American Oil and Gas Reporter - September 2021 - 61

events with poorly constrained locations.
This was achieved by only including
events detected by at least one vertical
and one horizontal array. All events with
misfits larger than 2.5 milliseconds were
excluded as well.
Figure 3 displays the representative
events for the Boxwood 1H, the first well
to be treated and the farthest from the
Bitterroots wells (3,000 feet to the east).
Microseismic activity during the treatment
of the 1H's toe-half extends toward the
depleted region associated with the Bitterroot
wells. Conversely, events from
the heel-half stages all are near the treatment
well, the pattern seen when stimulating
virgin reservoir rock.
The Boxwood 2H was the next well
to be completed and the closest to the
depleted region, which was only 1,100
feet eastward. The Boxwood 2H's fractures
preferred to grow eastward toward
the Bitterroot wells. However, the heel
stages' stimulation triggered events to
the west between the Boxwood 1H and
2H. This reversal in the direction of
half-length asymmetry is compelling evidence
that, absent a nearby depleted
zone, the Boxwood 2H preferentially
developed length toward the recently
completed Boxwood 1H.
This suggests that microseismic events
tend to be triggered on rocks that are
easier to refracture because they are in
previously fractured zones, rather than
virgin reservoir, regardless of the state of
depletion. The shallow Wolfcamp formation
was the primary conduit for communication
from the Boxwood 2H to the
Bitterroot wells, and the total event count
for the Boxwood 2H was higher than the
Boxwood 1H (more distant from the depleted
zone). The Boxwood 2H's treatment
also produced more events than the 1H's.
Boxwood 4H was the last well to be
completed, and microseismicity trends
from its treatment were distinctly different
from its predecessors, as seen in Figure
4. Most of the events associated with
the 4H were located between the Boxwood
1H and 2H, regardless of position
relative to the Bitterroot wells. This implies
pressure walls were created on
both sides of the 4H by the stimulations
of the 1H and 2H, effectively containing
length development.
Additionally, low-magnitude microseismic
events were recorded in shallow
formations at the top of the Bone Springs,
indicating this containment encouraged
more height growth. Interestingly, an
aseismic zone was observed in the middle
and bottom sections of the Bone Springs,
where few, if any events were triggered.
This lack of events was not associated
with any geometry limitations, as the
hypocenter was well constrained.
Finally, all three Boxwood wells displayed
a similar absence of westward
length development beyond the Boxwood
1H, toward Thresher 55-1-12 Unit A
16H. Events were restricted to the area
of Boxwood 1H, 4H, 2H and Bitterroot
1H and 2H. This indicates that these regions
were easier for microseismic event
development, since there was no former
cause that could prevent hydraulic fracturing
from progressing westward.
Imaging Results
Four volumes were created for the
microseismic imaging analysis: two derived
from P-waves and two from fast Swaves.
These volumes exhibit higher resolution
and detail than surface seismic
volumes. Major reflectors were observed
in the microseismic volumes, and these
reflectors can be tied with surface seismic
and well logs.
Despite differences in microseismic
pattern between the heel-half and toehalf
stages of the Boxwood wells, the
fracture density observed in the northern
and southern volumes are similar. This
is likely since all events selected as
sources for the microseismic volumes
were associated with the Boxwood 4H
stimulation, which captures the subsurface's
state after the treatment of Boxwood
1H and 2H. The northern volume appears
to be more continuous than the southern
volume, which has phases being terminated
multiple times.
We recognize that microseismic events
have grown toward the Bitterroots wells
because this region is easier to refracture.
It is evidenced by the different imprint
from toe and heel stages for Boxwood
1H and 2H, where the heel stages show a
more common microseismicity pattern
of a virgin reservoir rock treatment.
We also believe the distinct behavior
of microseismic events from Boxwood
4H comes from a pressure wall created
by the treatments of Boxwood 1H and
Boxwood 2H, which forced stimulation
to be triggered between those wells.
About the shallow events in the Bone
Spring Lime, we infer they occurred because
of the pressure wall formed by the
stimulation of Boxwood 1H and 2H.
Once more fluid is pumped into the subsurface,
it increases the pressure of the
surroundings and encourages the fracture
to grow upward.
A fiber cable was deployed in the
Boxwood 55-1-12 Unit 5PH monitor
well. Frac hits were detected on it during
the treatment of the Boxwood 4H for
stages close to the monitor well. This
observation agreed with the microseismic
results and confirmed our hypothesis.
Editor's Note: For more detail on
this study, including figures showing the
microseismic imaging volumes, see " Microseismic
at HFTS2: A Story of Three
Stimulated Wells, " a paper originally prepared
for presentation at the 2021 Unconventional
Resource Technology Conference.
The study is based on work supported
by the Department of Energy under
Award Number DE-FE0031577. The authors
would like to thank Vladimir Grechka
(1962-2021) for his contributions, as well
as his generosity, friendship and dedication
to teaching and mentoring.
BO HOWELL has served as Borehole
Seismic LLC's chief operating
officer since 2010. He began his oil
and gas career in 2007 with Halliburton
as a field engineer focused on wireline
and perforating services and borehole
seismic. Howell holds a B.S. in mechanical
engineering from The University
of Texas at Arlington.
ZHAO LI is a geophysicist with
Borehole Seismic LLC. He joined the
company in 2017 after earning a Ph.D.
in geophysics and seismology from the
University of Houston for work focusing
on poroelastic wave equations.
DENISE FURTADO is a geophysicist
at Borehole Seismic LLC. In that
role, she is involved in analyzing microseismic
data and creating, implementing
and upgrading analysis and
interpretation procedures. Before Borehole
Seismic, Furtado worked for BP
as a geoscientist in Rio De Janeiro.
COLLIN STRAUS is vice president
of operations at Borehole Seismic LLC.
He joined the company in 2011 as an
operations geologist before moving
into business development. Straus graduated
from the University of Colorado
at Boulder with B.S. degrees in both
geology and environmental studies.
SEPTEMBER 2021 61

American Oil and Gas Reporter - September 2021

Table of Contents for the Digital Edition of American Oil and Gas Reporter - September 2021

Contents
American Oil and Gas Reporter - September 2021 - Intro
American Oil and Gas Reporter - September 2021 - Cover1
American Oil and Gas Reporter - September 2021 - Cover2
American Oil and Gas Reporter - September 2021 - Contents
American Oil and Gas Reporter - September 2021 - 4
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American Oil and Gas Reporter - September 2021 - Cover3
American Oil and Gas Reporter - September 2021 - Cover4
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