Assessing rock physics and seismic characteristics of the Gassum Formation in the Stenlille aquifer gas storage – A reservoir analog for the Havnsø CO₂ storage prospect, Denmark

The rock physics and seismic characteristics of prospective CO₂ storage sites are important inputs to assess the feasibility of seismic reservoir characterization and monitoring of the CO₂ plume. In this paper, geophysical data from the Stenlille aquifer gas storage in Denmark are used as a reservoir analog to the Gassum Formation at the Havnsø CO₂ prospect. Rock physics and pore fluid modeling of the in situ natural gas and CO₂ saturation was performed to obtain an initial understanding of the seismic-reservoir relationship and to model the seismic sensitivity for various pore fluid scenarios. Then, a 3D seismic survey at Stenlille was used as input to a Bayesian inversion for litho-fluid classification to outline the injected natural gas distributions in a probabilistic manner. Although only post-stack seismic data were available, the gas injected into the different reservoir zones of the Gassum Formation yields a low acoustic impedance signature that could be interpreted from the inversion results. The predictions were consistent with well log data, reported gas withdrawal and injection volumes and conforms with the geological structure. This implies good conditions for seismic reservoir characterization and monitoring CO₂ saturation changes in the Gassum Formation for geological settings similar to the Stenlille aquifer.