Model-Based Acoustic Impedance Inversion for Mapping Gas Plumes in the Stenlille Gas Storage Aquifer

Underground gas storage plays an important role in reliable and secure energy supply. In Stenlille, Denmark, such a facility has been operated by Gas Storage Denmark since 1989. The gas is stored in stratigraphic units named zone 1-6, consisting of sandstones interbedded by more fine-grained material in the Triassic-Jurassic Gassum Formation (Laier and Øbro 2009). The geometry of the reservoir is further complicated by faults that divide the gas zones into compartments; some segments of the faults are sealed, while others are connected and may lead to gas migration of gas (Bredesen 2022). Understanding the fluid migration routes and detailed reservoir characterization is pivotal for the operation efficiency and security of the gas storage facility.
In the Stenlille area, 20 deep wells were drilled from 1980 to 2009, and 3D seismic data acquired in 1997 has recently been reprocessed yielding a higher continuity of reflectors in the Gassum Formation interval. This study performs model-based acoustic impedance inversion, and the inverted model is interpreted for fluid content, lithology, porosity, cementation, and pressure variations. The different acquisition times of borehole log and seismic data yield mismatching results of the inverted seismic traces and the log data due to changes in gas content and pressure. Methodologically, this is challenging, but it also provides a valuable time dimension to the data that reduces the ambiguity of acoustic impedance interpretation, as the differences can be ascribed to fluid and pressure variations rather than lithology, cementation, and porosity effects. The gas injection/extraction history, rock physics modelling, and mapping of potential uncertainties and errors also aid the interpretation. The results indicate that we can distinguish zones 1, 3 and 5 and interpret the extent of the gas plumes in zone 3 and 5 using the proposed approach. Further, it is found that zones 1 and 3 are connected by a fault with an offset leveling the two zones. The results also suggest that porosity, clay content and water saturation are the properties that mainly control the acoustic impedance in the Gassum Formation in Stenlille.

In addition to natural gas storage, the Stenlille structure is planned to be the first Danish onshore storage for CO2 starting from 2026 (Gas Storage Denmark, n.d.). Since natural gas has acoustic properties similar to CO2 (Bredesen 2022), the current work also serves as a feasibility study for imaging and monitoring CO2 reservoir using 3D-seismic post-stack data.