Monitoring CO ₂ injection into a fluvial brine‐filled sandstone formation at the Snøhvit field, Barents Sea

500 ktons of CO2 injected during 16 months has been monitored by well pressure and 4D seismic data. Clear 4D responses can be observed as amplitude increases and time delays at the lower part of the reservoir, up to at least a km away from the injector. CO2 saturation, increased water pressure and cooling of the rock all contribute to the change in seismic response, and it is challenging to separate the effects. Pressure sensitivity on seismic velocity is interpreted to be higher than expected. The observed pressure buildup and falloffs during shut-ins constrain reservoir volume in good communication with the well and the permeability distribution. Together these data provide a picture of a more heterogeneous reservoir than originally expected. By constraining models by both seismic and pressure data, more confidence is obtained.