Gas Hydrate Equilibrium Measurements for Multi-Component Gas Mixtures and Effect of Ionic Liquid Inhibitors



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Qatar holds the world's third-largest proven reserves of natural gas at 885 trillion cubic feet according to a recent report. Because of its desert climate, gas hydrate formation may seem an unlikely event in Qatar. However, its natural gas reservoirs are located 80 km offshore, in the North Field, and the production of liquefied natural gas (LNG) depends on reliable flow from offshore wellheads to onshore processing facilities. Classical methods for inhibiting hydrate formation are used in order to prevent pipeline plugging but changing gas concentrations and operating conditions make flow assurance quite challenging in the North Field. Between 2008 and 2011, sudden temperature drops near gas pipelines caused various incidents of gas pipeline blockage by hydrates, with a loss of US$ 10 million per day due to lost production for almost 4 weeks. Such unplanned shut downs jeopardize the reliable export of LNG to end users.

This work presents the recent investigation on synthetic multi-component gas mixtures whose compositions are typical of Qatari natural gases with initiatives aimed at helping producers minimize costs, optimize operations, and prevent interruption of gas flow in offshore drilling and production. In addition, it presents hydrate inhibition data from a newly commissioned micro bench top reactor, a high-pressure autoclave and a rocking cell. The conditions for hydrate formation for pure methane and carbon dioxide were also measured, for validation purposes. The measured data were compared with literature results and those of a commercial simulator, HydraFLASH?. Upon validation of the calibration data and determination of the apparatus uncertainty, results for hydrate formation equilibrium points for Qatari natural gas sample were collected and compared to HydraFLASH? predictions. Different percentages of 2-hydroxy-N,N,N-trimethylethanaminium chloride, also known as choline chloride ionic liquid, were used as hydrate inhibitor for the same gas mixture. The ionic liquid?s inhibition performance was compared to that of classical thermodynamic inhibitors (e.g. methanol). Ionic liquid inhibition showed (0.7 ? 1.8) oC and (2 - 2.6) oC shift in the hydrate equilibrium curve with 1 wt. % and 5 wt. % of choline chloride respectively. While the inhibition performance of 1 wt. % and 5 wt. % of methanol, obtained using HydraFLASH? software, were 2.8 oC and 4.4 oC respectively.