Alternative EoS-based model for predicting water content, metastable phases and hydrate formation in natural gas systems

Abstract

Methods for determination of water content in natural gas are a very important asset for the oil and gas industry. However, predicting water content can be a challenging task in some conditions. This is the case when acid gases are present in natural gas mixtures or when ice or hydrate formation is taken into account.In this work, we propose a methodology based on modified cubic equations of state (Peng-Robinson and Soave-Redlich-Kwong) as an alternative of greater accuracy than other standard approaches and simpler than SAFT or other more complex equations of state. The modifications used are a three-parameter alpha function and new binary interaction parameters. The parameters were regressed from bubble point experimental data for several compounds and mixtures. Then, the model was extrapolated to liquid water, ice and hydrate formation conditions. The calculations were compared to other common methods for water content prediction. The modified equations of state performed successfully, despite presenting some limitations.Finally, we used the alternative model to discuss the role of metastable phases in hydrate formation. We studied phase equilibrium of water + methane mixtures to extract information about hydrate formation in total absence of free water. Results show that there is a critical water mole fraction for every binary mixture, above which hydrates will not form directly from the vapor phase. In lower mole fractions, surface phenomena have to be taken into account in order to fully understand hydrate formation.