Influence of supercritical, liquid, and gaseous CO2 on fracture behavior in sandstone

Storing CO2 in geological formations can reduce the amount of greenhouse gases in the atmosphere. In order to explore the fracture behavior of caprock during CO2 geological storage, three caprock adsorption experiments for supercritical, liquid, and gaseous CO2 were designed. The fracture toughness of mode I, mode II, and mixed‐mode I/II of sandstone before and after the experiment was tested, and X‐ray diffraction (XRD), X‐ray fluorescence (XRF), and scanning electron microscopy (SEM) were used to examine the fracture mechanical characteristics of sandstone. Results showed that the pure mode I fracture toughness of sandstone immersed in supercritical CO2 (ScCO2), liquid CO2, and gaseous CO2 for 30 days decreased by 27.89%, 11.01%, and 17.43%, respectively, compared to nonimmersed sandstone. Pure mode I fracture toughness was more sensitive than mixed‐mode I/II and pure mode II fracture toughness to the various CO2 phase states. Furthermore, the ability of sandstone to resist fracture and failure was significantly reduced by the adsorption of CO2 in different phases. The effects of the different phases were in the following order: ScCO2 > gaseous CO2 > liquid CO2. The decreased ability of sandstone to resist fracture was primarily due to the geophysical and chemical reactions between CO2 and minerals, the alteration of minerals, or the formation of new substances. SEM observations showed that liquid CO2 adsorption caused the sandstone to undergo intergranular fracture, and the adsorption of gaseous CO2 promoted the occurrence of transgranular fractures. In particular, due to the ScCO2 adsorption, various fracture forms such as intergranular fractures, transgranular fractures, and mutual coupling fractures existed simultaneously. Sandstone exhibited numerous fractures and pores, and fracture resistance was weakened. Results of this study have important significance for evaluating the stability and safety of CO2 geological storage. (1) Experiments on sandstone adsorbed by CO2 of three phase states were performed. (2) The adsorption of ScCO2 minimized the ability of sandstone to resist fracture. (3) Geophysical and chemical reactions affect sandstone mechanical behavior.