Method for improving recovery efficiency of heavy oil reservoir through horizontal well fracturing auxiliary viscosity reducer-CO2 huff and puff

The invention relates to a method for improving the recovery efficiency of a heavy oil reservoir through horizontal well fracturing assisted viscosity reducer-CO2 huff and puff, and the method improves the recovery efficiency through a viscosity reducer-CO2 huff and puff exploitation mode on the basis of fracturing. Firstly, fracturing fluid containing a viscosity reducer is injected into a shaft, a reservoir is fractured to form one or more fractures, then the viscosity reducer is injected into the reservoir for viscosity reduction of thickened oil, and the injected fluid enters an oil layer along the fractures; and a CO2 slug is injected into the well, so that the sweep range of the viscosity reducer is widened. A thick oil system mainly comprises asphaltene, and the viscosity reducer enters an oil layer to destroy an aromatic ring pi-pi bond accumulation structure in the asphaltene, so that the viscosity and interfacial tension of the asphaltene are reduced. Crude oil is changed into oil-in-water emulsion