Physical simulation experiments of a downhole upgrading process showed that the use of a hydrogen donor additive (tetralin) in the presence of methane (natural gas) and mineral formation under cyclic steam injection conditions led to an increase of at least three degrees in API gravity of treated extra heavy crude oil, a threefold viscosity reduction and an approximate 8% decrease in the asphaltene content with respect to the original crude. A continuous bench scale plant was used at different temperatures (280-315 degrees C) and residence times (24-64 h) for carrying out kinetic studies. A reaction model involving four pseudo-components (light, medium, heavy and asphaltene fractions) was used and the kinetic parameters (pre-exponential factors and activation energies) were determined. Using these data, compositional-thermal numerical simulations were carried out and validated using the bench scale data. The results showed a good match between the calculated and experimental degrees API gravities of the upgraded crude oil (average relative error 4%). Using the previous model, the downhole upgrading process was numerically simulated under cyclic steam injection conditions. The simulation runs showed the production of 12 degrees API upgraded crude oil, accumulated over a 70-day cycle. However, a reduction in the percentage of conversion of tetralin was observed (0.8%) in comparison with the bench scale experiments (3%), which was attributed to gravitational segregation of the steam coupled with low mixing efficiency of the hydrogen donor with the extra heavy crude oil at reservoir conditions.