CO_2 can effectively improve the oil recovery of spontaneous imbibition in tight oil reservoirs. In this paper

the influence mechanism of CO_2 on the imbibition of tight oil is studied. The experimental equipment of carbonic acid solution was designed to conduct the experiments at different pressures

different permeabilities and residual oil saturations. The results show that

CO_2 can increase the spontaneous permeability and extraction degree. Increasing the pressure can enhance the mass transfer speed of CO_2 and improve the permeability. Lower permeability will hinder the diffusion of CO_2 in the core and weaken the ability of CO_2 to increase the spontaneous permeability. The existence of bound water will affect the seepage capacity and consume a part of the CO_2

to reduce the permeability and absorption speed

but the bound water is generally distributed at the blind end and invalid pores

so the influence on the total extraction degree of seepage and absorption is small. The traditional view is that capillary force is the main driving force of spontaneous permeation. However

this study found that increasing the CO_2 pressure can reduce the interfacial tension of oil and water and reduce the capillary force

and the higher the permeability

the greater the capillary radius and the lower the capillary force. Therefore

the effect of CO_2 on capillary force is not the main mechanism to improve the spontaneous imbibition in tight reserve. The micro-flow experiment indicates the dissolution

diffusion and expansion are the main mechanisms to improve the imbibition rate and recovery efficiency of tight oil.