煤粉侵入对支撑裂缝渗透率的动态影响规律

Dynamic influence of coal fine intrusion on propped fracture permeability

煤粉侵入煤层水力压裂支撑裂缝容易导致裂缝堵塞,造成支撑裂缝导流能力和煤储层渗透率降低,进而严重影响煤层气井的稳定排采及产量。分别建立煤层气井停井前与重启后煤粉侵入支撑裂缝渗透率动态演化模型,并利用煤岩导流性能测试系统进行煤粉连续流动与间歇流动条件下的煤粉侵入支撑裂缝实验,验证了模型的正确性,研究了煤层气井停井与排采流速对支撑裂缝渗透率时空演化的影响规律。研究表明:随煤粉运移时间增加,支撑裂缝渗透率骤降后缓慢衰减,且沿煤粉运移方向,支撑裂缝内部空间孔隙损失率逐渐降低,裂缝内部渗透率沿煤粉运移方向逐渐升高。随煤粉持续侵入支撑裂缝,煤层气井关井重启后支撑裂缝孔隙损失率较停井前孔隙损失率更大,渗透率不能恢复至停井前。排采流速越慢,煤粉侵入支撑裂缝渗透率衰减速度越慢,渗透率越高,支撑裂缝渗透率受煤粉侵入影响在低排采流速下对流速变化更为敏感,缓慢排采流速下停井对支撑裂缝渗透率伤害更低。煤粉沉积系数越大,煤粉侵入过程中支撑裂缝近端渗透率越小,远端渗透率越大,而弥散系数越大,支撑裂缝远端渗透率越小,裂缝近端渗透率受弥散系数影响极小,煤层气井停井对支撑裂缝近端渗透率伤害更严重。煤粉侵入过程中支撑裂缝近端渗透率衰减速度更快,裂缝远端渗透率衰减速度更缓慢,沉积系数变化对支撑裂缝近端渗透率影响显著,而弥散系数变化对支撑裂缝远端渗透率影响更显著。

Coal fine intrusion into hydraulic propped fractures of coal seam easily leads to the blockage of the fracture,resulting in the decrease of the conductivity and the reduction of coal reservoir permeability,which seriously affects the stable discharge and production of coalbed methane well.In this paper,the permeability dynamic evolution models of coal fine intrusion into propped fractures were established before the shutdown and after the restart of coalbed methane well,and the experiments of coal fine intrusion into propped fracture under continuous and intermittent flow conditions were carried out by using the coal-rock conductivity test system,which verified the correctness of the models and studied the in-fluence of coalbed methane well stoppage and drainage velocity on the permeability spatial and temporal evolution laws.The results indicate that with the increase of coal fine migration time,the permeability of propped fracture decreases slowly after a sudden drop,and along the direction of coal fine migration,the spatial pore loss rate of propped fracture de-creases gradually,resulting in the permeability of fracture decreases along the direction of coal fine migration.With the continuous coal fine intrusion into propped fracture,the propped fracture pore loss rate after the shut-in and restart of coal-bed methane wells is larger than that before the shutdown,and the permeability cannot be restored to the level before the shutdown.The slower the flow rate of drainage,the slower the permeability attenuation rate of coal fine intrusion into propped fracture,and the higher the permeability.During the coal fine invasion into proppant fracture at the low drainage flow rate,the fracture permeability is more sensitive to the flow rate change,and the less damage to the propped fracture permeability caused by the well shutdown.In the process of coal fine intrusion,the larger the deposition coefficient of coal fine is,the smaller the proximal fracture permeability and the larger the distal fracture permeability.The larger the diffu-sion coefficient is,the smaller the distal fracture permeability of the propped fracture is.The permeability of the proximal fracture is very little affected by the diffusion coefficient,and the damage to the proximal fracture permeability is more serious.In the process of coal fine intrusion,the proximal fracture permeability declines faster,while the distal fracture permeability declines slowly.The deposition coefficient changes have a significant influence on the proximal fracture per-meability,while the diffusion coefficient changes have a more significant influence on the distal fracture permeability.