Miscible CO₂ soaking alternating gas (CO₂-SAG) flooding is an improved version of CO₂ flooding, which compensates for the insufficient interaction of CO₂ and crude oil in the reservoir by adding a CO₂ soaking process after the CO₂ breakthrough (BT). The transmission of CO₂ in the reservoir during the soaking process is controlled by the pore throat structure of the formation, which in turn affects the displacement efficiency of the subsequent secondary CO₂ flooding. In this work, CO₂-SAG flooding experiments at reservoir conditions (up to 70 °C, 18 MPa) have been carried out on four samples with very similar permeabilities but significantly different pore size distributions and pore throat structures. The results have been compared with the results of CO₂ flooding on the same samples. It was found that the oil recovery factors (RFs) when using CO₂-SAG flooding are higher than those when using CO₂ flooding by 8–14%. In addition, we find greater improvements in the RF for rocks with greater heterogeneity of their pore throat microstructure compared with CO₂ flooding. The CO₂ soaking process compensates effectively for the insufficient interaction between CO₂ and crude oil because of premature CO₂ BT in heterogeneous cores. Moreover, rocks with a more homogeneous pore throat microstructure exhibit a higher pressure decay rate in the CO₂ soaking process. The initial rapid pressure decay stage lasts for 80–135 min (in our experimental cores), accounting for over 80% of the total decay pressure. Rocks with the larger and more homogeneous pore throat microstructure exhibit smaller permeability decreases because of asphaltene precipitation after CO₂-SAG flooding, possibly because the permeability of rocks with a more heterogeneous and smaller pore throat microstructure is more susceptible to damage from asphaltene precipitation. However, the overall permeability decline is 0.6–3.6% higher than that of normal CO₂ flooding because of the increased time for asphaltene precipitation. Nevertheless, the corresponding permeability average decline per 1% oil RF is 0.11–0.34%, which is lower than that for CO₂ flooding, making the process worthwhile. We have shown that CO₂-SAG flooding has the potential to improve oil RFs with relatively less damage to cores, especially for cores with small and heterogeneous pore throat microstructures, but for which severe wettability changes due to the CO₂ soaking process can become significant.

中文翻译：

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孔喉微观结构对致密CO ₂互渗交替气驱致密砂岩储层的影响

CO ₂混溶均质交替气驱（CO ₂ -SAG）是CO ₂驱替的改进版本，它通过在CO ₂突破后增加CO ₂均浸过程来补偿CO ₂与原油在储层中相互作用不足。BT）。在浸泡过程中，CO ₂在储层中的传输受地层的孔喉结构控制，进而影响随后的二次CO ₂驱替的驱替效率。在这项工作中，CO ₂-SAG驱油实验是在四个渗透率非常相似但孔径分布和孔喉结构差异很大的样品上进行的，在储层条件下（最高70°C，18 MPa）。将结果与相同样品上的CO ₂驱油结果进行了比较。结果发现，使用CO ₂ -SAG驱油时的采收率（RFs）比使用CO ₂驱油时高8-14％。此外，我们发现与CO ₂驱油相比，岩石的孔喉微观结构具有更大的异质性，从而在RF方面有了更大的改进。CO ₂浸泡过程可有效补偿CO ₂之间的相互作用不足和原油，因为异质岩心中的CO ₂ BT过早。而且，具有更均匀的孔喉微观结构的岩石在CO ₂浸泡过程中表现出更高的压力衰减率。最初的快速压力衰减阶段持续80-135分钟（在我们的实验堆芯中），占总衰减压力的80％以上。孔隙喉微观结构较大且较均匀的岩石表现出较小的渗透率，这是由于CO ₂ -SAG驱替后沥青质的沉淀所致，这可能是由于孔隙喉结构微观结构较不均一且较小的岩石的渗透率更容易受到沥青质沉淀的损害。但是，总体渗透率下降比正常CO高0.6–3.6％₂驱油，因为增加了沥青质沉淀的时间。但是，每1％的油RF相应的渗透率平均下降幅度为0.11-0.34％，低于CO ₂驱油的渗透率下降幅度，这一过程值得考虑。我们已经表明，CO ₂ -SAG驱油具有改善油RF的潜力，对岩心的损害相对较小，特别是对于具有细小且不均一的孔喉微观结构的岩心，但是由于CO ₂浸泡过程而导致的严重润湿性变化会变得很明显。