: According to the almost all Middle East reservoirs with high depth, carbonated and fractured characteristics, a model is required to estimate heavy oil recovery from low permeability, low porosity and low thermal conductivity of such reservoirs. So a non-thermal recovery technique seems to be more successful than a thermal technique in some situations. One of the cold recovery methods is vapor extraction called VAPEX. VAPEX is the solvent analogue of the steam assisted gravity drainage (SAGD) process and uses essentially the same well configuration. A solvent vapor is injected instead of steam and the viscosity reduction occurs due to the dissolution of the solvent in the oil. In the VAPEX process, tow horizontal wells are drilled inside reservoirs for solvent injection and oil production. The main mechanism of VAPEX is governed by mass transfer of the solvent into heavy oil and consequent oil viscosity reduction and gravity drainage. The objective of this research was to develop a semi-analytical and numerical method to predict the effects of the main parameters in the VAPEX process. The semi-analytical method bases on the solvent chamber growth of VAPEX under unsteady state, constant pressure and isothermal conditions. In this method, the oil and solvent interface was divided into a number of segments, and oil drainage for each time step was calculated. The other method bases on the compositional simulation in the VAPEX process. Therefore a two-dimensional, three-phase numerical model reservoirs with formulation details, numerical solution method, and computational results was described for simulation of VAPEX process in heavy oil. The model included the effects of three-phase relative permeability and pressure-dependent fluid properties and also interphase mass transfer of solvent in heavy oil was considered. The three-phase mass balance equations were solved sequentially using finite difference method, hence, it was able to simulate VAPEX with different diffusion and dispersion coefficients. Predictions of the two models were validated against experimental data and other available models. After this, the presented models were used for extraction of heavy oil from an Iranian reservoir according to the reservoir characteristics, and several cases were run in order to study the effects of solvent, bed permeability, initial oil viscosity, bed height and diffusion coefficient on the oil production rate. Finally, it was concluded that for the stabilized stage of production, oil rate is a function of bed height to the power of 0.6847 and the stabilized production rate is proportional to the square root of the matrix permeability, moreover, the horizontal interface movement with lower permeability will have a more uniform velocity. Comprehensive and comparative studies together with extensive sensitivity analysis from 0.0178 to 0.00178 cc/s as solvent injection rates were conducted to specify the dependency of the oil recovery, breakthrough time and SOR to injection rate. On contrary of general opinion the increasing of diffusion value doesn’t always cause significant effects on production rate. At the end of study, a proper plan was suggested such that a high solvent rate is injected at the beginning of the process afterward it is reduced to decrease the requirement of solvent and eventually makes the operation be more economical. Keywords: VAPEX, heavy oil, numerical modeling, reservoir simulation, oil recovery, solvent injection, diffusion.
