煤层气井排采中断后恢复模式对支撑裂缝内煤粉运移的影响OA北大核心

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 in-terruption on coal fines migration in propped fractures.The permeability evolution law of propped fractures under the one-stage re-covery 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-sta-ge recovery mode and the step-by-step recovery mode can increase the permeability of gas phase and liquid phase beyond the pre-inter-ruption 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 in-terruption on the permeability of propped fractures,the first-stage recovery mode should be adopted after the interruption of drainage.