In this study, we compare the changes in CO₂ saturation and pressure drop for two modes of injections: (i) a displacement of brine by supercritical CO₂ injection (1-to-1 displacement), and (ii) a forced co-current injection of supercritical CO₂ and brine at the same flow ratio (co-injection), respectively, in a laboratory core-flooding experiment using a Berea sandstone sample. The Berea sandstone sample showed a weak bedding structure that consists of high- and low- porosity layers. The main flow direction was set perpendicular to the bedding layers. We utilized the X-ray CT technique to image the rock interior and obtain the information of local porosity and saturation of each voxels in a three dimensional volume. The results show that the co-injection needs a much higher pressure drop to maintain the constant flow rate than the 1-to-1 displacement at similar saturation degrees. Moreover, the co-injection shows significant fluctuations in saturation and pressure drop, whereas the 1-to-1 displacement shows more gradual and monotonous changes. The spontaneous fluctuations in saturation and pressure drop during co-injection are basically coincident in temporal pace, and can be explainable by the intermittent flow of brine and CO₂ as shown in differential saturation images. Furthermore, the X-ray images show that the CO₂ mainly flows through built flow pathway during the 1-to-1 displacement; whereas the CO₂ flows near uniformly and does not strictly rely on pore size and capillarity during the co-injection. CO₂ saturation distributes more uniform among image pixels during the co-injection. These dissimilarities between the co-injection and 1-to-1 displacement suggest differences in fluid flow mechanism between them. In the 1-to-1 displacement, the pathway of CO₂ flow was created by the forward motion of CO₂ over the capillary pressure force. CO₂ preferred to first percolating through large-size pores and gradually expanded the percolation region while the CO₂ saturation grew. In this process, the displaced brine mainly flowed in its remained phase-pathway–less phase interference occurred. The connection of flow pathway for CO₂ naturally satisfied the maintaining of the flow rate. In contrast, during the co-injection, both phases flowed in the pore space. The connection of the phase-pathway was affected by the phase snap-off effect. The transport efficiency of such a partially disconnected flow-pathway was significantly lower than the 1-to-1 displacement case, leading to that much higher drive force of pressure drop was necessary to let the fluids flow at setting rates. Nevertheless, the reachability of CO₂ to low-porosity sites during the co-injection was higher than during the 1-to-1 displacement. Our findings are important for understanding of co-injections in applications of relative permeability measurement, and enhanced efficiency in capillary trapping, usage of pore space in CO₂ geological storage and in oil recovery.

在这项研究中，我们比较了两种注入方式的CO ₂饱和度和压降的变化：（i）通过超临界CO ₂注入驱替盐水（以1比1驱替），以及（ii）强制共注入当前注入的超临界CO ₂在使用Berea砂岩样品的实验室岩心驱油实验中，分别以相同的流量比（共注入​​）注入盐水和盐水。Berea砂岩样品显示出由高孔隙率和低孔隙率层组成的弱层理结构。主流动方向设置为垂直于铺垫层。我们利用X射线CT技术对岩石内部成像，并获得三维空间中每个体素的局部孔隙率和饱和度信息。结果表明，在相似的饱和度下，与1：1排量相比，共注入需要更高的压降来维持恒定的流量。此外，共注入在饱和度和压降方面显示出明显的波动，而一对一置换显示出更多的渐进和单调变化。如图₂所示，如差分饱和度图像所示。此外，X射线图像显示，CO ₂在1对1位移过程中主要流经已建立的流动路径；而在共注入过程中，CO ₂几乎均匀地流动并且不严格依赖孔径和毛细作用。在共注入过程中，CO ₂饱和度在图像像素之间分布更均匀。共注入和一对一排量之间的这些差异表明它们之间的流体流动机理不同。在1对1位移中，CO _2的流动路径是由CO ₂在毛细管压力作用下的正向运动产生的。一氧化碳₂首选先通过大孔进行渗滤，然后在CO ₂饱和度增加的同时逐渐扩大渗滤范围。在此过程中，置换后的盐水主要以其残留的相径流过，因此不会发生相干。CO ₂的流路连接自然满足了流量的保持。相反，在共注入过程中，两个相都在孔隙空间中流动。相位路径的连接受相位咬合效应的影响。这种部分断开的流路的传输效率显着低于“一对一”排量的情况，导致必须有更高的压降驱动力才能使流体以设定速率流动。尽管如此，CO ₂的可达性在共注入过程中，低孔隙度位点的位移要高于1对1位移过程中的位移。我们的发现对于理解相对渗透率测量应用中的共注入，提高毛细管捕集效率，利用CO ₂地质存储中的孔隙空间和石油采收方面的效率非常重要。