Vertical seismic profiling using a daisy-chained deployment of fibre-optic cables in four wells simultaneously – Case study at the Ketzin carbon dioxide storage site

The geological storage of carbon dioxide is considered as one of the measures to reduce greenhouse gas emissions and to mitigate global warming. Operators of storage sites are required to demonstrate safe containment and stable behaviour of the storage complex that is achieved by geophysical and geochemical monitoring, combined with reservoir simulations. For site characterization, as well as for imaging the carbon dioxide plume in the reservoir complex and detecting potential leakage, surface and surface-borehole time-lapse seismic monitoring surveys are the most widespread and established tools. At the Ketzin pilot site for carbon dioxide storage, permanently installed fibre-optic cables, initially deployed for distributed temperature sensing, were used as seismic receiver arrays, demonstrating their ability to provide high-resolution images of the storage formation. A vertical seismic profiling experiment was acquired using 23 source point locations and the daisy-chained deployment of a fibre-optic cable in four wells as a receiver array. The data were used to generate a 3D vertical seismic profiling cube, complementing the large-scale 3D surface seismic measurements by a high resolution image of the reservoir close to the injection well. Stacking long vibro-sweeps at each source location resulted in vertical seismic profiling shot gathers characterized by a signal-to-noise ratio similar to gathers acquired using geophones. A detailed data analysis shows strong dependency of data quality on borehole conditions with significantly better signal-to-noise ratio in regions with good coupling conditions.