Influence of supercritical, liquid, and gaseous CO2on fracture behavior in sandstone

Storing CO(2)in geological formations can reduce the amount of greenhouse gases in the atmosphere. In order to explore the fracture behavior of caprock during CO(2)geological storage, three caprock adsorption experiments for supercritical, liquid, and gaseous CO(2)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 CO(2)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 CO(2)phase states. Furthermore, the ability of sandstone to resist fracture and failure was significantly reduced by the adsorption of CO(2)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 CO(2)and minerals, the alteration of minerals, or the formation of new substances. SEM observations showed that liquid CO(2)adsorption caused the sandstone to undergo intergranular fracture, and the adsorption of gaseous CO(2)promoted the occurrence of transgranular fractures. In particular, due to the ScCO(2)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 CO(2)geological storage.