This paper examines non-isothermal effects during carbon dioxide injection into a depleted gas reservoir for storage and EGR (enhanced gas recovery). We illustrate Joule–Thomson behavior with VHD (viscous heat dissipation) effects during reservoir repressurization. The equations of state for density (empirically extended ideal gas equation) and dynamic viscosity of the real gaseous mixture are tested with numerical and experimental data. We consider the real behavior of the mixture by using energy and distance parameters. The energy conservation equation is solved to account for gas expansion with convection and conduction processes. Additionally, we present temperature profiles of the reservoir column during gas expansion and discover that the Joule–Thomson process and VHD decrease reservoir temperature, with a decrement in temperature of the reservoir depending strongly on medium porosity. Isothermal pressure and temperature calculations are compared with existing one-dimensional analytical solutions for model verification.