Estimation and modeling of coal pore accessibility using small angle neutron scattering

• Apply small-angle neutron scattering (SANS) to quantify pore accessibility in coal matrix.
• Propose and validate a pore accessibility model using SANS results.
• Estimate pore accessibility for two different rank coals.
• Pore accessibility and pore radius has a power-law relationship.

Gas diffusion in coal is controlled by nano-structure of the pores. The interconnectivity of pores not only determines the dynamics of gas transport in the coal matrix but also influences the mechanical strength. In this study, small angle neutron scattering (SANS) was employed to quantify pore accessibility for two coal samples, one of sub-bituminous rank and the other of anthracite rank. A theoretical pore accessibility model was proposed based on scattering intensities under both vacuum and zero average contrast (ZAC) conditions. The results show that scattering intensity decreases with increasing gas pressure using deuterated methane (CD4) at low QQ values for both coals. Pores smaller than 40 nm in radius are less accessible for anthracite than sub-bituminous coal. On the contrary, when the pore radius is larger than 40 nm, the pore accessibility of anthracite becomes larger than that of sub-bituminous coal. Only 20% of pores are accessible to CD4 for anthracite and 37% for sub-bituminous coal, where the pore radius is 16 nm. For these two coals, pore accessibility and pore radius follows a power-law relationship.