Quantitative micro mechanical and pore structural characterisation of coal before and after freezing

• Micro-CT analyses show the porosity of coal sample increases after freezing. • Elastic modulus of coal increases after freezing measured by Nanoindentation. • Changes of porosity and elastic modulus of different coal types vary after freezing. Altering the coal pore spaces and mechanical properties is of great significance in increasing the production of coalbed methane. Freezing has been used to change the pore structure in coal beds. This study uses micro computed tomography (μCT) and nanoindentation experiments to study the microstructure of coal samples before and after being frozen by liquid nitrogen. The change of porosity is analysed layer-by-layer. The change of volume fraction and quantity of pores with different diameters are counted and analysed. Pore network model is used to analyse the pores structural connectivity and permeability. In addition, the changes of elastic modulus and hardness are analysed based on nanoindentation experiments, and the relationship between the two is studied. The μCT results showed that the porosity of coal samples increases to varying degrees after freezing, with the maximum value being 41.23% and the minimum value being 17.93%. After coal is frozen, the volume fraction of large pores (>10 μm) increases, and the number of pores with the diameter less than 5 μm decreases. The results of nanoindentation experiments showed that elastic modulus of three coal samples increases after freezing, while one decreases. In addition, there is a positive correlation between the elastic modulus and hardness of coal samples.