Experimental Study of Impact of Dewatering Induced Coal Fines on Coal Permeability

摘要

Coal fines are small coal particles produced during Coal Seam Gas (CSG) production and may plug fluid flow paths, resulting in significant reduction in CSG productivity. This study aims to employ laboratory experiments to examine coal permeability variation induced by coal fines generation and migration during the dewatering stage. One coal sample from Bowen Basin, Australia, was tested under different pressure differences. The evolution of coal permeability and the amount of produced coal fines were monitored during the experiments. The pressure difference was increased from the initial 0.8 MPa to 1.5 MPa through 7 steps to investigate its impact on the characteristics of coal fines and the corresponding permeability change, and to obtain the critical velocities for a range of particle sizes. Every 5 mL effluent was collected and analysed in terms of particle size distribution and volume. The results revealed that for this sample, the sizes of coal fines ranged from 1 urn to 14 urn. Initially the coal permeability dropped dramatically due to excessive coal fines generation, with 74.1% reduction for the case of 0.8 MPa pressure difference, followed by gradual decline with fluctuations. It was observed that the production of large coal fine particles was accompanied by significant permeability fluctuations. This was attributed to the counteraction between formation damage (cleats plugging and coal fines settlement) and coal fines removal from the samples (widened cleats). Although no clear correlation between particle size and pressure difference was observed, the critical particle size was positively related to the flow velocity. The velocity followed parabolic fashion against the pressure difference. For this sample, keeping the differential pressure between 1.1-1.3 MPa can reduce the permeability damage induced by coal fines. This study delivers fundamental understandings of coal fines generation and migration behaviours during the dewatering stage, which can provide useful guidelines to implement effective dewatering strategies to minimize production loss induced by coal fines.