A Simple Correlation To Predict the Hydrate Point Suppression in Drilling Fluids

Abstract

Abstract Gas hydrate formation during deep-water offshore drilling is a well recognized operational hazard. At water depths in excess of 1000 ft [305 ml, the sea-bed ambient conditions of pressure and temperature become conducive for hydrate formation. This can impose serious well control difficulties during the containment of a gas kick. Salts and glycerol are the main hydrate inhibitors commonly used in water-based muds. To determine the required salt and/or glycerol concentration for a particular drilling fluid, costly and time consuming experiments are required This paper presents a simple correlation to predict the hydrate point suppression using mixtures of salts and glycerol. The correlation is based on rigorous thermodynamic principles and represents a substantial improvement over Hammerschmid's correlation. Example calculation is provided on how to utilize the proposed correlation. Introduction Gas hydrates are inclusion compounds that form when water and natural gas come in contact under certain conditions of high pressure and low temperature. Water, through hydrogen bonding, forms solid cages that physically entrap gas molecules smaller than n-pentane. Methane, ethane, propane, butanes, CO2, N2, H2S, and their mixtures are known hydrate formers. The hydrate structure is kept thermodynamically stable through van-der-Waals type interaction between the water molecules forming the cages and the gas molecules inside them. Natural gas hydrates are known to form two distinct crystal structures, I and II. Lately, structure H has been identified. An extensive review of gas hydrates has been given by Sloan. Although hydrates were known for over 160 years, their importance to the oil and gas industry was not realized until the 1930's. Hammerschmidt discovered that the solid compounds that frequently plugged the gas flow lines during cold weather were not ice but hydrates. As a result of his discovery, two techniques to prevent hydrate formation have emerged; dehydration and chemical inhibition. The latter was a direct utilization of the well known depressing effect of alcohols, glycols, and salts on the freezing point of water. Hydrate formation in drilling fluids represents a relatively new experience resulting from deep-water offshore drilling. Two case histories of hydrate formation during offshore drilling have been reported in the literatures. The first case happened while drilling offshore the U.S. west coast at water depth of 1150 ft [350 m] with a sea water temperature of 45F[7 C] at the mudline. The second case took place while drilling a well offshore in the Gulf of Mexico at water depth of 3100 ft [945 m] with mudline temperature of 40F[4 C]. In both cases hydrates plugged the choke and kill lines during the containment of a gas kick. Two reasons make the choke and kill lines and the BOP stack potential locations for hydrate formation during the containment of a gas kick and intermittent circulation interruption. The first reason is that the BOP stack and the choke and kill lines are located at the mudline where the ambient temperature is at its lowest. The second is the low thermal mass of the choke and kill lines and the BOP stack which makes them coot quickly to the mudline temperature during stop of circulation. As a consequence to hydrate formation, the water is extracted from the drilling fluid to form the hydrate structure. P. 287^