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Supercritical CO 2  (SC-CO 2 ), as a new type of energizing fluid, has been widely applied in fracturing and acidizing treatment on tight sandstone, shale and carbonate formation. However, the effect of SC-CO 2  on limestone's pore structure is unclear. To explore the effect, SC-CO 2  reactor featuring high-temperature and high-pressure was used to carry out soaking experiments of SC-CO 2 -brine-limestone and SC-CO 2 -limestone respectively. With the help of NMR and SEM, this paper studies the effect of SC-CO 2  on limestone's pore structure when the soaking time is different. It is found that in the experiment of SC-CO 2 -brine-limestone, SC-CO 2  will react with brine to produce carbonic acid, which would react with calcite to augment limestone's porosity and permeability. Yet at the same time, Ca 2+  and Mg 2+  in the brine deposit continuously, which would lessen limestone's porosity and permeability. Besides, it is the generation of large pores that leads to augmentation of porosity, and more large pores are produced as time increases. While in the experiment of SC-CO 2 -limestone, volume expansion caused by transforming SC-CO 2  into gaseous CO 2  also augments limestone's porosity, and the augmentation is due to small pores. However, the increase of porosity in experiments without brine is much lower than that with brine.

Effect of supercritical CO 2 on limestone's pore structure based on NMR and SEM