Measuring and testing – Liquid analysis or analysis of the suspension of solids in a...
Reexamination Certificate
1999-12-22
2001-05-01
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
C073S061450, C073S061470, C073S019030, C073S019010
Reexamination Certificate
active
06223588
ABSTRACT:
The present invention relates to methods and apparatus for detecting and/or measuring dew point (DP) or bubble point (BP) temperatures and/or pressures, and particularly, but not exclusively, for measuring the dew point or bubble point temperatures and/or pressures of hydrocarbon mixtures.
The dew point (DP) or bubble point (BP) is the point (in terms of pressure and temperature) at which, in a single phase (liquid or gas) fluid, a phase transition, (i.e. liquid to gas, or gas to liquid) occurs. The DP or BP of a fluid is an important indicator for determining the properties and/or composition of a fluid sample, and thus also its likely behaviour under certain conditions. For this reason it is important to be able to measure the DP or BP of a fluid, in many applications. One application is in relation to natural oil reservoirs where it is often desirable to be able to measure the DP or BP of a hydrocarbon fluid, either by analysing a fluid sample in a laboratory, or by carrying out in situ analysis of downhole fluids.
One problem faced in attempting to measure DP and BP temperature and/or pressures is finding an effective way of detecting the precise onset of the phase change in the fluid to be tested which occurs at the DP or BP i.e. detecting a DP or BP phase transition. Previously, the most common method of measuring BP pressure involved containing a representative two-phase (liquid and gas) sample of a fluid to be tested within a high pressure vessel and reducing the confining volume stepwise by injecting measured amounts of mercury into the vessel and recording the pressure at each volume, after shaking the vessel (to achieve equilibrium). Alternatively, a high pressure piston vessel is used to contain the sample and the piston is used to reduce the volume therein. By plotting measurements of volume against pressure, the point at which the fluid in the vessel changes from two phases (gas and liquid) to one phase (liquid) is detected as a marked change in the slope of the graph of pressure versus volume (due to a significant change in the compressibility of the pressure vessel contents at the BP). The pressure at which the slope changes is taken to be the bubble point pressure for the particular temperature of the vessel contents. However, where the fluid being tested is very volatile this change in slope may not be so marked and may be difficult to identify.
The most common methods of measuring DP pressures have involved visual identification of droplets of liquid formed at the DP. For fluids which exhibit what is commonly known as “retrograde condensation” behaviour, a fluid sample is contained within a high pressure vessel and is compressed to a point where it is a single, gaseous phase. The pressure in the vessel is then reduced stepwise, with the vessel being shaken after each volume reduction (to achieve equilibrium).
When the fluid reaches the DP pressure (for the particular temperature of the vessel contents), droplets of liquid can be observed coming out of the fluid and accumulating at the lowest point in the cell, and for some fluids a colour change will also be observed as the fluid approaches the dew point. For fluids which exhibit more “conventional” condensation behaviour, the process is modified in that the vessel is filled with a single, gaseous phase, sample of the fluid at low pressure and the pressure is then increased stepwise until droplets of liquid are observed. Such visual identification is error prone and often produces inaccurate measurements of dew point pressures.
It is an aim of the present invention to substantially avoid or minimise one or more of the foregoing disadvantages.
According to a first aspect of the invention we provide apparatus for detecting a dew point or bubble point phase transition in fluid, the apparatus comprising: a piezoelectric crystal sensor formed and arranged to resonate at a variable frequency which is dependent upon physical properties of fluid in contact with the sensor; 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 sample of fluid in contact with said sensor is varied, so as to detect a substantial change in said resonant frequency and/or in the rate of change in resonant frequency with change in the varying one of the temperature and pressure, occurring at a dew or bubble point phase transition of the sample of fluid, whereby a said dew or bubble point phase transition may be detected.
An advantage of the apparatus according to the invention is that it enables highly accurate detection of the dew point, or bubble point, phase transition in a fluid to be achieved. The detection of the DP or BP does not require any visual identification of the formation of droplets of liquid, or bubbles of gas, often used in other methods for detecting DP, or BP, and which can lead to problems and errors in the detection and subsequent calculation of the DP or BP. Moreover, only a relatively small amount of fluid is required i.e. a sufficient amount of fluid to contact, preferably to surround, the piezoelectric crystal sensor which may be very small e.g. of the order of 10 mm diameter, is required. Other advantages include extreme versatility of the sensor: the sensor may be is used at all, or at least most, temperatures and pressures likely to be encountered when measuring dew or bubble point temperature/pressure measurements on oil field reservoirs. The piezoelectric crystal sensor incorporated in the apparatus is also relatively inexpensive in comparison with some other sensors incorporated in the prior known types of apparatus.
For the avoidance of doubt, the dew point (DP) phase transition is defined herein as the appearance of a liquid phase (e.g. droplets) in a gas, and the bubble point (BP) phase transition as the appearance of a gaseous phase (e.g. bubbles) in a liquid. It will be understood that at the DP or BP phase transition the single phase fluid may become (at least temporarily) a two phase fluid (i.e. a gas/liquid mixture).
The apparatus preferably also includes at least one of temperature measuring means and pressure measuring means formed and arranged for measuring those of the temperature and pressure of the sample fluid in contact with the piezoelectric crystal sensor which are varied in use of the apparatus.
Preferably, the piezoelectric crystal sensor is an acoustic wave sensor selected from the group consisting of thickness-shear-mode (TSM) devices, surface-acoustic-wave (SAW) devices, acoustic-plate-mode (APM) devices and flexural-plate-wave (FPW) devices.
The piezoelectric crystal sensor preferably comprises a quartz crystal microbalance (QCM). The QCM conveniently comprises an AT-cut quartz crystal sandwiched between excitation electrodes to which a driving signal may be applied to generate a transverse shear wave across the thickness of the crystal. Such a QCM can be made to oscillate even when immersed in fluid (gas or liquid) and will resonate at a frequency which is related to properties such as the density and viscosity of the surrounding fluid. Any change of phase in the fluid will significantly change the resonant frequency of the QCM, and/or the rate of change in resonant frequency of the QCM with change in temperature or change in pressure.
The signal analyser means is preferably adapted to control the driving signal supplied to the excitation electrodes and may be adapted to, for example, analyse the phase of an electrical impedance or gain of the sensor so as to detect a resonant condition of the sensor (occurring at a resonant frequency of the sensor). Similarly, the resonant condition could be detected by monitoring, for example, current, voltage or electrical conductance of the sensor so as to detect a resonant condition thereof. In use of the apparatus, the signal analyser means is advantageously adapted to produce and detect a resonant condition of the sensor at a predetermined number of different pressures, or
Burgass Rhoderick William
Danesh Sayed Ali
Kalorazi Bahman Tohidi
Todd Adrian Christopher
Heriot-Watt University
Wiggins David J.
Williams Hezron
Wolf Greenfield & Sacks P.C.
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