Summary Low-salinity water (LSW) flooding is an attractive enhanced oil recovery (EOR) option, but its mechanism leading to EOR is poorly understood, especially in carbonate rock. In this paper, we investigate the main reason behind two tertiary LSW coreflood tests that failed to demonstrate promising EOR response in reservoir carbonate rock; additional oil recovery factors by the LSW injection were only +2% and +4% oil initially in place. We suspected either the oil composition (lack of acid content) or the recovery mode (tertiary mode) was inappropriate. Therefore, we repeated the experiments using an acid-enriched oil sample and injected LSW in the secondary mode. The result showed that the low-salinity effect was substantially enhanced; the additional oil recovery factor by the tertiary LSW injection jumped to +23%. Moreover, it was also found that the secondary LSW injection was more efficient than the tertiary LSW injection, especially in the acid-enriched oil reservoir. In summary, it was concluded that the total acid number (TAN) and the recovery mode appear to be the key successful factors for LSW in our carbonate system. To support the conclusion, we also performed contact angle measurement and spontaneous imbibition tests to investigate the influence of acid enrichment on wettability, and moreover, LSW injection on wettability alteration.

中文翻译：

---

总酸值及采收方式对碳酸盐低盐度提高采收率的影响

小结 低盐度水 (LSW) 驱是一种有吸引力的提高采收率 (EOR) 选择，但其导致 EOR 的机制知之甚少，尤其是在碳酸盐岩中。在本文中，我们调查了两次三次 LSW 岩心驱替试验未能证明储层碳酸盐岩中的 EOR 反应良好的主要原因；LSW 注入的额外石油采收率最初只有 +2% 和 +4% 的石油。我们怀疑油成分（缺乏酸含量）或采收模式（三级模式）不合适。因此，我们使用富含酸的油样重复实验，并以二级模式注入 LSW。结果表明，低盐度效果明显增强；第三次 LSW 注入的额外采收率跃升至 +23%。而且，还发现二次LSW注入比三次LSW注入效率更高，特别是在富酸油藏中。总之，得出的结论是，总酸值 (TAN) 和回收模式似乎是我们碳酸盐系统中 LSW 成功的关键因素。为了支持这一结论，我们还进行了接触角测量和自吸试验，以研究酸富集对润湿性的影响，此外，LSW 注入对润湿性改变的影响。