Study on the Applicability of Reservoir Fractal Characterization in Middle-High Rank Coals with NMR: Implications for Pore-Fracture Structure Evolution within the Coalification Process

In order to evaluate the applicability of the pore-fracture structure fractal characterizations in coal reservoirs and confirm the internal relationships between the porosity, permeability, coal metamorphic grade, and pore-fracture structure, the pore-fracture features of 21 middle-high rank coal samples from Anhe, Jiaozuo, and Huaibei coalfields in northern China were investigated using a low-field nuclear magnetic resonance (NMR). All the coal samples are characterized by low moisture content (M-ad), low and medium ash yield (A(ad)), and high vitrinite (V) in coal maceral. The adsorption space fractal dimension (D-A) is positively correlated with the Langmuir volume (V-L) under the three-peak transverse relaxation time (T-2) spectrum. The fractal dimension of all effective T-2 points under saturated water (D-NMR) is positively correlated with V-L and the adsorption pore volume, but negatively correlated with the volume ratio of seepage pores and fractures. The free flow space fractal dimension (D-M) is negatively correlated with the porosity of full saturated water (Phi(F)) and the porosity of movable water (Phi(M)). There is a negative correlation between Phi(F) and the seepage space fractal dimension (DS) in the coal samples with one-peak and two-peak T-2 spectra, but a positive correlation can be found with the three-peak T-2 spectrum. Therefore, it is necessary to consider the types of T-2 spectral peak as a prerequisite to analyze the correlations between pore-fracture parameters and NMR fractal dimensions. With the increase of coal rank, the adsorption pore content, Phi(F), and bulk volume immovable (BVI) fraction first increase and then decrease, whereas the seepage pore content, fracture development, bulk volume movable (BVM) fraction, and BVM/BVI first decrease and then increase. The inflection points of these changes correspond to the maximum vitrinite reflectance (R-o,R-max) at 2.6-2.8%, which would be attributed to the third coalification jump. Generally, D-A is the fractal dimension representing the coal pore surface, and D-S and D-M are closely related to the pore structure. Furthermore, D-NMR not only represents the roughness of the pore surface but also the complexity of the pore structure.