Experimental study of supercritical CO2-H2O-coal interactions and the effect on coal spontaneous imbibition characteristics

The interaction among supercritical CO2 (ScCO2)-H2O-coal can impact the pore structure of coal, thereby bringing about changes in the spontaneous imbibition (SI) characteristics of coal. This is of great significance for the effect of hydraulic fracturing and the increase in coalbed methane production. The changes in the pore structure and imbibition characteristics of coals with different degrees of metamorphism after ScCO2-H2O treatment were studied via nuclear magnetic resonance technology. The experimental results indicate that ScCO2-H2O treatment can increase porosity, enhance pore connectivity, and significantly increase the imbibition rate. It also alters the proportions of different pores. The proportions of micropores, microfractures and fractures increase, while the proportions of small pores, mesopores and large pores decrease. This change is more significant in low-rank coal. During the SI, the main contribution to the imbibition volume of coal samples is consistent with the proportion of pore structure; hence, there are differences in the extent of changes in the SI characteristics between low-rank and high-rank coals. The numerous etched pores produced by ScCO2-H2O treatment exhibit self-similarity, reducing the roughness and complexity of the pores while enhancing connectivity. The emergence of a large number of microfractures and fractures has a minor impact on the change in SI volume and rate but affects the mode of SI.