Fractal Analysis on Heterogeneity of Pore–Fractures in Middle–High Rank Coals with NMR

Because of the complex nature of coal reservoirs, no determined method can clearly assess their pore–fracture structure. This paper combines a fractal method with the transverse relaxation time ( T 2 ) of nuclear magnetic resonance (NMR) to study pore–fractures in a coal reservoir. Based on 10 coal samples with vitrinite reflectance ( R o,m ) in the range 1.32–2.43%, the pore size/volume distribution from mercury intrusion porosity (MIP) and NMR was compared, revealing that a strong correspondence exists, and that the correspondence for pore diameter of >100 nm is better than that for pore diameter of <100 nm. Pore diameter of 100 nm is also the division for adsorption and seepage pore, which corresponds to 0.29 ms of T 2 value. Furthermore, fractal dimension and pore–fractures by NMR are discussed. The pore–fractures in coal can be divided into four types based on the T 2 spectrum of NMR: micropore, mesopore, macropore, and fracture, corresponding to <0.29, 0.29–12, 12–100, and >100 ms of T 2 values, respectively. Fractal dimensions for these four types of pore–fractures (D1, D2, D3, and D4) were acquired and ranged from −2.84 to −2.41, from 2.71 to 2.89, from 2.73 to 2.95, and from 2.87 to 2.99, respectively. However, D1, −2.84 to −2.41, for micropore does not conform to the definition of fractal theory because of bulk fluids and diffusion in internal field gradients in micropore. D NMR, D MIP, and D N are fractal dimensions by NMR, MIP, and N 2 (77 K), presenting a relationship of D NMR < D N < D MIP . D NMR shows different correlations with the percentages of different pore–fractures. Strong correlations between percentages of pore–fractures and vitrinite reflectance ( R o,m ) show a huge change at R o,m = 1.8–2.0%, which indicates a third coalification jump. When permeability is over 0.025 mD (millidarcy), permeability also presents a positive relation with fractal dimension D NMR . Therefore, the NMR fractal method of studying the heterogeneity of pore–fractures in middle–high rank coals has both feasibility and practicability. The NMR fractal method provides a more reasonable and reliable way to characterize coal pore–fracture structure qualitatively. Meanwhile, the NMR fractal method provides a new perspective to explore the process of coalification jump.