利用CT扫描研究低渗透砂岩低速水驱特征

 利用CT 扫描技术、核磁共振技术(NMR)和恒速压汞,研究了低渗透砂岩岩心微观孔隙结构及低速水驱特征,讨论了束缚水和流速对水驱特征的影响。研究结果发现,低流速情况下由于沿程阻力差异较小,岩心内部产生类活塞式驱替,油水界面十分清晰,两相区域十分狭窄,含水饱和度剖面十分陡峭且几乎与运动方向垂直。在低速驱替过程中,水相直接通过大孔隙而把大量的剩余油封闭在中小孔隙当中,而且一旦注入水突破,靠增加注入速度及注入时间无法降低剩余油含量。这是因为岩心高孔喉比使得卡断和绕流效应增强,导致采出程度非常低下。与无束缚水相比,束缚水的存在改善了油相的流动性,且类活塞式驱替使得稳定的隔断无法形成,从而提高了驱油效率。

Low rate waterflooding in low permeability sandstones with CT scanning

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.