Experimental study of changes in fractures and permeability during nitrogen injection and sealing of low-rank coal

To investigate the influence of nitrogen injection and sealing (N2IS) on the fracture and permeability of coal reservoirs, two low-rank coal samples from the Fukang (FK) and Wudong (WD) mining areas in the southern margin of the Junggar Basin, China, were collected and used for N2IS experiments. A Chandler Model 6100 Formation Response Tester (FRT) from the Chandler Company was used for N2IS experiments with sealing times of 6, 18 and 36 h and a sealing pressure of 584 psi for the samples, and the real-time permeability changes were monitored during the N2IS process. An NDP-605 NanoDarcy Permeameter was used to test the permeability with different effective stresses before and after N2IS, and the difference in pore distribution before and after N2IS was determined by low-field nuclear magnetic resonance (NMR) for the samples. X-ray CT (μCT) was used to observe the changes in the distribution of fractures before and after N2IS. The results showed that the permeability of FK increased by 43.75%, 91.67% and 162.99% and that of WD increased by 13.96%, 49.92% and 73.68% after 6, 18 and 36 h of sealing time, respectively. For each N2IS period, the permeability increased with increasing sealing time, while the change rate gradually decreased, as demonstrated by a curve similar to the “Langmuir isotherm adsorption” curve. The stress sensitivity of permeability increased after N2IS, and the decrease in permeability with increasing effective stress was greater than that of non-N2IS-treated samples. Low-field NMR showed that the amplitude of 100% water saturation had no obviously changes after N2IS, which means that no new fracture formed after that. Meanwhile, X-ray CT images also showed that no new fracture formed after N2IS, but the connectivity is enhanced. Further analysis showed that N2IS dispersed fracture fillers and opened up the fractures of the coal reservoir, resulting in an enhancement in permeability, but also increased the degree of fracture closure, which caused an increase in the stress sensitivity of the permeability.