Multi-Parameter Viscoelastic Full Waveform Inversion (Q-FWI) for CO2 Saturation Monitoring: Leveraging Velocity and Attenuation Attributes

Abstract Monitoring injected CO2 saturation is critical for carbon capture and storage (CCS) projects, but its quantification remains challenging due to complex fluid-rock interactions and the subtle seismic signatures of partial saturation. Saturation analysis is an integral aspect, necessitating the use of all available information for estimation. Gas-bearing reservoirs with higher gas saturation often exhibit high attenuation, a key property we leverage. Here, we present an integrated approach combining a physics-guided rock physics model with a multi-parameter, hierarchical viscoelastic full waveform inversion (Q-FWI) to quantify and characterize subsurface CO2 saturation. For this purpose, we adopt a modified-CPET rock physics model to theoretically link elastic (Vp, Vs, ρ) and viscoelastic (Qp, Qs) properties to saturation states. By simulating extreme fluid distribution processes (drainage and imbibition), our approach addresses the scarcity of reliable attenuation measurements and characterizes the distinct effects of partial saturation. We then introduce a multiparameter hierarchical FWI workflow to estimate these subsurface properties. Our Q-FWI sequentially inverts for Vp, Vs and ρ, before estimating Qκ and Qμ simultaneously. The algorithm yields promising results on the synthetic Sleipner model with a multi-component OBN setup, demonstrating its accuracy. The final inverted models for the elastic and viscoelastic subsurface properties (Vp, Vs, ρ, Qκ, and Qμ) delineate the plume geometry and are used to jointly interpret CO2 saturation using Vp, Qp, well logs, and core data-based relationships using a most sensitive regime-dependent strategy based on their sensitivity to different saturation ranges. By rigorously integrating seismic wave imaging (Q-FWI) with rock physics modeling, we establish a robust foundation for quantitative CO2 saturation analysis in complex geologic environments. Our strategy extends readily to applications such as enhanced oil recovery and hydrogen storage.