Full-Wave Inversion of DAS Borehole Seismic Data for CO2 Injection Monitoring: A Feasibility Study on CRC3 Survey Data from Otway Field.

Geological carbon sequestration involves large-scale injection of carbon dioxide into underground geologic formations used as reservoirs. Changes in reservoir physical properties resulting from the CO 2 injection and migration can be monitored using time-lapse seismic data. Conventional analysis of time-lapse surface and/or borehole seismic data provides approximate quantitativeinformation on CO2 saturation from the changes of the acoustic impedance contrasts in the reservoir. In this paper, we evaluate the ability of the multiparameter elastic fullwave inversion method (FWI) applied to DAS borehole seismic data to obtain an accurate and reliable estimation of the underground physical properties in the well vicinity.We applied the multi-parameter elastic FWI to the CRC-stage 3 survey at Otway site. In a first step, we solved preprocessing issues associated with DAS data and we built a heterogeneous multi-parameter earth model in order to qualitatively explain the data (P- and S-wave velocities, density, VTI anisotropy parameters, compressive and shear Q factors for attenuation). In a second step, we inverted the well data. The estimated models, results of the FWI for DAS data are compared to those obtained with conventional geophone-type data for single shot OVSP datasets. We also appraised the capability of the multi-parameter FWI for walkaway DAS data. The time-lapse issues are not addressed in this paper.We point out that the geophone and the DAS data provide about the same information even if the recorded physical quantities are not the same and the radiation pattern of receivers are not equivalent. We conclude that the FWI is able to extract information from the well seismic data allowing to obtain an accurate and reliable information for several physical fields, the S-wave velocity being the estimated field with the best resolution. The FWI results are reliable in the well vicinity up to 200 m from the well for the actual acquisition geometry.