Improved Correlations Predict Hydrate Formation Pressures or Temperatures for Systems With or Without Inhibitors

Abstract

Abstract Gas hydrates are a well-known problem in the oil and gas industry that cost millions of dollars in production and transmission pipelines. Of the thermodynamic models in the literature, few can predict the hydrate formation temperature or pressure for complex systems which include inhibitors. Two new correlations can calculate the hydrate formation pressure or temperature for single components or gas mixtures, with or without inhibitors. These correlations are applicable to temperatures up to 90 °F and pressures up to 12,000 psi. The results show an average absolute percentage deviation of 15.93 in pressure and an average absolute temperature difference of 2.97 °F. Introduction Gas hydrates are ice-like crystalline structures with gas components such as methane and carbon dioxide as guest molecules entrapped into cavities formed by water molecules. Whenever a system of natural gas and water exists at specific conditions, especially at high pressure and low temperature, we expect the formation of hydrates. In the oil and gas industry, gas hydrates are a serious problem in production and gas transmission pipelines because they plug pipelines and process equipment. By applying heat, insulating the pipelines and using chemical additives as inhibitors, we can keep the operating conditions out of the hydrate formation region. To remediate problems caused by hydrates, it is important to calculate the gas hydrate formation temperature and pressure accurately. This is more complex when the system includes alcohols and/or electrolytes. Hammerschmidt(1) first found that the formation of clathrate hydrates could block natural gas transport pipelines. Since then, the oil and gas industry has been more willing to investigate the problem. This paper presents two new correlations that can predict the gas hydrate formation pressure at a given temperature (p-correlation) or the gas hydrate formation temperature when pressure is available (T-correlation) for a single component or a mixture of gas with or without inhibitors. The work focuses on gas hydrate formation at the three-phase equilibrium (liquid water, hydrocarbon gas and solid hydrate). The developed correlations are applicable to a range of temperatures up to 90 °F and pressures up to 12,000 psi. The capability of these correlations has been tested for aqueous solutions containing electrolytes such as sodium, potassium and calcium chlorides (NaCl, KCl and CaCl2) lower than 20 wt% and inhibitors such as methanol lower than 20 wt% and ethylene glycol (EG), triethylene glycol (TEG) and glycerol (GL) lower than 40 wt%, since the use of higher amounts of these inhibitors is neither practical nor economic. In addition, these correlations may not be appropriate in some cases with high concentrations of inhibitors. The results show an average absolute percentage deviation of 15.93 in pressure and an average absolute temperature difference of 2.97 °F. To make the correlations easy to use, we programmed them with Visual Basic (program is available upon request). From gas compositions, the inhibitor concentrations and either temperature or pressure of the system, a user can calculate the hydrate formation pressure or temperature as quickly as clicking a key.