2-D acoustic VTI full waveform inversion for CCS monitoring

Abstract:

These days many geophysicists have been working not only for oil and gas exploration but also for CO₂ monitoring for CCS (Carbon Capture and storage). When CO₂ is injected and stored to the target layer, it changes the physical properties of subsurface media like p-wave velocity, density and so on. Seismic method is one of the most widely used geophysical methods for CO₂ monitoring, because it can delineate physical properties of subsurface media. To prevent CO₂ from leaking out of reservoirs, most target areas require caprocks, and shale often acts as a caprock. However, shale has a strong anisotropic property. Without considering the anisotropic property of subsurface media, interpretations of seismic monitoring data can distort the CO₂distribution or movement in the subsurface media. For computational efficiency, seismic data interpretation based on acoustic VTI (Vertical Transversely Isotropic) wave equations has been commonly done although it does not consider the shear waves.

To investigate the importance of considering anisotropic properties in acoustic FWI (full waveform inversion) for CO₂ monitoring, we compare results obtained by the acoustic VTI FWI with those of the conventional acoustic FWI for isotropic case in the frequency domain. Both methods are based on the node-based finite-element method. Numerical examples show that neglecting anisotropic properties of subsurface media can distort distribution of CO₂ and degrade reliability of subsurface image obtained by FWI.

Acknowledgements

This work was supported by the Human Resources Development program (No. 20134010200510) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Trade, Industry, and Energy and by the “Development of Technology for CO₂ Marine Geological Storage” grant funded by the Ministry of Oceans and Fisheries of Korea.