The micro-occurrence state and the mobility of irreducible water in coal is important in achieving high coalbed methane (CBM) production. Displacement system and computed tomography experiments were conducted using high-rank coals collected from the Qinshui Basin of China. The registered wet and dry computed tomography images were subtracted to provide differential images, within which the contrast between the different features is high. The results show that the volume distribution of irreducible water in coal fractures follows the power law. With the displacement pressure increasing from 0 to 1 MPa, the irreducible water saturation in semibright coal changed from 93.10% to 20.08% and that in semidull coal changed from 73.62% to 9.35%. Based on the water volume and shape factor, the single irreducible water cluster (pore-scale droplets) was classified into four shapes: network shape, patchy shape, corner shape, and isolated shape. The network-shaped irreducible water cluster has a large volume, has a complex structure, and occupies several fractures. The patchy-shaped irreducible water is distributed mainly in the narrow-apertured or dead-end fractures. The corner-shaped irreducible water is small, and the structure of corner-shaped irreducible water is simpler. The isolated-shaped irreducible water is made up of small and fragmented droplets that can not block the fractures even stacking together. In the present study, the network-shaped irreducible water volume proportion in both samples changes from 94.27% to 71.17% for semibright coal and from 87.93% to 49.78% for semidull coal. Therefore, the network-shaped irreducible water plays a dominant role at different displacement pressures. Larger irreducible water is easily transformed into smaller irreducible water with increased displacement pressure. The corner- and isolated-shaped irreducible waters are higher in number than the network- and patchy-shaped irreducible waters. Compared to semidull coal, the better fracture connectivity in semibright coal causes a larger reduction of the network-shaped irreducible water volume with increased displacement pressure. Although we tested only two samples, the test shows that the mobilization of irreducible water for different shape types also varies with different lithotypes. The significant reduction of network-shaped irreducible water improves the CBM relative permeability in coal. This study develops the understanding of the irreducible water micro-occurrence state, which can be studied further and include capillary pressure distribution, irreducible water mobilization, and CBM production promotion.

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