Multiscale Fine Characterization of a Coal Pore–Fracture System Based on SEM, CT, and NMR in the Jingbian Block, Ordos Basin

Abstract

To achieve an accurate and comprehensive characterization of the multiscale pore–fracture characteristics of Permian coal in the Jingbian Block, Ordos Basin, a combination of scanning electron microscopy (SEM), X-ray computed tomography (CT), and nuclear magnetic resonance (NMR) techniques was utilized. With these experiments, the mineral composition, pore size distribution (PSD), porosity, and connectivity of pores in coal samples were characterized through qualitative and quantitative methods. The results show that the SEM experiments enabled qualitative identification of pores and mineral types. The coal samples primarily contained gas pores, cell pores, intercrystalline pores, and moldic pores, and clay minerals were the predominant fracture fillings. The 3D reconstruction of the CT experiments shows that the pores and fractures generally expand horizontally, while the minerals show obvious bedding expansion characteristics. Moreover, the estimation of full-size porosity in coal samples can be achieved by combining CT and NMR experiments. The full-size porosity of samples G11-5-1, G11-5-6, G11-5-9, and G11-5-11 was 8.93%, 9.11%, 10.45%, and 11.63%, respectively. The connectivity differences are primarily determined by the throat development degree and the connected pore–fracture count. Samples with more connected pores and larger throat radii exhibit excellent connectivity.