Abstract

Natural gas hydrates significantly impact the economics of development of off-shore hydrocarbon resources in deep water. Methods for avoiding hydrates that depend on kinetics of hydrate formation may replace thermodynamic methods. Hydrate growth rates measured on a quiescent drop of water were found to agree qualitatively with rates measured in a vigorously stirred reactor when normalized with respect to the area of contact between the gas and the liquid, or liquid suspension of hydrates. The Gibbs free energy change for hydrate formation is shown to be a good estimate of driving force for hydrate growth. Hydrate growth rates per unit area correlate with the driving force. Driving force calculations suggest that hydrate structures other than the thermodynamically favored structures may form.

Economic and Environmental Implications

Natural gas hydrates negatively impact the economics of drilling, production, processing, and transportation. The impact is seen in the cost of insulation, heating, thermodynamic inhibitors, and other means for avoiding hydrates, and in the cost of lost time, production, and/or capital investment when drills or surface facilities are jammed by hydrate plugs

The cost of hydrate prevention is a significant obstacle to development of off-shore hydrocarbon resources in deep water and in regions like the North Sea. Off the U. S. Gulf Coast, water temperatures drop to 35 to 40 F in 2000 feet of water. Sub-sea production facilities with pipelines to centrally located platforms in such cold conditions must be designed to avoid hydrate formation.

The most practiced approaches for avoiding hydrate formation rely on the thermodynamics of hydrate formation. Pressure, temperature, and composition of the hydrocarbon or aqueous phase are controlled or modified to keep out of the hydrate forming region on a phase behavior diagram. For example, addition of methanol, ethanol, or glycols to the aqueous phase shifts hydrate equilibrium behavior to lower temperatures. In some North Sea operations, 10 to 20% methanol is added to the aqueous phase to avoid hydrate formation. An example of thermodynamic shifting as calculated with CSMHYDRATE is shown in Fig. 1.

There is growing concern for the environmental impact of methanol and glycols used for hydrate prevention. The potential economic ramifications of this concern could exceed the capital and operating costs discussed by Sloan.

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