In order to quantitatively evaluate water blocking damage of tight reservoirs in flowback period, based on the relationship between capillary pressure and relative permeability with water saturation, Darcy's formula was used to characterize gas-water two-phase flow equations in invaded areas, and a quantitative evaluation model of water blocking damage of fracturing fluid in tight reservoir was established. The water saturation, relative permeability and permeability damage degree of invaded areas in flowback period were solved by the Taylor's nonlinear solution method, and the effect of fracturing fluid invasion depth, fracturing fluid viscosity, flowback and production differential pressure and pressure sensitivity were analyzed. The results show that, water blocking damage starts to gradually be relieved once the pressure differential of flowback and production exceeds the capillary force. The higher the differential pressure between flowback and production, the more effectively water blockage can be relieved and the less damage water blockage can cause. It is challenging to release the water blockage at the flowback period when the invasion depth, viscosity, and stress sensitivity coefficient of the fracturing fluid are high. The higher the damage degree of the water blockage, the lower the recovery of gas will finally be.

To study adsorption mechanism of polar substance of crude oil in water flooding reservoir and its influence on surface wettability of reservoir, a sandstone skeleton model represented by quartz, a non-polar substance model represented by hexane and polar substance models represented by toluene, resin and asphaltene are constructed. Competition adsorption process and wet condition of four crude oil components and water molecules on sandstone reservoir surface are studied with molecular simulation. It shows that non-polar substances of crude oil are more easily desorbed than the polar substances when water competes with the four components for adsorption on quartz mineral surface. And as time goes by, polar substances are gradually adsorbed on the rock mineral surface, while non-polar substances move away gradually. Electrostatic force acts as adsorption and van der Waals force acts as repulsion in adsorption process. Finally, based on wettability experiment results, influence of different crude oil compositions on wettability was explained in mechanism, that is, the more polar substance in the oil component, the more resin and asphaltene, the strong the oil wettability of rock surface is. And it is more difficult to change the wettability to hydrophilic during water flooding. The conclusions are of great significance in the understanding of contributing factors of the recovery in water flooding reservoirs.