A new multi-component kinetic model was developed to simulate the non-equilibrium gas ex-solution process in the heavy oil bulk volume. The model assumes that bubble formation starts by populating microbubbles in the oil phase when depressurization starts. Microbubbles have flexibility to form either bigger bubbles or free gas phase depending on depletion rate. When the pressure depletion rate is extremely low, microbubbles directly move to the gas phase. When the depletion rate is fast, the microbubbles aggregate into bigger bubbles. As a result, the model has flexibility to shift from non-equilibrium to equilibrium behavior simulating the full spectrum of foam formation behavior. The kinetic model was tuned by using a thermal reservoir simulator. The equilibrium K-values for the kinetic model were generated by tuning the Peng-Robinson equation of state. In the numerical simulator, the variables for the kinetic model were adjusted to match the experimental data for a heavy oil. In the proposed kinetic model, dispersed bubbles in the oil phase increase its compressibility and reduce its density, however, in the new model, there is no significant change to the oil phase viscosity. The new model is able to represent non equilibrium gas-exsolution laboratory data and reveals that there must be a minimum excess concentration of the dissolved gas in the oil phase to drive the bubble formation reaction. (C) 2017 Elsevier Ltd. All rights reserved.