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EN
The brittleness index can be indirectly converted by elastic parameters which are obtain by pre-stack amplitude variation with offset inversion and extended to the whole work area. However, indirect conversion will bring cumulative errors. In order to improve the accuracy of obtaining the brittleness index, the exact Zoeppritz equation including different brittleness indices is derived. Before inversion, we analyzed the characteristics of the brittleness index under the changes of brittle minerals, porosity and organic matter content through rock physics model, and selected the brittleness index most suitable for the work area. Based on the Bayesian framework, we introduce the Limited-memory Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) method to invert theoretical and actual data. Theoretical data inversion results demonstrate this method can achieve good results in both PP- and PS-wave joint inversion as well as only PP-wave inversion. To further verify the effectiveness of the algorithm, the brittleness index of actual data is directly inverted by using the studied algorithm and process. The inversion results of the borehole-side trace are in good agreement with the brittleness index calculated by log data. Both theoretical and practical data prove the feasibility of our proposed method.
EN
Causes and effects of global warming have been highly debated in recent years. Nonetheless, injection and storage of CO2 (CO2 sequestration) in the subsurface is becoming increasingly accepted as a viable tool to reduce the amount of CO2 from the atmosphere, which is a primary contributor to global warming. Monitoring of CO2 movement with time is essential to ascertain that sequestration is not hazardous. A method is proposed here to appraise CO2 saturation from seismic attributes using differential effective medium theory modified for pressure (PDEM). The PDEM theory accounts pressure-induced fluid flow between cavities, which is a very important investigation in the CO2-sequestered regime of heterogeneous microstructure. The study area is the lower Tuscaloosa formation at Cranfield in Mississippi, USA, which is one of the active enhanced oil recovery (EOR), and CO2 capture and storage (CCS) fields. Injection well (F1) and two observation wells (F2 and F3) are present close (within 112 m) to the detailed area of study for this region. Since the three wells are closely situated, two wells, namely injection well F1 and the furthest observation well F3, have been focused on to monitor CO2 movement. Time-lapse (pre- and post-injection) log, core and surface seismic data are used in the quantitative assessment of CO2 saturation from the PDEM theory. It has been found that after approximately 9 months of injection, average CO2 saturations in F1 and F3 are estimated as 50% in a zone of thickness ~ 25 m at a depth of ~ 3 km.
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