A number of heavy oil reservoirs under solution gas drive show anomalously good primary performance. Foamy oil behaviour is believed to be one of the reasons. Currently, numerical simulation of primary depletion in foamy oil reservoirs is based primarily on empirical adjustments to the conventional solution gas drive models. This paper presents a numerical model including the rate processes related to the nucleation, bubble growth and coalescence. The rate of nucleation is assumed to be instantaneous; the rate of bubble growth is a function of supersaturation and time; and the rate of coalescence is proportional to the amount of gas bubbles dispersed in the oil phase. The two-phase flow of oil and gas is modelled with the normal two-phase relative permeability-saturation relationship. The dispersed gas is assumed to flow with the oil as if it was a part of the liquid phase. The model satisfactorily matched the primary depletion tests in a sand-pack. It was observed that the solution gas drive recovery factor increased dramatically as the rate of pressure decline was increased. This model adequately accounts for the rate processes under solution gas drive in heavy oil reservoirs. It can be extended to investigate the effects of various process parameters on oil recovery and it may provide more reliable prediction of field performance. It also provides a tool to evaluate the significance of dynamic processes under various operation conditions.