Summary
CO2 injection into an oil reservoir at low temperatures (less than 120 °F) can form three hydrocarbon phases—a vapor phase, an oil-rich liquid, and a CO2-rich liquid phase. Most available reservoir simulators cannot handle three-hydrocarbon-phase flash, and the use of two-phase flash may cause significant numerical instability. The issue has been recognized in the industry for a long time. Studies to include three-hydrocarbon-phase flash in compositional simulations exist in the literature. However, this approach results in substantial increases of model complexity and computational cost; thus, it may not be realistic for practical applications (at least for now). In this work, we propose a new pressure/volume/temperature (PVT) modeling procedure to eliminate the three-hydrocarbon-phase region for reservoir-fluid/CO2 mixtures at low temperatures and to study its implication for flow simulation. In our method, the acentric factors of pseudocomponents are adjusted to eliminate the three-hydrocarbon-phase region, which was not considered in any of the previous studies. Then, the experimental data for reservoir-fluid PVT, CO2 swelling test, and minimum miscibility pressure are also matched by adjusting further binary-interaction coefficients, volume-shift parameters, and critical volumes of the pseudocomponents. The procedure is applicable for cases with relatively small three-phase regions (e.g., some fields in west Texas), and can be applied with any PVT simulation software and conventional two-hydrocarbon-phase simulators. The method is considered for two sector models from oil fields in west Texas, with fine-scale (more than 600,000 gridblocks) and upscaled models. Compared with the standard characterization, in which the three-hydrocarbon phases exist, the new fluid model significantly improves the stability of flow simulation, demonstrating the robustness and efficiency of the new procedure. One can view the method as a practical approximation to field-scale simulations of CO2 injection at low temperatures.
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