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

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• 作者：Zhongqiu Ouyang ; Dameng Liu ; Yidong Cai ; Yanbin Yao
• 刊名：Energy & Fuels
• 出版年：2016
• 出版时间：July 21, 2016
• 年：2016
• 卷：30
• 期：7
• 页码：5449-5458
• 全文大小：562K
• 年卷期：0
• ISSN：1520-5029

文摘

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 (T₂) 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 T₂ 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 T₂ spectrum of NMR: micropore, mesopore, macropore, and fracture, corresponding to <0.29, 0.29–12, 12–100, and >100 ms of T₂ 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 N₂(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.