FWI time-lapse monitoring of CO₂ injection using VSP at CaMI FRS: a feasibility study

Ninoska Amundaray, Kristopher A. Innanen, Marie Macquet, Donald C. Lawton

Full waveform inversion (FWI) holds a strong potential for reservoir monitoring due to its proven capabilities to determine high-resolution subsurface models. An expanding branch of reservoir monitoring, seismic time-lapse, aims to image variations in the elastic properties of selected formations due to carbon dioxide (CO₂) storage. Several datasets and technologies are being assessed to verify secured containment of the gas, among others, vertical seismic profiles (VSP). Previous investigations for the Containment and Monitoring Institute Field Research Station (CaMI FRS) suggest changes in the bulk and shear moduli in shallow reservoir for a CO₂ injection program of a maximum 1664 tons. In this paper, we modeled two sensor dispositions with two different source arrangements of VSP, to evaluate the performance of an FWI algorithm at this geological setting. A multiscale approach with nine frequency bands (4-8 Hz, 4-12 Hz, 4-16 Hz, 4-20 Hz, 4-24 Hz, 4-28 Hz, 4-32 Hz, 4-36 Hz and 4-40 Hz) was utilized to evaluate three stages of gas injection from a baseline to a five-year period. Inverted models converged towards the true solution suggested from previous projections. Time-lapse results reproduce a reduction of P-wave velocity for near and far offsets, registering a maximum 17% decrease at reservoir levels. At these depths, model resolution does not appear to be particularly influenced by the amount and distribution of receivers, as much as it is controlled by number of modeled sources.