Experimental investigation on the impact of coal fines generation and migration on coal permeability

Measurements of the coal fines production and the impact of these fines on the permeability of two coals from the Bowen Basin, Australia, were performed at different flow conditions (single-phase water or gas, two-phase water and gas) and pressure conditions. The fines collected from each coal samples ranged in size from 1 μm to 14 μm. For both coal samples, during the first 50 h, the permeability decreases from 0.005 mD and 0.048 mD by 60.9% and 85%, respectively, followed by gradual decline with fluctuations. By the end of water injection, the permeability drops by 88% and 89%, respectively. This phenomenon is attributed to the counteraction between formation damage (cleats plugging and coal fines settlement) and breakthrough of coal fines from the samples (widened cleats). It was found that coal fines volumetric production is proportional to the third power of flow velocity once the flow paths for coal fines are established. The critical flow velocities of coal fines production for both samples were also obtained. For hydrophobic coal, water-drive-gas two-phase flow introduces abrupt permeability loss due to coal fines generation and migration. Furthermore, pauses (well shut-in) in the experiments cause slight permeability drops. A comparison between the two samples indicates that narrower and less connected cleating system results in more frequent coal fines generation and migration, resulting in significant permeability fluctuations with general decreasing trend. Tortuosity of the cleats can enhance the deterioration in permeability by coal fines behaviours. This study delivers fundamental understandings of coal fines generation and migration during the CSG production process, and useful guidelines are suggested to be implemented in the field to minimize production loss induced by coal fines behaviours.