Toward quantitative CO2 storage estimates from time-lapse 3D seismic travel times: An example from the IEA GHG Weyburn–Midale CO2 monitoring and storage project

Time-lapse seismic interval travel time differences are determined during 7 years of CO2 injection within the Weyburn oil field, Saskatchewan, Canada. Travel time difference maps are used in conjunction with geophysical and geological logs, and depth dependant fluid properties to provide upper bounds on the amount of CO2 that may reside within various geological intervals. Calculated travel time sensitivity values increase from 0.01 ms/m to ∼0.25 ms/m moving upward from the reservoir depth (1450 m) to the shallowest aquifer considered (600 m). The seismic-based apportionment of CO2 shows the fraction of CO2 within or below the reservoir increasing from at least 62–70% after 1 year of injection to at least 92–94% after 7 years of injection, whereas the proportion of the total injected CO2 in the zone immediately overlying the reservoir decreases from a maximum of 30–36% to ∼5–6% over the same time period. The estimates of CO2 quantities residing within this zone are significantly overestimated due to clearly identified pressure effects in this zone. The maximum estimated proportion of CO2 residing in either interval above the regional sealing formation is ≤1%. The maximum amount of CO2 potentially residing above the regional seal by 2007 is <56, 000 tonnes, with a maximum of 0.31 Mtonnes just above the reservoir, and 4.6–4.7 Mtonnes in the reservoir-containing interval.

► 4D seismic data are used to apportion injected CO2 to geological intervals.
► The maximum amount of CO2 residing in intervals above the regional seal is <56,000 tonnes.
► This compares with 4.6–4.7 Mtonnes stored in the injection reservoir.