NUMERICAL ANALYSIS OF PRESSURE RESPONSE OF FORMATION INDUCED BY OCEAN TIDES DURING CO₂ GEOLOGIC STORAGE

Abstract

 It is well known that the pore pressure fluctuates due to the earth tide and ocean tide. In the Tomakomai CCS conducted in Tomakomai City, Hokkaido, pore pressure fluctuation due to ocean tides were detected from the pore pressure data measured at a CO₂ injection well. It has been reported that after CO₂ injection, the amplitude decreased and the time lag to ocean tide occurred. This phenomenon is due to the increase in the compressibility of pore fluid induced by inflow of CO₂. The phenomena can be explained by mechanical equilibrium under ocean tide loadings and flow of pore fluids in the reservoir. This indicates the possibility of a monitoring method to estimate the CO₂ behavior in the reservoir by analyzing the variation of pore fluid pressure due to ocean tide. For this purpose, in order to deal with three-dimensional mechanical equilibrium and CO₂ movement, a numerical analysis for fluid-geomechanical problem is suitable. However, there has been no example in which the relationship between the change in the pressure fluctuation and the movement of the CO₂ plume has been investigated by numerical analysis. Therefore, in this study, we have developed a fluid-geomechanical coupled analysis program (SWN3D) based on the theory of poroelasticity in the presence of two-phase flow. We have conducted a simulation for the reduction of amplitude and time delay of the water pressure response during CO₂ injection. As a result, it was confirmed that the decrease in amplitude and time delay of the pore pressure fluctuation appeared in the numerical analysis results. In this analysis, the amplitude and time delay of the pore pressure fluctuation change in conjunction with the CO₂ behavior in the reservoir. That indicates possibility that the CO₂ behavior in the reservoir can be monitored by analyzing the pore pressure fluctuation due to ocean tide.