Depths to Which Frozen Gas Fields May Be Expected - Footnotes

Abstract

Four items related to my article, "Depths to Which Frozen Gas Fields (Gas Hydrates) May Be Expected," (April, 1971, JPT, Pages 419-423) are worthy of further discussion. First, gases in shallow horizons in the permafrost country, if present when the earth cooled, might well have transformed into hydrates before the reservoir reached 32 degrees F, and so could now be partially or wholly in hydrate form within the present permafrost layer. Hydrates in Liquid-Liquid Systems The second note is that gas hydrates can form from gases dissolved in liquids and do not require a gas phase as such. Gaseous constituents from either a phase as such. Gaseous constituents from either a gas phase or a liquid phase dissolve in water, and it is the dissolved gas that forms the solid hydrate. Early work with the methane-propane-water system had four phases in equilibrium, including a hydrocarbon liquid in contact with water. Removal of the gas phase at equilibrium would have left a hydrocarbon liquid, water liquid, and hydrate. Liquid propane at pressures above those reflected by the propane at pressures above those reflected by the three-phase vapor pressure curves forms hydrate. So one would expect that gaseous constituents dissolved in crude oil would dissolve also in connate water, and that at the appropriate temperature and pressure would solidify as hydrates. Vapor-solid equilibrium constants, y/x8 - Kv8, were devised to predict hydrate points in much the same way that dew point is calculated. The hydrate point corresponds to the temperature and pressure where the following relationship holds: .........................(1) where y is the gas-phase hydrocarbon mol fractions and xs is the hydrate phase (water free) mol fractions. Should one have a liquid phase composition and wish to make a similar prediction, he can use the vapor-liquid equilibrium constant to relate the vapor and hydrocarbon liquid compositions: y = KvLxL. Thus, the point at which hydrate forms from the hydrocarbon liquid phase becomes: .................(2) It would follow that crude oils containing dissolved gas when present in the earth during the cooling process could have that dissolved gas leave the crude process could have that dissolved gas leave the crude oil to form hydrates. Then the vapor-liquid bubble point of the crude oil would be lowered, and Eq. 2 point of the crude oil would be lowered, and Eq. 2 would be satisfiedScauzillo, in measuring hydrate formation conditions with crude oil-natural gas systems, derived results that do not appear to coincide with this analysis. Hydrates in Ocean Sediments Recently it was discovered that methane hydrates are present in the sediments at the bottom of the oceans. present in the sediments at the bottom of the oceans. Because pressure increases with depth in the ocean at some 44 lb per 100 ft of depth, and because the temperature is in the neighborhood of 43 degrees F at depths of 3,000 ft or more, it is clear that methane is in the pressure-temperature range for hydrate formation. Accordingly, it is not unexpected that any methane, as well as heavier hydrocarbon gases, generated in the ocean sediments should be in hydrate form in such sediments. P. 557