Characterization of the Pore Structure and Fluid Movability of Coal-Measure Sedimentary Rocks by Nuclear Magnetic Resonance (NMR)

Analyzing and mastering the pore structure and fluid movability characteristics of coal-measure sedimentary rocks is significant for the safe and effective development of unconventional resources. In this work, nuclear magnetic resonance (NMR) experiments were carried out on three common fine sedimentary rocks (i.e., shale, mudstone, and sandstone) from a coal-measure stratum in northern China. NMR transverse (T₂) of the water-saturated and centrifuged rock samples are compared and analyzed. Moreover, the pore size distribution (PSD) and the free-fluid volume index (FFI) of the investigated samples are discussed. Results have shown that the shale and mudstone samples are mainly dominated by adsorption pores with a diameter of 0.01–1 μm, while the sandstone samples are dominated by seepage pores with a diameter of 1–100 μm. The FFI results calculated by the cutoff and the area methods are 11.15–77.62 and 7.56–75.96%, respectively. There are good correlations between FFI and porosity, permeability, and reservoir quality index (RQI). Also, the effects on FFI are different on various kinds of clay minerals. The contents of illite and chlorite are negatively correlated with FFI, while kaolinite is positively correlated with FFI.