Thomas H. Wilson and Art Wells

1. The Kirtland Shale primary seal (caprock) Fruitland Formation reservoir zone B. A. A) Migrated stack line passing through the injection well (COM A ING 1) and two production wells (COM A 300 and STATE COM). B) Enhanced view reveals potential faults and fracture zones within the Fruitland Formation and Kirtland Shale. The VSP was run in the center COM A ING 1 well. baseline monitor Baseline - monitor Case 1 0’ 3000’ Velocity in coal section increased 10% Case 1 Post Injection travel time advance monitor Baseline - monitor baseline 0’ 3000’ Case 2 Velocity of individual coals decreased by 10% Case 2 Post Injection travel time delay 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 andArt 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 Original and modified logs for the fast model (above). Synthetic AVO response computed for the fast case (above right) 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. 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. Baseline Monitor 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/2007/07047henthorn /index.htm & http://www.geo.wvu.edu/~wilson/netl/ HenthornWilson&Wells -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 Laboratory’s on-going research in carbon sequestration under the RDS contract DE-AC26-04NT41817-6060404000 and URS subcontract No. 2010-SC-RES-30033-023. We’d 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 Partnership’s 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 O’Doud (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.