Influence of Fracturing on a Coal Structure during Coalbed Methane Stimulation

The impact of fracturing on coal seams includes not only mechanical alterations but also physical and chemical alterations. The coupling of these alterations plays an important role in the recovery of coalbed methane (CBM). ¹³C nuclear magnetic resonance (¹³C NMR), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and molecular models were conducted on coals with different degrees of fracturing to study the alterations in the coal structure during CBM stimulation. The ¹³C NMR results show that some aliphatic chains and oxygen-containing functional groups were shed, and some aliphatic rings were broken due to the effects of fracturing, which cause an increase in the relative content of aromatic carbon. The HRTEM and XRD results indicate that fracturing will result in a decrease in the interlayer spacing d, an increase in the stacking height L, and a slight increase in the layer size L. Moreover, the orientation distribution in fractured coal was more intensive. The construction of molecular models also verified the variation of surface functional groups and interlayer spacing. Based on these analyses and molecular models, the alteration mechanism of functional groups and aromatic structures under fracturing was demonstrated. This study clarifies the alteration of the coal structure by fracturing and has important implications for the recovery of CBM.