Effects of lumping on the numerical simulation of thermal-compositional-reactive flow in porous media

Abstract

In this work, we study the influence of using different lumping strategies on the thermal recovery of an extraheavy oil. Numerical simulation of thermal recovery processes typically requires advanced thermodynamic equilibrium computations to model the phase behavior and displacement. Those models rely on compositional descriptions of the oil using up to tens of components. 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. In the context of reactive transport, most reaction schemes usually use at most four hydrocarbon components. However, the impact the lumping process has on the displacement processes can be hard to estimate a priori. We focus on 1D, 3-phase combustion tube-like numerical simulations of In-Situ Combustion (ISC) displacement processes. These thermal-compositional-reactive simulations 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. ISC cases involve multiple fronts moving downstream, including a steam front, a reaction/temperature front and multiple saturation fronts. First, we show that using a small number of components does not allow for an accurate estimation of the phase behavior of an extra-heavy oil. Using the typical reaction-based descriptions of a few hydrocarbon components (1-4) leads to inaccurate phase envelopes, for multiple compositions encountered in the displacement process. Then, we illustrate that under hot air injection without reactions, the displacement results do not capture the physical phenomena. Lumping heavy components together overestimates the size of the oil banks and gives inaccurate speeds for multiple fronts. Finally, in the presence of exothermic oxidation reactions, more components are needed to accurately capture the evaporation of medium and heavy components due to the tighter coupling and higher temperatures.