Earthquakes response analysis of CO2 storage site using hydraulic-mechanical coupled analysis by NMM

A big earthquake causes strong seismic wave with long duration. In the case of the 2011 off the Pacific Coast of Tohoku Earthquake, duration of strong motion is more than six minutes and long-period seismic wave causes large displacement in wide area of the Japan island. We have little knowledge about the effects of such long period strong motion on fluid saturated layers at depth. Consequently, evaluating the integrity of CO2 containment during earthquakes is one of the important issues in Japan. We developed a method to evaluate the pore pressure build up due to deformation and fluid migration caused by an earthquake, and the integrity of the containment of the storage sites. This method evaluates the integrity by the safety factor response from the effective stress solved by the hydraulic-mechanical coupling analysis using NMM (Numerical Manifold Method). Then we applied the method on the anticline and flat models to study the safety factor response (see Fig. 1). The drained boundary is defined at the upper boundary. The viscose boundary is attached at both the left and right sides as a Lysmer damper (Kuhlemeyer, R.L. and Lysmer, 1973). The fixed boundary is given at the bottom boundary. Fig. 2 shows the Mohr-Coulombs failure envelope defined by the effective stress. Fig. 3 shows the input horizontal accelerations. Fig. 4 shows the results of the local safety factor response at the Caprock (blue line: anticline structure, red line: horizontal structure). It is confirmed that the local safety factor response of the horizontal structure is smaller than that of the anticlinal structure and becomes more dangerous, especially after 19 seconds, when it is reduced by about 40%. Fig. 5 shows the results of the local safety factor response at the CO2 storage site (blue line: anticline structure, red line: horizontal structure). As in the case of the Caprock, the local safety factor response of the horizontal structure is smaller than that of the anticlinal structure and is on the dangerous side, especially after 19 seconds. In this dynamic analysis, reasonable results were obtained under effective stress conditions. The dynamic analysis by NMM is considered to be one of the methods that can contribute to the evaluation of the stability of the ground in the future preliminary evaluation stage of CO2 storage sites.