Time lapse vsp

1
Time-lapse seismic simulations of potential CO2
induced changes in acoustic properties of coals
for MMV
NETL ORD Work Planning Session, Hyatt Regency,
Pittsburgh May 3-4,2010
Thomas H. Wilson and Art Wells
Objective Evaluate potential AVO and time
lapse response to CO2 injection. Present
simulations for alternative cases in which CO2
increases and decreases coal zone velocity.
Determine the potential for time lapse AVO
observations in CMP and VSP records.
Abstract We are involved in site
characterization and MMV efforts at pilot tests
in the San Juan and Appalachian basins. In the
San Juan Basin, CO2 injection began July 30th of
2008 and continued through August 14th of 2009.
During the 12 month injection period
approximately 256 MMCF, equivalent to nearly
18,407 short tons of CO2 (319 MMcF) were injected
into the Fruitland coals. Time-laps vertical
seismic profiles (VSP) were collected to
evaluate the potential of this method to image
the CO2 flood one on June 3rd and 4th of 2008,
and the post injection survey, September 17th,
2009 (a month after CO2 injection was completed).
Processing of these data are still in progress.
WVU is also involved in another coal
sequestration effort here in the Appalachian
Basin through ZERT with CONSOL and NETL. Time
lapse seismic imaging of the flood front will
also be evaluated on that site. Some published
studies suggest that acoustic impedance of coal
should increase because of coal swelling. Xue and
Ohsumi (2005), for example, make detailed
measurements of swelling strain and waveform
traveltime changes for the Kushiro Coal,
Hokkaido. They note a 10 increase in P-wave
velocity at 2.5 MPa (362psi) and perhaps up to
12.7 at 12 MPa (supercritical). Nishimoto et al.
(2008) report only 2.2 increase in Vp at 12MPa
under supercritical conditions. McCrank (2009)
notes that CO2 injection into the Ardley Coal,
Alberta produces a 10 reduction in velocity
attributed to increased coal plasticity after a 9
month CO2 soak. In this study we calculate
AVO variations in CMP gathers using full
solutions to the Zoepritz. CMP gathers are used
as a proxy for the VSP, with short to long
offsets corresponding roughly to upper to lower
borehole sensor locations. Two possible scenarios
are modeled 1) CO2 injection reduces coal
velocity and 2) CO2 injection increases coal
velocity. The results suggest that in both cases
significant time lapse response occurs due to
relative delay or advance in the pre-post arrival
times.
Zero offset VSP used as TD curve to adjust sonic
sonic and density travel time data used to
generate AVO synthetics.
Structure base of Fruitland
SWP Vertical Seismic Profile
Three offset VSPs and one zero offset VSP were
collected at the site prior to CO2 injection and
1.3 years later following completion of
injection. The source point locations are shown
on the QuickBird image at left (green squares).
Presence of archaeologically sensitive areas at
the site limited our choice of offsets. The image
at left also shows locations of wells, NETL
tracer and soil gas sample points and tiltmeters
Conclusions Time lapse responses in common
midpoint (CMP) gathers were computed as a proxy
to estimate vertical seismic profile (VSP)
response for two cases. In one case, the velocity
is increased by 10 throughout the entire
Fruitland coal section including 10 to 15 foot
zones above and below the Fruitland coals. The
rationale for this case is that coal swelling in
response to CO2 injection increases Vp (Xue and
Ohsumi, 2005). The swelling strain is assumed to
produce a pressure halo that increases velocity
of intervals sandwiched between the coals and
intervals bounding the coal zones. Amplitude
variation with offset (AVO) was evaluated for the
positive event just below the top of the upper
Fruitland coal at 0.6045 s. The amplitude
variation for this event drops with offset (upper
graph at left). The post-injection drop (middle
left) is greater. Thus the baseline minus monitor
difference increases with offset. The
post-injection arrival time is advanced 1 ms.
In the second case, coal velocity is decreased
10. The decrease is limited to the coals. In
this case the post-injection zero offset
amplitude is much greater as expected. The travel
time is delayed in this case by 0.5 ms. The post
injection AVO drop is also much greater so that
an increase in amplitude with offset is again
observed in the baseline minus monitor difference
(see graphs lower part of left panel). The
increase in amplitude with offset is nearly the
same for both high velocity and low velocity
cases. In this study, time lapse differences
in two CMP attributes were evaluated 1) AVO, and
2) travel time delay or advance. Travel time
delay or advance is a discriminating attribute
whereas the difference in AVO is not. The
simulations suggest that differentiation between
the two cases can be obtained in CMP gathers or
VSP through cross-equalization of seismic
response above the injection zone followed by
careful analysis of travel time differences
between events in the baseline and monitor
surveys arising from within and beneath the
injection zone.
Original and modified logs for the fast model
(above). Synthetic AVO response computed for the
fast case (above right)
Source point locations
Comparison of monitor and baseline VSPs for
source offset D northwest of the injection well.
Displays represent intermediate stage processing
results (Jitendra Gulati with Schlumberger).
Baseline and monitor surveys were
cross-equalized before differencing. Although
subtle, the initial interpretation suggests that
interval velocities may have increased in
response to CO2 injection.
Monitor
Baseline
Baseline - Monitor

