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• Non-equilibrium swelling experiments were carried out of HO-gas systems. • Image analysis technique was developed to quantify the effectiveness of foamy oil. • A novel model is proposed to calculate the non-equilibrium properties of foamy oil. • The variation of apparent density/compressibility can be divided into three stages. • CH 4 is preferable for inducing foamy oil behavior than CO 2 and C 3 H 8. The foamy oil phenomenon has a positive influence on cyclic solvent injection (CSI) processes for oil recovery because it affects the properties of heavy oil (HO). However, there is still a lack of mathematical methods and basic experimental data for the physical properties of foamy oil during CSI processes. The effects of injection gas type and mole fraction on foamy oil behavior and non-equilibrium properties are still not well understood. In this study, a new type of non-equilibrium swelling experiment was proposed to determine the properties of foamy oil induced by CH 4 , CO 2 , and C 3 H 8. The effects of gas type, mole fraction, and pressure were analyzed experimentally, and foamy oil behavior was observed by utilizing a high-pressure cell that was designed specifically for this purpose. Then, a mathematical model was developed to calculate the non-equilibrium properties of foamy oil. The results show that higher pressures are needed to dissolve and disperse gas in HO to induce foaming, and the lifetime of foamy oil increases with increasing mole fraction of injected gas. The formation of foamy oil can effectively swell HO, and the swelling factor (SF b) and pseudo-swelling factor (SF pb) increase gradually with increasing mole fraction of injected gas. Compared to CO 2 and C 3 H 8 , CH 4 is advantageous for inducing foamy oil phenomena. Lower apparent density and higher compressibility were observed in the HO-CH 4 systems than in the HO-CO 2 and HO-C 3 H 8 systems at the same gas mole fractions. [ABSTRACT FROM AUTHOR]