Prediction of gas-hydrate formation conditions in production and surface facilities

Abstract

Gas hydrates are a well-known problem in the oil and gas industry and cost millions of dollars in production and transmission pipelines. To prevent this problem, it is important to predict the temperature and pressure under which gas hydrates will form. Of the thermodynamic models in the literature, only a couple can predict the hydrate-formation temperature or pressure for complex systems including inhibitors. I developed two simple correlations for calculating the hydrate-formation pressure or temperature for single components or gas mixtures. These correlations are based on over 1,100 published data points of gas-hydrate formation temperatures and pressures with and without inhibitors. The data include samples ranging from pure-hydrate formers such as methane, ethane, propane, carbon dioxide and hydrogen sulfide to binary, ternary, and natural gas mixtures. I used the Statistical Analysis Software (SAS) to find the best correlations among variables such as specific gravity and pseudoreduced pressure and temperature of gas mixtures, vapor pressure and liquid viscosity of water, and concentrations of electrolytes and thermodynamic inhibitors. These correlations are applicable to temperatures up to 90ºF and pressures up to 12,000 psi. I tested the capability of the correlations for aqueous solutions containing electrolytes such as sodium, potassium, and calcium chlorides less than 20 wt% and inhibitors such as methanol less than 20 wt%, ethylene glycol, triethylene glycol, and glycerol less than 40 wt%. The results show an average absolute percentage deviation of 15.93 in pressure and an average absolute temperature difference of 2.97ºF. Portability and simplicity are other advantages of these correlations since they are applicable even with a simple calculator. The results are in excellent agreement with the experimental data in most cases and even better than the results from commercial simulators in some cases. These correlations provide guidelines to help users forecast gas-hydrate forming conditions for most systems of hydrate formers with and without inhibitors and to design remediation schemes such as: · Increasing the operating temperature by insulating the pipelines or applying heat. · Decreasing the operating pressure when possible. · Adding a required amount of appropriate inhibitor to reduce the hydrateformation temperature and/or increase the hydrate-formation pressure.