Pore structure of low permeability sandstones and characteristics of the corresponding low rate waterfloods have been investigated in virtue of CT scanning technology, nuclear magnetic resonance and rate-controlled mercury. The paper also addressed the effects of initial water and flow rate on characteristics of waterfloods. It was found that low rate can induce quasi-piston-like displacement where the saturations profile was steep and almost vertical to the forward direction and the water front was narrow and distinct. The reason for this flooding pattern may be the non-pronounced differences of flooding resistances. Notably, water passed directly through large pores and immediately blocked considerable oil in small and middle pores to be residue, though water was preferential to those pores. Moreover, once water breaks through, increasing flooding rate or flooding time hardly reduces the oil remaining inside the samples. It brings a low oil recovery probably due to the high pore-throat ratio which may result in enhancement of bypassing and snap-off. The results confirmed that the presence of initial water enhanced sweep efficiency substantially. On one hand, because water had previously occupied the small pores, the subsequent oil can only invade relatively large pores and became more movable. On the other hand, stable collars cannot form due to the steep front, which may suppress the snap-off.

CT scanning  /  nuclear magnetic resonance  /  rate- controlled mercury  /  low permeability reservoirs  /  quasi- piston- like displacement