A pragmatic techiiique has been developed to experimentally and theoretically quantify the nonequilibrium phase behavior of alkane solvent(s)-CO2-heavy oil systems under reservoir conditions. Experimentally, constant -composition expansion tests have been conducted for alkane solvents-CO2-heavy oil systems at constant -volume expansion rates with a PVT setup to simultaneously measure volume and pressure change of the aforementioned systems. Theoretically, mathematical formulations have been developed to quantify the amount,of the evolved gas as a function of time based on the real gas equation, while mathematical models on compressibility and density of the oleic phase mixed with the entrained gas (i.e., foamy oil) are respectively formulated. The required equilibrium time and exponential coefficients associated with gas -bubble growth are determined once the deviation between the experimentally measured pressure volume profile and the theoretically calculated one has been minimized. In addition to effectively capturing the main features of foamy oil during expansion processes, its nonequilibrium fluid properties (i.e., compressibility and density) are determined as a function of the amount of the entrained gas in the liquid phase. For compressibility, a sudden change is located at the pseudobubble point pressure rather than at the thermodynamic bubble point pressure at which gas bubbles start to form. The density of the foamy oil is then found to decline at different rates when pressure is decreased from its initial value to the pseudobubble point pressure. For CO2-heavy oil systems (binary system), the difference between the pseudobubble point pressure and the maximum pressure after the pseudobubble point pressure shows a monotonic decline, whereas, for CO2-C3H8 heavy oil systems (ternary system), it reaches a peak with an increase in temperature.