Utilization of wave attenuation in the time-lapse sonic logging at the Nagaoka site for a near well monitoring of CO2 migration

This paper discusses the seismic wave attenuation observed by time-lapse sonic logging at the Nagaoka CO2 injection site, in consideration of the many geological on-site experimental results asserting the occurrence of amplitude attenuation, as well as reduction of P-wave velocity, by CO2 invasion. The amplitude attenuation of seismic wave can be caused by a variety of physical phenomena and that the seismic data is obtained in a noisy environment, where acquisition of reliable amplitude data is technically challenging. In this paper we analyzed time-lapse sonic logging data at the Nagaoka site for evaluating the amplitude attenuation by CO2 invasion. We re-examined the semblance analysis conducted in the site using more robust methods and evaluated data quality by utilizing the obtained coherency as the quality of the waveform data. Then, we examined the P-wave amplitude from the envelope in time and spectrum domain. Although the amplitude at each logging was erroneous, averaging several logging runs revealed the amplitude attenuation. Moreover from the spectrum of P-wave, the peak frequency changed depending on the degree of CO2 saturation. We also examined the relationships that exist among the CO2 saturation, velocity reduction, and amplitude attenuation at the Nagaoka site. These relationships exhibited a dependency pattern similar to the model expected from a squirt flow. As a whole, we confirm the amplitude attenuation in the time-lapse sonic logging data, indicating that P-wave attenuation is applicable for a reliable evaluation of CO2 saturation, and for the early detection of CO2 leakage along the well.