Mechanisms and control measures of incipient motion of coal fines induced by moving gas bubbles in coalbed fractures

Key words:  coalbed methane well  coal fines  moving bubbles  coal fines incipient motion  coal fines migration

Abstract:

In this study, a mechanical model for the incipient motion of coal fines induced by moving gas bubbles in fractures of coalbed was developed based on capillary mechanics, hydrodynamics, extended-Derjaguin-Landau-Verwey-Overbeek(DLVO) theory and Johnson-Kendall-Roberts(JKR) contact theory, which can be used to reveal the micro-mechanical mechanisms of the incipient motion of coal fines during coalbed methane production. Additionally, control measures for the movement of coal fines produced during the early stage of gas production in coalbeds were also proposed. The results illustrate that, during the early stage of gas production, the capillary force exerted on coal fines by moving gas bubbles is several orders of magnitude greater than the hydrodynamic forces, which is the dominant force to induce the incipient motion of the coal fines. Clay fines can be mobilized more easily by moving gas bubbles than the organic matter fines, and they are mainly migrated by the receding interfaces through rolling mechanisms. The incipient motion and migration of coal fines can be effectively controlled by decreasing the differential pressure of production and the gas-water interfacial tension, as well as by increasing the surface hydrophilicity of the fractures and the adhesive force between coal fines and fractures.