellus.
Proximal cross-well events also developed
upward through time with larger magnitude proximal events developing in the
upper 400 feet of the 800-foot-high column
of events. The magnitude distribution extended from -2.8 to -1.6. Events with
magnitudes greater than -2.0 were located
in the upper half of the columnar cluster
(Figure 3). This also held true for the
subsets of proximal cross-well events.

In-Zone Microseismicity
For initial testing, events produced by
different hydraulic fracture stages with
separations of less than 75 feet were extracted. Microseismicity produced by hydraulic fracture treatments of wells 4 and
5 revealed that hydraulic fracturing was
concentrated primarily in the reservoir
intervals. Of the total 93,545 joules of
energy released during treatments of wells
4 and 5, 81,376 joules were released inzone and 12,169 joules (about 13 percent)
were released in the out-of-zone cluster.
Out-of-zone and in-zone seismicity
are separated by a relatively quiet region
(an aseismic zone) in which very little
microseismic activity was observed. Using
outcrop analogs, it has been suggested
that these aseismic zones may correspond
to zones in which smaller fractures develop
and produce lower-magnitude events below the monitoring array's detection
threshold. The aseismic zone extends
roughly through the Sonyea formation
down to the Tully Limestone (Figure 1).
Proximal in-zone cross-well events
produced while hydraulically fracturing
wells 4 and 5 were associated with an
energy release of 13,073 joules, or 16
percent of the total in-zone energy release
of 81,376 joules. For scale, the well pipe
display widths in Figure 4 are approximately 50 feet. The proximal event dis-

American Oil and Gas Reporter - July 2015

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