低速非达西流动与泥质岩孔隙流体超压的形成

泥质岩层中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,当成岩作用进入到中—晚期以后,泥质岩层启动阻力增大,流体不能流动,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,孔隙流体超压就必然存在。启动阻力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、砂质含量的减小而增大,孔隙流体超压也随上述因素的变化与启动阻力等量增大。

THE NEW VIEWP01NT ON THE MECHANISM AND INFLUENCE FACTORS OF FORMING FLUIDE ULTRA-PRESSURE IN PORES OF MUDSTONE

It follows the law of low-velocity seepage that the water in pores of mudstone flows outward, after mud-sediment has been changed stone, the water in pores of middle part of mudstone layer is not able to flow outward if the starting resistance by the water film forming is not overcomed, the water is preserved in the pores so as to form undereompaction of the mudstone layer, the ultra-pressure forms by the water in the pores undertakes the part weight of the superjacent stratum, therefore, starting resistance is the fundamental reason that the fluid in pores of mudstone layers forms ultra-pressure. In the early stage of diagenesis, the starting resistance is lesser, the fluid in pores is able to flow, the ultra pressure is made up starting resistance and viscous resistance. In the middle later period of diagenesis, the starting resistance increases, the fluid is not able to flow, the viscous resistance is equal to zero, the ultra-pressure value is equal to starting resistance. Because starting resistance is the characteristic of mudstone, there is the ultra-pressure of fluid in the pores is inevitable if there are neither cracks nor faults. Starting resistance increases with increase of thickness of mudstone layer, and increase of buried depth, and heightening of diagenesis degree, and decrease of sandy content, the ultra-pressure of fluid in pores increases as much as starting resistance with the change of above factors.