Control Mechanisms of Macromolecular Compositions and Structures of Coals on the Evolution of Nanopores during Coalification
Tong Liu, Baiquan Lin, Shuxun Sang, Wei Yang, Ting Liu, Shiqi Liu, Sijian Zheng, Tianfu Wang
Abstract
Nanopores have a crucial bearing on the adsorption, desorption, diffusion, and seepage characteristics of coalbed methane. This paper discussed how macromolecular compositions and structures of different-rank coals influence the evolution of 0.3–321 nm pores by various means, including N 2 adsorption, CO 2 adsorption, Fourier transform infrared spectroscopy, X-ray diffraction, and Raman spectroscopy. The following beneficial findings were obtained. With the third coalification jump ( R o = 1.2–1.3%) as the dividing line, the evolution of different-sized nanopores in coal changes prominently from being dominated by aliphatic structures to being dominated by aromatic structures. Prior to the third coalification jump (0.5% < R o < 1.2–1.3%), the volume of nanopores is positively correlated with the aliphatic side chain length CL al and the oxygen enrichment coefficient IO and negatively correlated with the degree of condensation DOC and aromaticity AR. At this stage, the degradation and removal of aliphatic compounds result in the disappearance of a large number of aliphatic structural pores. Moreover, affected by aromatic structure condensation, physical compaction, and bitumicarb blockage, the volumes of 0.3–321 nm pores witness a rapid decrease. After the third coalification jump (1.2–1.3% < R o < 2.5%), the evolution of micropores within 0.3–1.5 nm is largely subjected to the basic structural unit (BSU) structure of microcrystals, especially the average lateral size L a . The volume of mesopores within 2–50 nm exhibits an excellent exponential function relationship with both L a and L a / L c of BSUs and a linear positive correlation with the disorder degree of macromolecules AD 1 /AG. With the increase of pore size, the influence of the macromolecular structure on the evolution of macropores within 50–321 nm gradually weakens. The evolution of 2–321 nm meso-macropores is comprehensively manipulated by the lateral growth, structural orientation, and disordered arrangement of large-sized BSUs and molecularly oriented domains (MODs), and the evolution mechanism is relatively complex. On the whole, as the metamorphic degree enlarges, the evolution of meso-macropores will gradually shift from microcrystalline growth to macromolecular structure orientation. At the end of the study, a microscopic molecular model of nanopore occurrence in different-rank coals was put forward, which clearly portrays how differently sized nanopores evolve when macromolecular compositions and structures change dynamically during coalification. These findings will contribute to giving deeper insights into the formation and evolution mechanisms of nanopores in coal and bear crucial implications for the study of methane storage and transportation mechanisms within nanopores in coalbed methane exploration and development engineering.