A New Four-Phase Flash Algorithm for Surfactant Flood Simulation

Abstract

Abstract In the recent surfactant flood simulations, there have been efforts to consider the well-known effect of oil composition on the surfactant phase behavior. In order to truly take this effect into account, hydrocarbon phase behavior needs to be coupled with the surfactant phase behavior to evaluate more accurate oil composition, especially when gaseous phase could exist in a reservoir during a simulation. Also, there is a strong need to have this coupling in a fully implicit, fully coupled (shortly, FIM) simulator in order to get all the benefits from the FIM numerical solution approach in surfactant flood simulations. In this study, therefore, we propose a new gaseous/oleic/microemulsion/aqueous phase flash algorithm to couple the hydrocarbon phase behavior with the surfactant phase behavior in our FIM simulator using a new set of non-linear solution variables. Instead of using the phase molar fraction which is one of the conventional natural non-linear solution variables in a FIM simulator, we use various concentrations/fractions along with pressure and phase saturation as input for the four phase flash algorithm. The concentrations/fractions are the non-linear solution variables newly introduced to our FIM simulator. With the new algorithm, we can easily consider the cation exchange on surfactant and the indirect effect of volume of surfactant adsorbed on rock in the four phase flash. We need little conversion work and don't need to locally solve equations for concentrations/fractions even if the variables are required. Also we can incorporate the original two-phase hydrocarbon flash and three-phase surfactant flash models into our four phase flash without modification. These are all the advantages over a four phase flash algorithm using the phase molar fraction as input from the non-linear solve. Moreover, as molar fraction of each component is a calculated intermediate input for the new flash algorithm, any simulator using non-linear solution variables related to component moles can easily adopt the algorithm. This actually allows us to implement the same algorithm with slight extension in one of our well and separator models which use the molar non-linear solution variables. We have successfully tested and validated the new four phase flash algorithm using our FIM simulator with both conventional natural and mixed non-linear solution variables. Also, we show the algorithm works well with the effect of cation exchange on surfactant phase behavior. The algorithm has great extensibility to adopt any hydrocarbon and surfactant phase behavior models in our FIM simulator with the mixed non-linear solution variables if the models use their own conventional input and output variable types.