Pore Structure in Coal: Pore Evolution after Cryogenic Freezing with Cyclic Liquid Nitrogen Injection and Its Implication on Coalbed Methane Extraction

Freezing and thawing cycles occur with cyclic liquid nitrogen (LN2) injection in coal. The freeze-thaw treatment damages the pore structure of coal and thus increases its permeability. In this study, NMR and strain monitoring were employed to investigate the changes in coal structure when the coal specimens were under cryogenic treatment using LN2. We classified freeze thaw process into four stages; stages I and III are dominated by seepage pore development, and stages II and IV are dominated by adsorption pore development. It was found that LN2 freeze thaw cycles can cause structural deterioration in the coal so as to improve both fracture density and overall permeability. The results demonstrate that the rate of increase of both the effective porosity and total porosity of the coal are positively correlated with the LN2 freezing time and the number of freezing cycles but negatively correlated with the residual porosity. For the same absolute LN2 freezing time, cyclic freeze thawing has a greater effect on the rate of growth of pore spaces and reduction of P-wave velocity in the coal compared with single freeze thaw treatment. It was also found that the number of freeze thaw cycles is a very important factor for the creation of larger pores, pores that can connect the fracture network. The results suggest that appropriate control of the number of freeze thaw cycles can result in effective fracturing of coal.