低速非达西流动及泥质岩孔隙流体超压的形成

Non-Darcy low-velocity fluid flow and ultra-pressure formation in pores of mudstone

泥质岩层中的孔隙水在向外排运的过程中遵循低速非达西流动规律。当泥质沉积物成岩以后 ,地层中部的孔隙水必须克服微孔隙对其产生的启动阻力才能向外排出 ;否则 ,地层水就被滞流在岩层孔隙中并造成泥岩的欠压实。具欠压实地层的孔隙水承担了部分上覆地层的重量 ,从而形成孔隙流体超压。因此 ,启动阻力是泥岩层形成孔隙流体超压的根本原因。在成岩作用的早期阶段 ,泥质岩层启动阻力较小 ,流体可以流动 ,流体超压由粘滞阻力和启动阻力两部分组成 ;当成岩作用进入中—晚期以后 ,泥质岩层启动阻力增大 ,流体难以流动或不能流动 ,粘滞阻力近似等于零 ,超压值等于启动阻力值。因为启动阻力是泥质岩层的固有特征 ,所以 ,在没有裂缝或断层的情况下 ,泥质岩层就必然存在孔隙流体超压。随着泥质岩层厚度的增大、埋藏深度的增加、成岩程度的增强及砂质含量的减小 ,启动阻力增大 。

The water in pores of mudstone flowing outward follows the law of low-velocity non-Darcy seepage.After mud-sediment changed into stone,the water flowing in pores of middle layer of mudstone has to overcome the starting resistance resulted from the water film forming in pores.Water is easily preserved in the pores and forms the undercompaction of the mudstone layer.The water in the pores of the undercompaction layer undertook a part weight of the super jacent stratum and resulted in the ultra-pressure of fluid in the pores.The starting resistance is a fundamental factor for causing the ultra-pressure of fluid in pores of mudstone layers.In the early stage of diagenesis,the starting resistance was small,and the fluid was easily flowing in pores.The ultra-pressure of fluid was made up starting resistance and viscous resistance.In the middle to later period of diagenesis,the starting resistance of mudstone layer increased,and the fluid was hard or impossible to flow in the pores.The viscous resistance is equal to zero,and the ultra-pressure value is equal to the starting resistance.Because the starting resistance can express the characteristics of the mudstone,the ultra-pressure of fluid must exist in the pores if there are neither cracks nor faults.The starting resistance increased with rising thickness of mudstone layer,buried depth and heightening of diagenesis degree and with the decrease of sandy content.The ultra-pressure value of fluid in pores increased as much as the starting resistance with the change of above factors.