Denuding Hydrocarbon Liquids of Natural Gas Constituents by Hydrate Formation

Abstract

Hydrate formation from gases dissolved in hydrocarbon liquids partially strips lighter components from the liquid phase. Experiments demonstrate why Alaskan North Slope crude oils found below the permafrost can be expected to be denuded of a substantial portion of their dissolved gases. Introduction Gas hydrates are crystalline, ice-like solids formed when certain light hydrocarbons and other low-molecular-weight, nonpolar substances are contacted with water. Previous work in this field, which has been extensively reviewed, bad the major thrust of defining conditions that allow industrial gas-processing units to operate free from deposition of solid hydrates. Gas hydrates are known to exist in natural gas fields in colder climates, and it is believed that they also can exist in oil fields, under permafrost, if suitable pressure-temperature permafrost, if suitable pressure-temperature conditions prevail. The majority of previous studies considered conditions for hydrate formation from pure gases and gas mixtures, although it is now known that a gas phase need not be present to form hydrates. That is, gas hydrates can be formed from liquids containing dissolved hydrate-forming gases. It follows that if hydrate-forming conditions exist in oil fields, the lighter hydrocarbons can be removed from crude oils, rendering the denuded crudes that are low in methane to isobutane constituents. Such denuded crudes will have no dissolved gases to expel the oil and will have high viscosity, making them difficult to displace. This problem can be further complicated by the blockage problem can be further complicated by the blockage of reservoir rock pores, which can reduce flow of oil to the recovery well. This study shows the denuding effect of hydrate formation on two condensates and one crude oil containing dissolved methane and propane gases, and is a continuation of our recent study on hydrate formation from methane-propane and methane-propane-decane liquid mixtures. Experimental Apparatus and Procedure The experimental apparatus, shown schematically in Fig. 1, consisted of an instrumented, high-pressure, glass-windowed cell immersed in a controlled temperature bath. In a typical run, each hydrocarbon component was charged quantitatively to the cell and the resulting mixture was pressurized by injecting water. After mixing by rocking the cell, the bubble-point curve was determined by a method described by Katz et al. Next, the four-phase (vapor, liquid hydrocarbon, liquid water, and hydrate) equilibrium point was measured by subcooling the system to initiate hydrate formation, bringing the cell to equilibrium conditions, and holding for 6 to 8 hours. Only a few hydrate crystals and a very small gas bubble were allowed to exist in equilibrium with the large hydrocarbon phase to insure small changes from the initial measured composition. To demonstrate denuding, all hydrate crystals were melted and the system was pressurized well above the bubble point at a temperature below the quadruple point. Large amounts of hydrates were formed. point. Large amounts of hydrates were formed. JPT P. 223