CO2启动盲端孔隙残余油的微观特征

低渗透油藏中存在大量的盲端孔隙,其中赋存的原油通常难以被水驱启动,成为水驱残余油的一种表现形式。建立玻璃刻蚀微观盲端孔隙模型,饱和油后开展室内实验模拟水驱、CO2非混相驱和CO2混相驱等开发过程,利用微观可视系统观测盲端孔隙中残余油在各种驱替方式下的启动特征。实验结果显示,水驱仅可以进入盲端孔隙较浅的区域,难以启动残余油;CO2非混相驱可深入盲端孔隙,部分残余油被启动,并沿内壁流出,进入主通道;CO2混相驱则可以驱替盲端孔隙深部的残余油,且随着CO2注入孔隙体积倍数的增大,基本可以将盲端孔隙中的原油驱替干净。分析认为,水驱仅依靠压力变化及流体弹性启动极少量的盲端孔隙残余油,而超临界CO2则可以不停地与残余油发生组分交换,通过流体间的传质作用进入盲端孔隙,从而启动大量残余油。因此,超临界CO2不仅对常规驱替介质波及的可流动孔隙具有较高的驱油效率,还可以启动盲端孔隙中的残余油,降低残余油饱和度,提高采收率。

Microscopic characteristics of residual oil in dead-end pores initiated by CO2

There are plenty of dead-end pores in low permeability reservoirs,in which crude oil is hardly initiated by water flooding. As a result,oil in dead-end pores is a kind of residual oil after water flooding. The microscopic dead-end model is made by glass etching. Laboratory experiments are conducted to simulate the reservoir development,including water flooding,CO2 immiscible flooding and CO2 miscible flooding under the condition of saturated oil. With the help of microscopic and visual system,the initiating characteristics of the residual oil in dead-end pores by different displacement methods are observed. Experimental results show that water could only enter the shallow area of the dead-end pores,and residual oil is hard to be initiated. Moreover,supercritical CO2 could penetrate into the dead-end pores,and parts of residual oil is initiated and driven out along the glass wall,and enters into major channel after CO2 immiscible flooding. Furthermore,the supercritical CO2 could displace the residual oil in deep pores at the dead end,and all of the residual oil could be displaced with the increase of the CO2 pore volume after CO2 miscible flooding. The analysis indicates that extremely limited residual oil in dead-end pores can be displaced with the help of pressure change and fluid compressibility by water flooding. However,the supercritical CO2 will exchange some components of the residual oil continuously,and enters into the dead-end pores through fluid mass transfer,and then a large amount of residual oil is displaced. Therefore,the supercritical CO2 not only has a high oil displacement efficiency in the live pores swept by conventional displacement fluid,but also could displace the residual oil in the dead-end pores. In other words,supercritical CO2 could reduce the residual oil saturation and enhance oil recovery.