Study on the Imbibition Law and Mechanism of Strong Emulsification System Based on the Experimental and Numerical Assessment
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摘要: 向压裂液中加入表面活性剂强化焖井过程中裂缝与基质间的油水置换，是提高致密油采收率的重要措施。普遍的观点认为润湿改性和相对高的界面张力（IFT）是实现渗吸采油的关键。近年来，室内实验和矿场实践均表明，具有超低IFT的表面活性剂体系也能实现渗吸，有效地动用基质中的原油。针对超低IFT体系渗吸机理仍不明确的问题，设计润湿改性和超低IFT乳化体系，明确了致密砂岩中的高效置换体系的特征和渗吸效果，结合数学模型揭示了两种体系渗吸的油水运移模式和主导机制。结果表明，两种体系的适用范围不同，润湿改性体系在0.1 mD的储层中渗吸效果更好，而超低IFT强乳化体系在0.01 mD和0.001 mD储层中的渗吸采收率更高。润湿改性体系的渗吸是以毛管力驱动的层流模式，随着渗透率降低，润湿相填充孔隙的速度快速降低，渗吸采收率从约45%大幅度降低至18%。超低IFT强乳化体系可形成微纳米粒径的乳液，具有特殊的乳化、乳液扩散机制，在渗吸初期，主要通过乳化和扩散效应实现渗吸采油，渗吸后期，IFT降低和润湿改性的层流模式逐渐发挥作用，采收率从约38%降低至23%。超低IFT强乳化体系的乳化、扩散机制适合用于更致密油藏的油水置换过程，在致密储层的压裂增产方面具有广阔的应用前景。Abstract: Surfactants are usually added to the fracturing fluids to enhance the imbibition between fractures and matrix, which is an important measure to improve oil recovery. The popular perspective believes that altering water-wet conditions and keeping a relatively high IFT value are the keys to achieving the imbibition process. Recently, both indoor experiments and oilfield practice have shown that the surfactant systems with ultra-low IFT can also achieve imbibition and effectively recover oil from the matrix. For the problem that the imbibition mechanism of ultra-low IFT system is still unclear, the wettability alteration system, ultra-low IFT system, and emulsification system are constructed, the characteristics of imbibition systems and imbibition recovery are clarified, the oil-water transport patterns and dominant mechanisms of wettability alteration system and ultra-low IFT system with excellent emulsification are revealed by using mathematical models. The results show that the applicability range of the wettability alteration system and ultra-low IFT system with excellent emulsification is different. The wettability alteration system has better imbibition effects under 0.1 mD, but the ultra-low IFT system with excellent emulsification performs better under 0.01 mD and 0.001 mD. The imbibition process of the wettability alteration system is a layer flow pattern dominated by capillary force, the rate at which the wetted phase fills the pores decreases rapidly with the decrease of permeability, and the imbibition recovery decreases significantly from about 45% to 18%. The micro-nano emulsions can be formed by the ultra-low IFT system with excellent emulsification, which has a special emulsification and diffusion mechanism. In the early stage, oil is mainly recovered by emulsification and diffusion, in the late stage, the layer flow pattern of IFT reduction and wettability alteration gradually works, and the imbibition recovery decreases from about 38% to 23%. The emulsification and diffusion mechanism of the ultra-low IFT system with excellent emulsification is suitable for the imbibition of tighter reservoirs, and has a broad application in fracturing and stimulation of tight reservoirs.
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Key words:
- Spontaneous imbibition /
- Ultra-low IFT /
- Emulsification /
- NMR /
- Tight reservoirs
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表 1 岩心样品参数
岩心 直径/mm 长度/mm 孔隙度/% 渗透率/mD H1 2.53 24.96 13.71 0.1631 H2 2.50 25.17 13.35 0.0951 H3 2.52 24.87 13.49 0.0995 H4 2.49 24.92 13.96 0.2236 H5 2.51 24.86 14.38 0.2321 M1 2.53 25.29 8.26 1.45×10-2 M2 2.53 24.80 8.04 1.21×10-2 M3 2.50 25.07 9.05 1.08×10-2 M4 2.48 24.84 7.27 2.10×10-2 M5 2.53 24.88 7.69 2.31×10-2 L1 2.51 25.01 5.74 2.05×10-3 L2 2.50 24.98 4.15 3.21×10-3 L3 2.49 25.11 4.64 1.96×10-3 L4 2.55 25.06 4.10 1.64×10-3 L5 2.48 24.97 4.76 1.14×10-3 
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表 2 表面活性剂与原油之间的IFT
溶液 IFT/(mN/m) 水 16.1320 0.5% AES 3.2790 0.5% AEO-9 3.1270 0.5% Sur-1 + 3% NaCl 0.0042 0.5% Sur-2 + 2% NaCl 0.0048
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表 4 岩心模型的初始参数
参数 数值 模型参数（r×θ×z） 1.875 cm×360°×9 cm 网格数（r×θ×z） 15×10×18 岩心区域参数（区域1） Φ2.5 cm×5 cm 孔隙度（区域1） 13%、8%、4% 渗透率（区域1） 0.1 mD、0.01 mD、0.001 mD 含油饱和度（区域1） 90% 孔隙度（区域2） 99.9% 渗透率（区域2） 999 mD 含水饱和度（区域2） 99.9%
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