The Influence of Coal Lithotype and Metamorphism on the Development Characteristics of Pore and Fracture in Coalbed Methane Reservoir in Ordos Basin and Qinshui Basin

Abstract

To study the characteristics of pores and fractures development in coalbed methane reservoir controlled by coal lithotype and metamorphism, a total of 26 coals from 16 coal mines in the Ordos Basin and Qinshui Basin were tested for kerogenic components, scanning electron microscopy, mercury intrusion experiments, low‐temperature nitrogen adsorption experiments, and Fourier transform infrared spectroscopy, which summarized and analyzed the coal lithotype and metamorphism constraints on the development of pores and fractures and came to the following conclusions: At the early stage of coalification, with the aliphatic hydrocarbon chains in coals are gradually shortened, and the branched chains and oxygen‐containing functional groups are continuously shed and reduced, resulting in asphaltenes filling the pores, which leads to an overall decrease in the porosity of coal and an increase in the inhomogeneity of the pore structure; at the transition stage of coals to high maturity, asphaltenes in coal are gradually reduced, the filling effect is reduced, aromatic lamellae are continuously oriented, and aromatic rings are continuously condensed. In the transition stage of coal to high maturity, the filling effect decreases, the aromatic lamellae keep orienting and arranging, and the aromatic rings keep condensing, which makes the porosity, pore volume, and specific surface area of coal rebound, and the nonhomogeneity of pore structure weakens; the density of microfracture decreases with the increase of coal rank, which may be the effect of compaction and asphaltene effect; the mirror group in coal is favorable to the development of pore fracture, while the inert group is opposite; the density of microfracture is positively correlated with the content of vitrite and vitrinertite, and negatively correlated with the content of inertite. The higher the content, especially the content of microscopic coal, the more developed the microfractures.