Detachment of coal fines deposited in proppant packs induced by single-phase water flow: Theoretical and experimental analyses

Abstract

Hydraulic fracturing is an effective approach to improve the productivity of coalbed methane (CBM) reservoirs. However, during hydraulic fracturing and CBM production, massive coal fines are deposited on the surface of proppants, leading to a sharp reduction in fracture conductivity. Herein, the detachment of deposited coal fines, caused by single-phase water flow in proppant packs, was investigated to elucidate the release mechanism of fines, which would be applied towards their effective removal in the field. First, coal fines detachment experiments, including permeability and effluent measurements, were conducted on proppant packs under saturated conditions. The experiments were performed with different sizes of coal fines and gradual/abrupt increases in flow velocity to study the effects of fines size and the variation pattern of flow velocity on the release of coal fines. The experimental results indicated that there existed a critical flow velocity (CFV) for the release of coal fines in proppant packs. The value of CFV decreased at first, and then increased with the increase in fines size. Additionally, both the amounts of fines produced and the increases in permeability were more pronounced under a gradual increase in flow velocity than those under an abrupt increase in flow velocity. Second, based on Happel's sphere-in-cell model, the forces/torques acting on the deposited coal fines were analyzed to illustrate the mechanisms underlying the release of fines in proppant packs. The theoretical results revealed that, with the increase in flow velocity, the release of fines began at the proppant midpoint and ended at the front and rear stagnation points. Theoretical CFV, corresponding to the detachment flow velocity of fines at the midpoint, first decreased with the increase in fines size and then increased. Theoretical CFVs were consistent with the experimental data, thus validating the proposed model. Finally, the implications of this research on the removal of coal fines in proppant packs were also discussed.