Evaluation of Compressibility of Multiscale Pore-Fractures in Fractured Low-Rank Coals by Low-Field Nuclear Magnetic Resonance

Evaluation of stress sensitivity of multiscale pores in coal seams stimulated by hydraulic fracturing or that disturbed by underground mining is significant to coalbed methane recovery and gas disaster control. In this work, two types of low-rank coals are selected to investigate the stress sensitivity of pore-fractures with different scales by the low-field nuclear magnetic resonance (LF-NMR) method. The compressibility changes of multiscale pores in intact coal cores and coal cores with self-supported and proppant-supported fractures under different loading paths are analyzed. The results show that the volume of pores decreases with the increasing confining stress, except some specific cases caused by water migration across pores with different scales. In the loading-unloading process, an unloading memory effect is observed. After three cycles of loading-unloading, the pore volume cannot recover to the initial value. The pore compressibility is not a constant but decreases with the increasing confining stress. Besides, the pore compressibility is also scale-dependent, and the compressibility of fractures can be 10 times greater than that of the micropores. The existence of the artificial fracture can obviously increase the compressibility of the coal, while the introduction of the proppants can significantly reduce the fracture compressibility. During the loading-unloading process, there exists a 2-30% loss of the compressibility, even when the confining stress decreases to the initial value. The compressibility measured by LF-NMR has a value and variation trend similar to that measured by the permeability-stress method, indicating that LF-NMR is an effective method for the compressibility tests of coal.

Automated summary

This study investigated the stress sensitivity of multiscale pores in coal seams using the LF-NMR method. The research found that the volume of pores decreases with increasing confining stress, except in cases caused by water migration. It was also discovered that pore compressibility is not constant, but decreases with increasing confining stress and is scale-dependent.