The current work presents a fully-compositional tool for modeling systems exhibiting two-phase (gas-water) flow. The tool couples complex hydrodynamic and thermodynamic models to describe the behavior of fluids flowing in a pipe.
Results show that the model is capable of predicting water condensation in the pipe, estimating pressure/temperature profiles, and handling different flow patterns and their transitions. Moreover, the model has the capacity of estimating the concentration of hydrocarbons, carbon dioxide, hydrogen sulfide, methanol, and other substances dissolved in the aqueous phase.
Hydrate formation is related to the presence of methane, ethane, propane, butanes and nitrogen in a gas stream, while acid is expected to form when significant amounts of CO2 or H2S are present in the aqueous phase. The model developed in this study provides production engineers with an excellent tool for evaluating the potential formation of hydrates and/or acids in piping systems.
Several cases were used to validate the model. Model predictions were compared against experimental data obtained by Eaton et al. (1967). The flow patterns and pressure profiles calculated by the model accurately matches experimental data. The pressure profile calculated is within 2.0% of the reported experimental values. Similarly, the average liquid holdup estimated was in good agreement with the values reported by Eaton et al. (1967).
The model is capable of predicting the initial water condensation point in the pipeline and the concentration of different substances in the aqueous mixture. The tool gives production engineers important information on how much hydrate/corrosion inhibitors to inject and where, thus saving in the design, operation and maintenance of pipeline systems.
Several flow assurance issues are related to the formation of undesirable substances (i.e. acids, hydrates, emulsions) inside a transport system. These problems can be properly addressed if the composition of the phases traveling inside the pipe is known. This information will allow engineers to assess the probability of these substances being formed and how to avoid them. The model presented herein provides engineers with a tool for understanding the flow mechanisms involved in two-phase (gas-water) flow in pipes, and help them make better assessments on the actions required to prevent those restrictions.
The Two-Fluid model (Hughes et al. (1976)) was originally developed and successfully implemented for problems in the nuclear industry. This approach was based on solving the mass, momentum, and energy conservation equations for pipes transporting water and air. This formulation provided the framework for integrating the thermodynamic behavior and the retrograde condensation effects in the simulation of gas/gas-condensate systems.
Since the late 80's, Penn State has systematically developed multiphase flow models for solving practical problems in the natural gas industry. These thermodynamic-hydrodynamic models have sought to address an operational problem while at the same time seeking a fundamental understanding of two-phase (gas-condensate) flow in pipes. The chronology of the studies performed at Penn State is described in Table 2.1.