• References
• Henthorn, B., Wilson, T., and Wells, A., 2007,
  Subsurface Characterization of a Carbon
  Sequestration Pilot Site San Juan Basin, NM
  Annual AAPG Convention, Proceedings CD. See also
  http//www.searchanddiscovery.net/documents/2007/0
  7047henthorn /index.htm http//www.geo.wvu.edu/
  wilson/netl/ HenthornWilsonWells -07AAPG.pdf
• McCrank, M., 2009, Seismic detection and
  characterization of a CO2 flood in Ardley Coals,
  Alberta, Canada M. S. Thesis, Department of
  Geoscience, Calgary, Alberta, 191p.
• Xue, Z., and Ohsumi, T., 2005, Experimental
  studies on coal matrix swelling due to carbon
  dioxide adsorption and its effect on coal
  permeability Shigen-to-Sozai, vol 121, no.6, p.
  231-239 (in Japanese with English abstract and
  figure captions).
• Xue, Z., and Ohsumi, T., 2003, Laboratory
  measurements on swelling in coals caused by
  adsorption of carbon dioxide and its impact on
  permeability of coal Coal Safety, no. 23, p
  36-43.

Acknowledgements This technical effort was
performed in support of the National Energy
Technology Laboratorys on-going research in
carbon sequestration under the RDS contract
DE-AC26-04NT41817-6060404000 and URS subcontract
No. 2010-SC-RES-30033-023. Wed like to thank
Dave Wildman and Donald Martello, our DOE-NETL
project managers, for their support and advice on
these efforts Scott Reeves and Brian McPherson
of the Southwest Regional Partnership for their
help in facilitating our involvement in the
Partnerships activities on their San Juan Basin
carbon sequestration pilot test and for allowing
us to use data collected as part of the pilot
effort and Ryan Frost.Tom Cochrane. Bill Akwari
and Craig Hartline of Conoco Phillips for helping
facilitate many of the activities on the site.
Appreciation is also extended to Dwight Peters,
Marcia Coueslan, Jitendra Gulati, Les Nutt and
Ric Smith with Schlumberger for critical
assistance in the design of the logging and VSP
acquisition, analysis and processing efforts at
the site. We also want to thank Bill ODoud
(NETL) SWP project manager.
Comparisons of synthetic baseline and monitor CMP
gathers showing time lapse responses for two
cases 1) increased velocity in the coal section
and 2) decreased velocity within individual
seams. Close up views of the Fruitland coal
section highlight differences observed in both
cases. AVO plots are presented for both cases.
The CMP gathers are used as a proxy for the VSP
response the response at longer CMP offsets
corresponds to deeper phones in the borehole VSP.