Hydrate Formation: Considering the effects of Pressure, Temperature, Composition and Water
- Jerome Joel Rajnauth (Texas A&M U.) | Maria A. Barrufet (Texas A&M University) | Gioia Falcone (Texas A&M U.)
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- Society of Petroleum Engineers
- SPE EUROPEC/EAGE Annual Conference and Exhibition, 14-17 June, Barcelona, Spain
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
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- 663 since 2007
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The main components in producing natural gas hydrate (whether for gas storage or for transportation), are water and natural gas, at low temperatures and high pressures. Each variable has a significant effect on the formation of gas hydrate. It is therefore critical to analyze the effect of each variable on hydrate formation to ascertain the best conditions required for a successful gas hydrate formation process.
This research evaluates the effect of these critical elements: temperature, pressure, gas composition, and water upon gas hydrate formation. This paper summarizes the findings of a sensitivity analysis using varying natural gas compositions. Results show that the composition of the natural gas can affect the temperature and pressure required for formation of the hydrate. Even more significant is the effect of impurities in the natural gas on the pressure temperature (PT) curves of the hydrate. Carbon dioxide, hydrogen sulfide and nitrogen are the main impurities in natural gas affecting the hydrate formation. At a particular temperature, nitrogen increases the required hydrate formation pressure while both carbon dioxide and hydrogen sulfide lower the required hydrate formation pressure.
The quantity of water required for hydrate formation is an important variable in the process. The water to gas ratio vary depending on the composition of the natural gas and the pressure. Generally the mole ratio of water to natural gas is about 6:1; however, to achieve maximum hydrate formation an incremental increase in water or pressure may be required. This is an interesting trade off between additional water and additional pressure in obtaining maximum volume of hydrate and is shown in this analysis.
Natural gas hydrates are ice-like crystalline solids formed from a mixture of water and natural gas subjected to high pressure and suitable low-temperature conditions. These conditions are found in the permafrost and under the ocean floor.
Hydrates consist of geometric lattices of water molecules containing cavities occupied by light hydrocarbons and other types of gaseous components for such as nitrogen, carbon dioxide, and hydrogen sulfide. Transporting gas in the form of a gas hydrate may prove to be very useful in capturing stranded natural gas to meet future small scale energy demand.
The objective of this work was to pre-design a system for transporting natural gas in hydrate form. As such a sensitivity analysis was used to investigate theoretically predicted hydrate formation PT boundaries from a thermodynamic model using a commercial simulator. The theoretical model was validated with selected data (Fig 1) to ensure that the predictions were reasonable.
In this analysis, twenty one natural gas compositions of typical wells mainly from Trinidad (Ministry of Energy, 2007) are analyzed in order to estimate the process conditions to convert and transport 5 mmscf/d of gas in hydrate state to neighboring islands (e.g.). The natural gas streams obtained from these wells were, generally, sweet gases (without H2S) and ranges in composition from C1 to C10. These samples are shown in Table 1.The most important design variables affecting the percentage of hydrate formed include: pressure, temperature, composition, and amount of water required to form hydrate.
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