页岩气藏纳米孔隙气体渗流特征分析

页岩气藏储层孔隙非常细小，国内外页岩孔隙半径主要集中在几个纳米到20个纳米之间，国内部分页岩孔隙半径小于10个纳米。页岩气藏生产受到纳米孔隙中的游离气和吸附于干酪根中吸附气两大主体气源影响，这2种气源气在生产中表现出4种机理。研究了纳米孔隙中气体分子克努森扩散、气体滑脱、达西渗流及吸附于干酪根中气体扩散4种机理下页岩气体渗透率及孔隙压力的变化情况，并以此建立圆柱管内平面单向稳定渗流数学模型。模型模拟结果表明页岩的表观渗透率远远大于达西渗透率，孔隙半径越小，则两者比值越大，当孔隙半径从20个纳米减小到几个纳米，两者比值将会从十增大到几十；孔隙压力越小，则两者比值越大，而当压力小于5MPa时，表观渗透率与达西渗透率之比明显增加1~2个数量级。随着压力降低，克努森扩散作用不断增强，相应的压力损耗不断增加，使得纳米管柱内平面单向从供给边缘到排液道的稳定渗流压力分布已不再是线性分布。干酪根中气体由于扩散速度慢、扩散量小而对压力影响不明显。
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Gas Flow Characteristics in Nanoscale Pores of Shale Gas

hale gas reservoir pore is very small.Pore radius of domestic and foreign shale mainly concentrates from several nm to twenty nm,and a part of domestic shale pore radius is less than 10nm.There are two main gas sources which are free gas in nano-pore and dissolved gas in kerogen,yielding four mechanisms during production.By researching the four mechanisms which are Knudsen diffusion,gas slippage,Darcy Law and dissolved gas diffusion in kerogen,a cylindrical tube and unidirectional steady seepage flow mathematical model is built to study shale gas permeability and pore pressure.The results show that apparent permeability is much larger than Darcy permeability.The smaller pore diameter is,the larger ratio of the two permeability will be.The ratio will increase by 1 to 2 orders of magnitude while the pore diameter varies from twenty nm to several nm.The smaller the pressure is,the larger the ratio will be.The apparent permeability is also one to two orders greater than Darcy permeability while the pressure lies below 5 MPa.The transient pressure is nonlinear distribution from supply edge to the discharge in nano-pore mainly because Knudsen diffusion in nano-pore gets stronger and depletes more pressure under lower pressure.Diffusion in kerogen has a weak effect on pressure depletion because of such low diffusion flux.