Compositional Effects in Thermal, Compositional and Reactive Simulation

Advanced thermodynamic equilibrium computations are required to model the phase behavior in the numerical simulation of thermal recovery processes. Such phase behavior models rely on compositional descriptions of the oil using up to tens of components to obtain accurate solutions. Lumping a large number of components into a smaller number of pseudo-components in order to reduce the computational cost is standard practice for thermal simulations. However, the impact the lumping process has on the displacement processes can be hard to estimate a priori. Thermal, compositional and reactive simulations such as In-Situ Combustion (ISC) exhibit a tight coupling between mass and energy conservation, through phase behavior, heat transport and reactions. We observe that depending on the number and type of lumped pseudo-components retained in the simulation, the results can exhibit modeling artefacts and/or fail to capture the relevant displacement processes. We illustrate that for several thermal hot gas injection processes, with and without reactions, the displacement results using a small number of pseudo-components do not capture the physical phenomena. Lumping heavy components together overestimates the size of the oil banks and gives inaccurate speeds for multiple fronts. Then, we consider the effects of different compositional interpretations of the lumped pseudo-species appearing in typical reaction schemes. Due to compositional effects, they strongly impact the behavior of the solutions through the intricate interplay between reactions, phase behavior and displacement effects. We allow a constant, mass-based fraction of the oil to react and modify which components are reactants. Due to molecular weight effects, the reaction rate is larger when light and medium components are allowed to react. In those cases, the fronts are faster and the oil banks bigger due to the increased displacement.