Information on the phase behavior of gas hydrate is vital for several mitigation techniques to control their formation in pipelines and surface facilities for efficient flow assurance during the production and transportation of natural gas and reservoir fluids. The pipeline environment (high pressure and low temperature) favors the formation of natural gas hydrate, which is one of the main elements in the flow assurance issues and this has to be taken in to account very seriously.
New thermodynamic computing approach is being developed for predicting the phase equilibria for vapor and solid hydrate. Vapor phase properties at different conditions can easily be calculated using equation of state, which is quite challenging for solid hydrate. The proposed hydrate model is an extension of Chen-Guo model, takes in to account the change in activity at every change in temperature. The new model assists in tuning the parameters accurately to different sets of experimental data.
To propose a model which predicts hydrate phase stability of a multi-component natural gas, consideration of hydrate structural changes and activities is necessary. Therefore besides careful selection of interaction parameters for n! combinations of guest gas molecules, selection of activity of water for a respective set of equilibrium pressure and temperature condition is an important step while computing equilibrium temperatures and pressures of hydrates formed in bulk phase. In this work, activity of water is calculated by Pitzer equation. An intricate mechanism of hydrate formation is followed and it is assumed that hydrate formed in the pores is in complete solid phase. The multi component natural gas mixture (six to ten gases) compositions are chosen in such a ways that a substantial amount of CO2 is present, which makes the activity of the hydrate formation increase from water activity slightly more (>1) due to their high solubility at higher temperatures.
The proposed equilibria phase prediction method is well validated examining eight sets of different natural gases from literature which were studied experimentally by different authors for equilibrium pressure and temperature. The developed method is found to predict the phase stabilities for a multicomponent natural gas hydrates system satisfactorily.
|Journal||Offshore Technology Conference-Asia|
|Publisher||Offshore Technology Conference|