Influence of recovery model of coalbed methane wells after drainage interruption on coal fines migration in propped fractures

In the drainage process of coalbed methane well, the interruption of drainage will induce a large amount of coal fines to stay in propped fractures, causing damage to the permeability of propped fractures. Based on the solid-liquid two-phase flow and gas-liquid-solid three-phase flow two patterns, experiments were carried out to investigate the effect of recovery mode after drainage interruption on coal fines migration in propped fractures. The permeability evolution law of propped fractures under the one-stage recovery mode and step-by-step recovery mode was compared, and the mechanism of coal fines migration under different recovery modes was revealed. The results show that the one-stage recovery mode can increase the permeability of propped fractures above the pre-interruption level under the condition of solid-liquid two-phase flow, but the step-by-step recovery mode is difficult to restore the permeability of propped fractures to the pre-interruption level. Under the condition of gas-solid-liquid three-phase flow, both the one-stage recovery mode and the step-by-step recovery mode can increase the permeability of gas phase and liquid phase beyond the pre-interruption levels, and the one-stage recovery mode presents a better improvement effect than the step-by-step recovery mode. Under the conditions of solid-liquid two-phase flow and gas-liquid-solid three-phase flow, the retention forms of coal fines in propped fractures are mainly solid-liquid interface and gas-liquid interface adhesion, which is the main reason for the different improvement effects of the permeability of propped fracture under a same recovery mode. The flow rate amplification of the first-stage recovery mode is much greater than that of the step-by-step recovery mode, resulting in greater wall shear stress and stronger gas-liquid interface disturbance effects. As a result, the promotion and discharge effect on retained coal fines is stronger, and the improvement effect on permeability is better. During the stages of single-phase water drainage and gas-water joint drainage, in order to suppress the impact of drainage interruption on the permeability of propped fractures, the first-stage recovery mode should be adopted after the interruption of drainage.