Study of Drainage and Percolation of Nitrogen–Water Flooding in Tight Coal by NMR Imaging
N2 flooding in pores and water percolation in fissure is analyzed in-depth to investigate the permeability reduction caused by coalbed methane displacement in low-permeability coal seam in China. Using low-field nuclear magnetic resonance imaging (NMRI), the transverse relaxation time spectrum (T2 spectrum) was carried out for tight intact and fractured coal. The T2 spectra were obtained for saturated samples containing adsorption pores (AP), percolation pores (PP), and migration pores (MP), as well as one-, two-, and three-dimensional spatial distributions of water content. The spatial distribution of the water in the displacement process of tight coal was finely characterized by low-field NMRI. Our results show that the water displacement by N2 in AP, PP, and MP requires a high to low pressure gradient and they play different functions of percolation. And the study revealed that there is no obvious gas–liquid interface during N2 flooding in tight coal. The results show that PP play an important role in water transfer and pressure transmission during the N2 flooding process, which assists in forming connected channels from nonpenetrative cracks. PP themselves can also form percolation channels and transmit pressure.