Background  The effective coalbed methane (CBM) exploitation using hydraulic fracturing wells is severely constrained by two major solid-phase production issues: coal fines blockage and proppant flowback. The mechanisms behind both issues are associated with complex interplays between geological conditions and engineering disturbances. This study presents a systematic review of the current status of domestic and international research on the mechanisms of the solid-phase production, as well as relevant prevention and control methods. Furthermore, future research directions are proposed.

Advances The solid-phase production in hydraulically fractured CBM wells essentially arises from flow-solid-chemical multi-field coupling processes under interplays between geological conditions and engineering disturbances. Specifically, coal fines generation is attributed to the synergistic effect between intrinsic factors (i.e., coal properties) and extrinsic factors (e.g., coal seam abrasion by drilling tools, fracturing impact, and CBM production-induced pressure differences). The coal fines migration is jointly governed by the critical flow velocity, particle-size distribution, and fracture geometry, with coal fines undergoing a dynamic evolutionary process from banded to blocky morphologies. Coal fines blockage results from geometric straining (e.g., the “one-sixth rule”) and physicochemical adsorption within proppant packs. Additionally, proppant flowback occurs when the hydrodynamic drag force exceeds the particle retention force and is comprehensively governed by the fracture closure pressure, the ratio of the fracture width to proppant particle diameter (i.e., the w/d ratio, a ratio of greater than 3 leads to lower proppant stability), and fracturing fluid flowback velocity. Regarding technologies for the prevention and control of solid-phase production, a multi-tiered technical framework has been developed, covering source control, process control, and end treatment. In this framework, the preventive and control measures against coal fines blockage include coal surface modification using chemicals, proppant optimization, the application of fracturing fluid additives, and the fine-scale control of drilling and production parameters. Meanwhile, primary preventive and control measures against proppant flowback include fracturing fluid infilling by fiber, proppant modification, and the optimization of fracturing parameters.

Prospects  There is an urgent need to address several critical challenges, including poorly understood microdynamic mechanisms underlying solid-phase production, a lack of mechanisms controlling coal-proppant-fracturing fluid multi-phase interfaces, and less complete geological-engineering integrated technologies for the prevention and control of solid-phase production. Future research should focus on (1) the exploration of mechanisms behind coal fines migration and retention using multi-scale dynamic imaging combined with intelligent algorithms; (2) the R&D of coal rank-specific interfacial modifiers for coordinated multiphase control; and (3) the construction of geology-engineering integrated intelligent prevention and control systems based on digital twin and real-time monitoring. These efforts will facilitate the shift from passive response to proactive control, thereby ensuring safe and efficient CBM production.