The Significance of a Detailed Phase Description in Simulations of Surfactant Flooding

Two simulation approaches for modelling surfactant floods exist in the literature. The main difference between the two is the description of the surfactant phase behavior. The first (detailed) approach includes a thorough representation of the surfactant ternary phase behaviour while the second (simplified) approach ignores the formation of a middle phase microemulsion. Several reasons support the use of a simplified two-phase approach including the commercial availability of this option, the ease of incorporating such option in existing waterflood simulators, and the relative ease of generating input data.

Therefore, the objective of this study is to investigate whether the two approaches differ in terms of their predictions. In other words, we ultimately want to know whether a simplified two-phase simulation approach is suitable for the evaluation and design of a given surfactant formulation in any reservoir and/or operational settings or whether we must account for the ternary phase behaviour. For this purpose, we use the University of Texas Chemical Flooding Simulator (UTCHEM) for evaluating both the simplified and detailed modelling options. Simplified models are also built in UTCHEM by diminishing the salinity window. This option was chosen in order to use the same simulator suite for the evaluation of both the detailed and simplified assumptions.

In this work, we first use a detailed surfactant three-phase simulation model that was previously generated in UTCHEM using laboratory data and calibrated against coreflood experiments to generate three simplified surfactant two-phase pseudo models that are equivalent in 1D. Their equivalency in 1D is demonstrated using analysis of variance (ANOVA). We later design two simulation-based experiments to evaluate the suitability of the simplified models for field-scale predictions. Essentially, we divide the problem into two slightly simpler parts. The first experiment looks at the evaluation of a surfactant flood under uncertainty and the second looks at the optimisation of the surfactant injection scheme under a single deterministic realisation. For each of those two simulation-based experiments, we use a 4 × 4 Graeco-Latin square design requiring 16 simulation runs. Beside the surfactant simulation model, three factors are investigated in each of those experiments. For the robust evaluation experiment, the additional factors are permeability, adsorption, and initiation. For the optimisation experiment, the additional factors are surfactant slug size, surfactant concentration, and the injection rate.

ANOVA results of both experiments suggest the surfactant models do not differ significantly. This conclusion is supported by Tukey comparisons and the main effects plots. Therefore, the results suggest that a surfactant two-phase model can reasonably approximate the actual ternary phase behaviour of surfactants. Consequently, such simplified two-phase models can be used to obtain reliable predictions for field scale simulations.