Transport and Retention of Coal Fines in Proppant Packs during Gas–Water Two-Phase Flow: Insights from Interface, Pore, to Laboratory Scales

The transport and retention of coal fines within proppant packs play a critical role in the recovery of coalbed methane (CBM). Their accumulation can lead to serious pore clogging and reduced pack conductivity. While previous research has primarily focused on the behavior of coal fines during single-phase water flow, there is a lack of understanding of their behavior during gas–water two-phase flow. The current study utilized visualization observations and theoretical analysis to investigate the multiscale behavior of coal fines within unsaturated proppant packs. A series of experiments were conducted to examine the transport and retention of coal fines within proppant packs under different gas-to-water flow rate ratios. The experiments involved visualizing the transport process of coal fines, as well as monitoring the displacement pressure and effluent concentration. The results showed that, during gas–water two-phase flow, gas–water interfaces (GWIs) frequently expanded and contracted, leading to the remobilization of coal fines retained on the solid–water interfaces (SWIs). The significant attachment of coal fines to GWIs hindered their mobilization and induced the formation of isolated gas bubbles, whereas the coal fines adhered to these bubbles and were retained within the proppant packs. At the laboratory scale, the distribution of retained coal fines was highly nonuniform, constructing preferential pathways for the smooth passage of gas, water, and coal fines. Furthermore, as the gas-to-water flow rate ratio increased, the amount of retained coal fines decreased and the area of preferential pathways increased. The interface-scale mechanisms governing the transport and retention of coal fines were analyzed using the force analysis. It was found that the attachment of coal fines to GWIs was chemically unfavorable, occurring passively during the removal of coal fines from SWIs by GWIs. The capillary force exerted by GWIs was identified as the primary driving force for the mobilization of coal fines, with coal fines being removed only during the contraction (advancement) of GWIs. Theoretical results were consistent with visualization observations. Overall, the findings from this study provide significant insights for effectively managing coal fines within proppant packs during gas–water two-phase flow, with implications for enhanced CBM recovery.