In view of tight oil reservoirs with no natural productivity (tight sandstone and carbonate reservoirs with matrixpermeability under overburden pressure of no more than 0.2mD (pore permeability less than 2mD)), a nanometer highefficient oil washing agent has been developed by combining nano-drop, high efficient oil washing surfactant andwettability reversal means, which can turn the disadvantage of the small pores and throats into advantage and realizefracturing effect enhancement by making use of the spontaneous imbibition.For the tight oil reservoir, large scale and high pumping rate multi-stage fracturing in horizontal wells and fracturediversion technique are recommended to smash the reservoir to create a fracture network of natural fractures andartificial fractures for oil and gas flow. The aim is to increase reservoir stimulation volume and the artificial fracture area.Meanwhile, nanometer high efficient oil washing agent is added in the fracturing fluid. By changing rock wettability, theoil-wet interface is turned water-wet, and the capillary force is changed from imbibition resistance to imbibition drive,boosting spontaneous imbibition. Therefore, the biobased solvent and surfactant can work in synergy to separate the oilfilm from pore and throat surface, realizing displacement of oil by water. Experimental data shows the application thenanometer oil displacement agent can enhance the oil displacement efficiency of core by12%. After fracturing, the wellshould be shut down for some time. The shut-down time should take the reservoir pressure and temperature, porestructure and connectivity and the imbibition capacity of the fracturing fluid etc into account, and can be worked out byspontaneous imbibition experiment and NMR etc to guide field operation.This technology has been used 10 well times in tight oil reservoirs in western China, including 3 well times of keyexploration well stimulation, and 7 well times of old well repeated fracturing, all the treatments have achieved goodeffect. Among them, one well worth special note, this well had no production before fracturing, but obtained a highproduction of 42t a day after fracturing. During the shutdown of well, the fracturing fluid in the artificial fracturenetwork can contact fully with micro-pores in the matrix and displace the oil in them through imbibition to the fracturesystem, then the oil can flow along the fracture network to the well bottom.The high efficient imbibition fracturing technology for tight oil reservoir is a revolution in fracturing. With fracturingfluid system and well shutdown different from traditional fracturing, this technology can enhance fracturing effect andmore importantly oil recovery. This paper has great guidance and reference significance for engineers and researchersengaged in tight oil development.
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