Changes in Pore Structure of Dry-hot Rock with Supercritical CO2 Treatment

Using supercritical CO2 as a fracturing or heat exchange medium can not only improve the thermal recovery efficiency of geothermal energy but can also store CO2 and reduce CO2 emissions. The pore structure characteristics are important factors affecting the permeability of the dry-hot rock (granite in this study), as well as the key parameters to predict any induced earthquakes. However, the influence of supercritical CO2 on the pore structure of granite is not clear. In this study, nuclear magnetic resonance, X-ray diffraction, and scanning electron microscopy were performed to study the effects of supercritical CO2 saturation on the granite over time. The results showed that many new dissolution pores are found on the surface of calcite after supercritical CO2 saturation, and the formation of secondary minerals can also be observed. The crystallinity of granite mineral particles decreased after supercritical CO2 saturation. In addition, the proportion of micropores increased, while the proportion of mesopores and macropores decreased compared to the unexposed granite. Supercritical CO2 saturation improved the connectivity among micropores, mesopores, and macropores, and longer saturation times in supercritical CO2 led to stronger microscopic heterogeneity in the granite. Furthermore, ultrasonic wave velocity results showed that the granite was damaged by the supercritical CO2 saturation. In the early stage of supercritical CO2 saturation, the damage coefficient of granite developed rapidly. With the increase of the saturation time, the damage coefficient of granite remained stable.