A Multi-Fibre Optic Sensing System for Cross-Well Monitoring at the Svelvik CO₂ Field Lab

Svelvik CO₂ Field Lab is a small-scale test site for cost-efficient testing and development of techniques for subsurface monitoring, with emphasis on CO₂ storage. The infrastructure was established in October 2019 and includes an injection well surrounded by four monitoring wells, all instrumented with a suite of sensors, including five different fibreoptic cables, outside the casings. Three flexible commercial cables (straight DTS, DSS and DAS) are installed in a continuous loop through all four wells without splicing, while two stiff cables provided by LBNL (a straight tactical cable and a helical wounded cable) had to be cut off at the well-ends. The fibres inside the cables were, however, spliced at the well-ends, enabling continuous connection of the wells in a future project. Strain rate raw data were recorded by the FEBUS A1-R DAS system during the first experimental campaign in October and November 2019. The campaign included conventional cross-well seismic acquisition with P- and S-wave sources, hydrophone chains and geophones. Data processing has started, with the DAS gauge length parameter currently being optimised. Further optimisation of the processing steps as well as analyses and comparison with conventional cross-well measurements will be performed in the near future.