Evolution of the pore structure in coal subjected to freeze−thaw using liquid nitrogen to enhance coalbed methane extraction

The permeability of coalbed methane (CBM) reservoirs is typically very low and it is challenging and essential to effectively increase coal permeability to maximize CBM recovery. An exploratory study on enhancing coal porosity/permeability using freeze−thaw cycling with liquid nitrogen (LN₂) was conducted. The changes of fracture and porosity in coal with the freeze−thaw treatment using LN₂ were evaluated using nuclear magnetic resonance (NMR). After freeze−thaw treatment, the coal pore size tended to increase and new pores/fissures were generated. The growth rate of the pore size was positively correlated with the LN₂ freezing duration. The effective porosity had a positive correlation with the freezing duration, but the correlation for residual porosity was negative. This means that the volume of irreducible fluid in the coal decreased while the amount of free fluid increased. Scanning electron micrograph studies indicated that the maximum fracture width in the coal samples grew from 5.56 μm at a T_freezing = 1 min to 100 μm for T_freezing = 60 min, matching the NMR findings. This study provides a scientific basis and guidance for engineering application of freeze−thaw using liquid nitrogen to enhance coalbed methane extraction.