Effects of Gas Composition and Geothermal Properties on the Thickness and Depth of Natural-Gas-Hydrate Zones

Abstract

Summary When natural gas and water contact at low temperature and high pressure, gas hydrates can form. In colder climates (such as Alaska, Northern Canada, and Siberia) and beneath the oceans, conditions are appropriate for gas-hydrate formation. This paper gives the depths and potential thicknesses of such hydrate formations as a function of the geothermal gradient, gas composition, and where appropriate, permafrost thickness. pressure gradient, and ocean-bottom temperature. Analysis of Reservoir Conditions Where Hydrates Are Likely To Form An accurate model for predicting hydrate equilibrium as a function of gas composition, temperature, and pressure has been used to generate hydrate-stability data. Figs. 1 through 6 present depth-vs.-temperature profiles for the earth and indicate hydrate-stability zones. The variables studied included permafrost thickness, geotherinal radients, gas composition, and pressure gradients on land and ocean depth, ocean-bottom temperature, and gas composition beneath the ocean. Except for the gas composition. these variables allow the pressure/temperature conditions in the earth to be determined. Together with gas composition, pressure and temperature determine whether hydrates are stable. These variables are cross-correlated so that any combination of permafrost thickness, gas composition, and geothermal permafrost thickness, gas composition, and geothermal gradient can be evaluated. Correlations presented in Figs. 7 through 11 show the effect that these variables have on hydrate-stability-zone thickness. All results were calculated with a computer program of excellent accuracy that calculates the minimum pressure required for hydrate stability given temperature, T, and gas composition. If the pressure calculated by the program is lower than the pressure determined from the pressure gradient in the pressure determined from the pressure gradient in the earth, the hydrates are stable; otherwise, the hydrates are not stable. Results of the depth/temperature studies are presented in Figs. 1 through 6 as outlined below. Table 1 shows the default values used for these calculations. These values are considered to be representative. Where differences in values are likely, the effects of changes in these values have been given in Figs. 1 through 11. The values studied are not the only possible conditions in the earth. The ranges of values used were chosen because they represent conditions where hydrates can form, but some values, such as geothermal gradient and average surface temperature, can vary with location. P. 1379