应用CT扫描技术研究低渗透岩心水驱油机理

Study on waterflooding mechanism in low-permeability cores using CT scan technology

通过CT扫描技术在线获得不同时刻岩心内流体饱和度的沿程分布信息,探索了低渗透岩心水驱油机理,并讨论了驱替速度(毛管数)、束缚水存在状况和非均质性对微观孔隙介质中流体渗流分布特征及微观驱油机理的影响。实验结果表明,当驱替速度较高时,含水饱和度增量沿程分布呈现对流式的直进形态,此时微观驱油机理以活塞式推进为主;当驱替速度较低时,毛管压力开始起作用,使含水饱和度增量沿程分布范围拓宽,在很小的注入孔隙体积倍数下出口端即会见水,此时微观驱油机理以卡断或爬行为主。另外,与无束缚水时相比,束缚水的存在使得含水饱和度增量沿程分布推进前缘变得更加平缓,这是因为预先存在于小孔隙中的水很容易被注入水补充聚集,在含水饱和度增量沿程分布推进前缘到达前有充足的时间形成稳定的隔断阻塞孔喉,因此束缚水的存在促进了卡断现象的发生。在强非均质性岩心中,含水饱和度增量沿程分布前缘的推进更加分散、均匀,这是因为孔隙介质的微观非均质性使得驱替产生的毛管阻力具有较大差异,使得指进和绕流成为主要的微观驱油机理。

To understand the waterflooding mechanism in low-permeability cores, the CT scanning has been applied to monitor the in-situ water saturation profiles along the core at different time points. The new method improves the accuracy.of measurement with CT values correction. The influence of injection rates (or capillary numbers), presence of connate water and heterogeneity of core samples on displacement characteristics of waterflood has been further studied. The saturation profiles show convective straight displacement at high flow rate (high capillary number) that is, the piston-like transport mechanism is favorable. While at low flow rate (low capilla number), the capillary forces broaden the saturation distribution and water breakthrough occurs at the outlet very quickly which makes the snap-off and crawling mechanism predominant. The presence of connate water lessens the steepness of imbibition fronts. It could be attributed to the pre-existing water phase in the small pores which supplies wetting fluid and forms the stable collars of water before the connected front catches up. Therefore, the presence of connate water promotes snap-off events. The advance of saturation front proceeds in a hyper-dispersive manner and the saturation profiles are almost uniformly distributed along the heterogeneous core samples.The microscopic heterogeneity increases the difference of capillary forces generated in the displacing process, which causes the bypassing and fingering to be the dominant oil entrapment mechanism.