The possibility of the monitoring using seismic attenuation for CO₂ geological storage

Abstract

  The monitoring of subsurface CO₂ migration and distribution is of great importance for the safety of assessment in the CO₂ geological storage project. Among the various monitoring techniques, seismic method has been selected as the most reliable method in many worldwide CO₂ storage projects. However, the velocity of the seismic attributes has ever been mainly utilized. On the other hand, an amplitude attenuation which is another significant seismic attribute has not ever been applied. Aiming more extensive utilization of the seismic attenuation, this paper provides the relationship between CO₂ saturation and the seismic attenuation. In cases where estimation of amount of CO₂ will be required in underground for the future CO₂ emission trading, utilization of the seismic attenuation in addition to the velocity can increase the accuracy for the estimating. At first, we present a relationship between CO₂ saturation and seismic velocity in the case of which CO₂ are distributed partially in the pore space of rock. Next, we provide a theoretical relationship between the seismic attenuation and CO₂ saturation by the standard linear solid model. We apply this theory to geophysical logging results of the Nagaoka site and predict the seismic attenuation. Also, we have found that the modified patchy theory is applicable to the Nagaoka site by our previous research. In order to validate the theoretical model and the methodology for the calculation, we applied to a laboratory dataset. This dataset is acquired using air, not using CO₂. However the model and calculation method can be applied regardless of the kind of fluid in pore space of rock. This data set provides the observed relationship between seismic velocity and CO₂ saturation and the observed between seismic attenuation and CO₂ saturation by using a same rock-core specimen. The result shows the good agreement between the computed attenuation by the model and the observed attenuation. Hence, we found that our model and the methodology are validated consequently. And we found that large seismic attenuation will be observed at the small CO₂ saturation. It is very useful to monitor the CO₂ leakage from the storage layer. Because it enables us to detect CO₂ leakage in the earlier stage of leakage due to being able to observe a small amount of CO₂ by using the attenuation.