Characterizations of full-scale pore size distribution, porosity and permeability of coals: A novel methodology by nuclear magnetic resonance and fractal analysis theory

Nuclear magnetic resonance (NMR) has been widely used to evaluate the pore size distribution (PSD) properties of coals. However, the NMR signal itself only provides a relative distribution of pore size. To calculate an absolute pore size distribution from the NMR data, the T2 cutoff value needs to be known. Meanwhile, the T2 cutoff value is an indicator to divide the irreducible fluid and movable fluid in porous rock and a key factor for permeability prediction. Conventionally, the T2 cutoff value is obtained by centrifugal experiments, the process of which is complicated and time consuming, and some T2 cutoff value prediction models are not suitable for coals with complex pore structures. Hence, a new method is needed for T2 cutoff value prediction. In this study, we performed scanning electron microscopy (SEM), low-temperature nitrogen adsorption/desorption (LTNA) and NMR experiments on 12 coal samples. The results of SEM and LTNA reveal the complex pore structures of the coals. According to the results from centrifugal experiments, the T2 cutoff value is in the range from 0.5-2.8 ms for subbituminous coals, whereas it is 15-32 ms for anthracite coals. We present a fractal theory based method for T2 cutoff value prediction. Using the estimated T2 cutoff values, we calculated the movable porosities, PSD and permeability for the selected coals. The results show that the proposed permeability model provides significantly better permeability estimation than classic (Coates and SDR) models. These methods are applicable not only for coals, but also for other unconventional gas reservoir rocks such as gas shales.