Time-lapse analysis of pressure transients due to ocean tides for estimating CO2 saturation changes

This study proposed and examined a practical technique for analysing CO2 storage responses using offshore pressure transients affected by the ocean tide. The gravitational attractions of the solar-system bodies cause ocean tides, and the pore pressure exhibits diurnal and semidiurnal fluctuations in response to such tidal phenomena. The pressure-fluctuation amplitude is related to the loading efficiency, which is a function of reservoir elastic properties and fluid saturations. Therefore, the loading efficiency can be used to estimate the in situ pore compressibility and the CO2 saturation. Applying the tidal-signal analysis in a time-lapse manner, one may see temporal changes in CO2 saturation and consequently describe the dynamic behavior of sequestered CO2. In the analysis at the offshore CO2 storage site in Tomakomai, Japan, a temporal decrease in CO2 saturation was detected during the shut-in period, which is caused primarily by CO2 migration away from the well. The proposed methodology essentially requires only continuous pressure data, which are routinely available during CO2 storage operations, and thus, can be a cost-effective and labour-saving monitoring technique.