Characterization of the stress sensitivity of pores for different rank coals by nuclear magnetic resonance

Nuclear magnetic resonance (NMR) experiments of stress sensitivity on the pore and fracture systems of coal samples with different ranks were performed. Pore compressibility was calculated based on the NMR results. The relationship between pore compressibility and effective stress was discussed and a mathematical model for pore compressibility was developed to describe the experimental data. The experimental results showed different characteristics of NMR T2 distributions, which were in good accordance to the diverse pore and fracture structures for the different rank coals. Medium and high rank coals have more developed pore space for adsorption, and the main peak of T2 spectra locate at the low T2 value section; while for the low rank coals, all the pores and fractures are well developed, with peaks corresponding to them are all obvious on the T2 spectra. Furthermore, the pore spaces showed different stress sensitivity for different rank coals. For low rank coals, seepage space changes dramatically as the confining pressure changes, and seepage space is the main controlling factor of stress sensitivity. As the metamorphism degree increasing, adsorption space becomes dominant in the pore and fracture structure of coals. Thus adsorption space in high and medium rank coals decreases significantly with the increase of the confining pressure, and stress sensitivity is controlled by their adsorption space, as suggested by the experimental results. The pore compressibility of the coals decreases with confining pressure increase and the experimental data can be accurately described by the new developed stress dependant pore compressibility model.