Investigation of microcrack activation in CO2 geological storage during shear slip via real-time ultrasonic monitoring

Abstract CO 2 geological storage (CGS) is a promising strategy for reducing CO₂ emissions into the atmosphere. However, the acidic environment caused by CO₂ injection can lead to the evolution of fracture networks and induce seismicity. This study conducted extensive real-time ultrasonic P-wave measurements on acid-etched basalt samples during the shear slip process. The experimental results reveal that microcracks within acid-etched samples are activated during shear slip and close again once the shear load is removed. The distribution density of the activated fracture increases with longer acid etching durations and higher shear rates but decreases with increasing normal stress. This study also employed sensor measurements, 3D scanning, and binarization analysis to explore the friction coefficient, worn area proportions, and morphological elevation changes on the contact surface during slip. The wear and friction characteristics of the simulated fault do not fully align with the activation trends of internal microcracks in the rock mass. The primary effect of acid etching is to sever the connections between particles rather than corroding entire particles. As acid etching progresses, the weakening of these connections promotes the formation and stability of microcracks during shear slip. These findings provide insights into the formation of interconnected fractures and the frequent occurrence of low-magnitude seismic events in CGS projects. Moreover, the study highlights the need to account for the weakening effects of crack activation on rock strength, particularly when human activities occur near faults and joint surfaces undergoing shear slip.