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.
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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History
- Original manuscript received:
26 August 2007
- Meeting paper published:
11 March 2005
- Revised manuscript received:
4 February 2009
- Manuscript approved:
6 April 2009
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