Micropore volume modification for coal using non-contaminating cryogenic liquid nitrogen and its impact on sorption and diffusion behaviors

Fracturing using liquid nitrogen (LN2) has been successfully applied in coalbed methane reservoirs stimulation in field since the 1990s. However, the mechanism of LN2 acting on the alteration of pore structure is still obscure. For moisture coal, the cryogenic treatment can provide both frost-heaving force and the expansion force to increase the pore volume and fracture aperture and density. Investigation on the evolution of pore structure predominately provides deep understanding of the CBM performance after cryogenic stimulation and potentially optimizes the fracturing plan in field. This study used the particle method to examine the change in pore structure due to LN2. The particle method eliminates the pre-existing fracturing network and ensures the transport of gas is purely driven by diffusion. In this study, we focus on the micro-pore modification by LN2 treatments. The coal particles were treated by LN2 immersion. The adsorption experiment was conducted to measure the sorption and diffusion behavior of coal. The results indicated that the micropore volume increased by LN2 treatments and thus sorption capacity and diffusion coefficient of coal were improved. This study provides a scientific justification for diffusion improvement and gas production enhancement for high “sorption time” CBM reservoirs.