Measuring and testing – Vibration – Resonance – frequency – or amplitude study
Reexamination Certificate
1999-12-27
2001-10-09
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
Resonance, frequency, or amplitude study
C073S061450, C073S061470, C073S053010
Reexamination Certificate
active
06298724
ABSTRACT:
The present invention relates to the detection of the dissociation of clathrate hydrates and in particular, but not exclusively, to the measurement of pressures and corresponding temperatures at which clathrate hydrates are found to dissociate.
Clathrate hydrates of gases are formed when, under favourable temperature and pressure conditions, gas molecules become encapsulated in crystalline structures of water. The water forms a cage-like structure around guest molecules. Chlorine hydrates were first discovered in 1810 and following this many compounds which form clathrate hydrates have been identified. In 1934 it was discovered that gas clathrate hydrates were causing blocking of natural gas transmission lines and for this reason research aimed at understanding and resolving this problem was initiated and continues today. Apart from being considered as a problem in the oil industry, clathrate hydrates are considered important for a number of reasons such as a potential source of energy, and for use in processes such as desalination and gas transportation.
Clathrate hydrates, especially in the oil industry, are often referred to as gas hydrates, or simply as hydrates. Gas hydrates of interest, particularly with respect to producing, transporting and processing of natural gas and petroleum fluids, are composed of water and the following eight guest molecules: methane, ethane, propane, isobutane, normal butane, nitrogen, carbon dioxide and hydrogen sulphide. Other guest molecules capable of forming clathrate hydrates include ethane, nitrous oxide, acetylene, vinyl chloride, methyl bromide, ethyl bromide, cyclopropane, methyl mercaptan, sulphur dioxide, argon, krypton, oxygen, xenon, trimethylene oxides and others. Clathrate hydrate formation is a possibility wherever water exists in the presence of such molecules, both naturally and artificially, at temperatures above 0° C. and below 0° C., when pressure is elevated.
It is primarily due to their crystalline, insoluble, non-flowing nature that hydrates have been of interest to industry. They are a source of problems, because they block transmission lines, plug Blow Out Preventers, jeopardize the foundations of deepwater platforms and pipelines, collapse tubing and casing, and foul process heat exchangers and expanders. Common methods of preventing hydrate formation are the regulation of pipeline water content, the use of special drilling mud compositions and the injection of Large quantities of methanol into pipelines. All these methods are costly and complex, so there is a need to know more about the likelihood of clathrate formation in a particular sample.
The point (in terms of pressure and temperature) at which, in a system containing a clathrate hydrate, the hydrate dissociates is known as the dissociation point.
The pressure at which a clathrate hydrate dissociates at a given temperature is referred to as the dissociation pressure (DP) for that temperature, while the temperature at which the hydrate dissociates for a given pressure is referred to as the dissociation temperature (DT) for that pressure. The DP and DT are important factors to be determined in order to identify and characterise the nature and properties of any clathrate hydrate.
Previously, the DP or DT of a given hydrate has been determined using methods dependent on visual identification of clathrate formation/dissociation, which are prone to human error and are inherently inaccurate. Measurements of the DT have also been made by mixing a test fluid with water, supercooling the mixture till the clathrate is formed, and then slowly warming the clathrate and detecting and/or measuring the increase in pressure which results from gas molecules escaping from inside the clathrate upon its dissociation. The latter method suffers from the problem of being extremely time consuming since the time taken for a system containing a sufficiently large sample of the (solid phase) clathrate to reach equilibrium at each desired temperature measurement is significant, in some cases a number of weeks.
It is an aim of the present invention substantially to avoid or minimise one or more of the foregoing disadvantages.
According to a first aspect of the present invention we provide an apparatus suitable for use in detecting the formation of, and/or the onset of dissociation of, clathrate hydrates, the apparatus comprising: a piezoelectric crystal sensor which is formed and arranged to resonate at a variable frequency which is dependent upon a mass loading on a deposition surface thereof; and signal analyser means formed and arranged for monitoring, in use of the apparatus, directly or indirectly, change in the resonant frequency of the piezoelectric crystal sensor while one of the temperature and pressure of a test sample in contact with the deposition surface of the sensor is varied, so as to detect a substantial change in said resonant frequency occurring upon the formation or dissociation of a clathrate hydrate on the deposition surface, whereby the formation or dissociation of said clathrate hydrate may be detected.
One advantage of the apparatus of the invention is that it enables the dissociation (or the formation) of clathrate hydrates to be detected very accurately. Unlike the aforementioned prior art, the invention does not rely on visual identification methods, or the detection of small pressure increases.
According to a second aspect of the invention we provide an apparatus for measuring dissociation point temperatures and pressures of clathrate hydrates, the apparatus comprising: a piezoelectric crystal sensor which is formed and arranged to resonate at a variable frequency which is dependent upon a mass loading on a deposition surface thereof; a pressure vessel having a pressure chamber defined therein, said piezoelectric crystal sensor being mounted in the pressure chamber, and the pressure vessel having inlet means via which a test fluid may be introduced into the pressure chamber of the vessel; temperature control means for controlling the temperature in the pressure chamber; pressure control means for controlling the pressure in the chamber; signal analyser means formed and arranged for monitoring, in use of the apparatus, directly or indirectly, change in the resonant frequency of the piezoelectric crystal while one of the temperature and pressure of test fluid in contact with the deposition surface of the sensor is varied, so as to detect a substantial change in said resonant frequency occurring upon the formation or dissociation of a clathrate hydrate on the deposition surface, whereby the formation or dissociation of said clathrate hydrate may be detected; and temperature measuring means and pressure measuring means for measuring the temperature and pressure in the chamber at least when the dissociation of said clathrate hydrate is detected.
An advantage of the apparatus of present invention is that only a relatively small amount of the test fluid is required since only a small amount of clathrate hydrate need be formed in the apparatus of the invention in comparison with the prior art techniques which rely on visual identification of clathrate formation and dissociation, or detection of pressure changes due to clathrate hydrate formation/dissociation, and thus require much larger amounts of clathrate hydrates to be present. In consequence, another advantage of the apparatus is that it enables the DT or DP to be measured relatively quickly in comparison with the afore-mentioned prior art methods which required relatively large samples of clathrate hydrate (in which equilibrium conditions for a given temperature and pressure can take a very long time to reach) in order for effects occurring at the dissociation point to be detected.
Said substantial change in the resonant frequency occurring in the resonant frequency may be of the order of a few hundred to a few thousand Hertz. The magnitude of the change may be greater or smaller than this, though, depending on the amount of hydrate formed on the crystal and/or where on the crystal surface the hydrate is situate
Burgass Rhoderick William
Danesh Sayed Ali
Kalorazi Bahman Tohidi
Todd Adrian Christopher
Heriot-Watt University
Saint-Surin Jacques
Williams Hezron
Wolf Greenfield & Sacks P.C.
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