Depths to Which Frozen Gas Fields (Gas Hydrates) May Be Expected
Authors: Katz Donald L. 
Published: December 2007

Because natural gas under pressure forms solid hydrates above the freezing point of water, it is quite normal for hydrates to form in that part of the earth's surface where the temperature is low enough. In the Arctic region there is a layer below the permafrost where natural gas is expected to occur in hydrate form.

Introduction

For more than 25 years, I have been concerned with the possibility that a layer of the earth contains natural gas in hydrate form. In areas where the earth's surface temperature is low, such as the Arctic with its permafrost, the superposition of earth temperature permafrost, the superposition of earth temperature pressure relationships with similar pressure-temperature pressure relationships with similar pressure-temperature curves at which gas hydrates form will indicate the thickness of the layer where gas would become a hydrate. The recent announcement by the Soviet Union of their discovery of frozen gas fields (solid gas hydrates) and the current interest in Arctic oil and gas production prompted the preparation of this paper. In a substantial layer of the earth below the paper. In a substantial layer of the earth below the permafrost, we may expect to find natural gas in permafrost, we may expect to find natural gas in hydrate form. I shall present here the considerations in predicting the depths of such a layer.

Gas Hydrates

Gas hydrates, which are solids resembling ice, were discovered in 1823 by Faraday while he was liquefying chlorine in the presence of water. They were later studied by French scientists in the 1890'S and rediscovered for the American natural gas industry by Hammerschmidt in 1934. Later, using the work of Deaton and Frost, my students and I conducted studies and presented methods for predicting conditions at which hydrates form. Crystalline structural considerations and high pressure work have been carried out in recent years.

Water and methane under pressure form a solid:

One explanation for hydrate formation is that the entrance of the gaseous molecules into vacant lattice positions in the liquid water structure, causes the water positions in the liquid water structure, causes the water to solidify at temperatures above the freezing point of water. The pressure-temperature conditions at which methane and natural gases form hydrates are shown on Fig. 1. Ethane, propane, and butane raise the hydrate formation temperature for methane, One percent of propane, the worst offender, raises the percent of propane, the worst offender, raises the temperature for methane from 41 degrees to 49 degrees F at 600 psia. Hydrogen sulfide and carbon dioxide also are relatively significant offenders in causing hydrates, whereas N2 and pentanes plus have no noticeable effect

Effect of Water Salinity

Fig. 1 applies to pure water and gaseous constituents. As in making ice cream in the country freezer, salt added to the water lowers the temperature at which hydrates melt or form. So the salinity of the water in contact with the gas is of consequence reducing the temperature some 5 degrees to 7 degrees F per 100,000 ppm of dissolved salt. Fig. 2 shows the behavior of methane with salt solutions.

JPT

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