Continuous gravity observation with a superconducting gravimeter at the Tomakomai CCS demonstration site, Japan: applicability to ground‐based monitoring of offshore CO2 geological storage

Gravimetric techniques are used for monitoring the distribution and migration of CO2 stored in geological formations. Superconducting gravimeters (SGs), because of their high sensitivity, can enable continuous ground‐based monitoring of offshore CO2 storage. For offshore monitoring purposes, gravity stations should be located near the seashore to maximize the signal of interest. We observed gravity continuously with an SG near the seashore at the Tomakomai carbon dioxide capture and storage demonstration site in Japan to assess variation of the observed gravity and to evaluate the applicability of this technique for monitoring of large‐scale offshore storage. Strong noise caused by strong winds and ocean waves was removed by low‐pass filtering. The noise did not fundamentally affect long‐term gravity changes. The observed gravity was affected strongly by shallow groundwater level changes but the effects were sufficiently corrected by expressing their effects as a summation of linear functions of groundwater level changes. Considering all possible effects on gravity, the standard deviation of the gravity residuals during 85 days of observation was minimized to 7.5–8.2 nm s–2. Based on the model setting of the Tomakomai site, the estimated annual gravity changes caused by industrial‐scale injection (1 MtCO2 year–1) into a hypothetical offshore formation (1 km depth) were –2.8 to –3.3 nm s–2 under different saturation conditions. The estimated changes exceed the observed standard deviation in 2.3–2.9 years. These results suggest that injection‐induced gravity changes can be detected using SG within a few years. Therefore, the present ground‐based gravimetric technique can contribute to long‐term monitoring of offshore storage. © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.