Vapour extraction (VAPEX) is a technique used to enhance heavy oil recovery. Many studies have been carried out on VAPEX on a macro scale since 1989. However, the number of studies conducted on a mega scale is limited. Solvent design and determination of injection rate are two important parameters that have not been clearly defined on a mega scale. In this study, the VAPEX process was simulated for a heavy oil reservoir to determine the optimum solvent injection rate and composition on a mega scale. Propane (C3) ranging from 5 to 50 mol% and butane (C4) from 5 to 40 mol% as solvents were mixed with methane (C1). A method of estimating injection rate on a mega scale was introduced. Furthermore, it was found that porosity, vertical permeability, and bulk volume size did not have a noticeable effect on injection rate, while injection height from reservoir top, injector length, and amount of solvent in the mixture significantly influenced injection rate. It was also observed that higher amounts of solvents in the mixture caused greater reduction in heavy oil viscosity. Conversely, oil recovery values were higher for the mixtures with lower amounts of C3 and C4. Moreover, C4 seemed more effective for heavy oil viscosity reduction than C3, while the oil recovery value by C3 was higher than C4.