Abstract Hydraulic fracturing is an important approach for stimulating the production of ultra-low-permeability oil reservoirs; however, the low flowback efficiency of the fracturing fluid (FF) may cause contamination of the groundwater and reduce oil recovery. This study experimentally investigates the possibility of improving the flowback efficiency of water-based FF and oil production through the achievement of supercritical water (SCW) conditions after the injection of FF in ultra-low-permeability oil reservoirs. Possible mechanisms are explored through measurement of the capillary pressure, visbreaking of oil, pore-fracture distribution, and mineral components. The results show that the FF flowback efficiency and oil recovery after the SCW condition treatment are an average of 10.2% and 19% higher than those after regular hydraulic fracturing (RHF), respectively. The lower capillary pressure after the SCW condition treatment indicates a lower flow resistance. Furthermore, the increase in the proportion of large pores ranges from 6.6% to 9.0% for the fracturing process after the SCW condition treatment, whereas the increase in large pores after RHF ranges from 2.8% to 4.5%. The increases in the porosity of the core samples after the SCW condition treatment are 0.87%, 1.08%, and 1.43%, which are markedly higher than those after RHF (0.33%, 0.24%, and 0.26%, respectively). The percentage of kaolinite in the rock samples increases, whereas that of interstratified illite–montmorillonite decreases after the SCW condition treatment, thus increasing the stability of the minerals in water. The reduced dynamic viscosity and lighter components of the recovered oil from the SCW condition treatment reveal the occurrence of visbreaking of the crude oil.

中文翻译：

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超临界水条件在超低渗透砂岩地层改善压裂液返排中的应用

摘要 水力压裂是超低渗透油藏增产的重要途径；然而，压裂液（FF）的低返排效率可能会导致地下水污染并降低石油采收率。本研究通过在超低渗透油藏中注入FF后实现超临界水（SCW）条件，通过实验研究提高水基FF的返排效率和石油产量的可能性。通过测量毛细管压力、油的减粘裂化、孔隙-裂缝分布和矿物成分探索可能的机制。结果表明，SCW条件处理后FF返排效率和采油率均值为10。分别比常规水力压裂 (RHF) 后高 2% 和 19%。SCW 条件处理后较低的毛细管压力表明较低的流动阻力。此外，SCW条件处理后压裂过程大孔隙比例增加6.6%~9.0%，而RHF处理后大孔隙比例增加2.8%~4.5%。SCW条件处理后岩心样品孔隙度增加0.87%、1.08%和1.43%，明显高于RHF处理后的孔隙率（分别为0.33%、0.24%和0.26%）。SCW条件处理后，岩石样品中高岭石的百分比增加，而层间伊利石-蒙脱石的百分比减少，从而增加了矿物质在水中的稳定性。