A METHOD OF MEASURING PRESSURE BY MEANS OF A PRESSURE GAUGE HAVING A RESONANT ELEMENT

Abstract: The invention relates to a method of measuring pressure in which an evacuated capsule (1) containing a resonant element (5) is placed in the fluid whose pressure is to be measured, a vibration characteristic of the element is measured, and the pressure is deduced from said characteristic. A resonant element is used which, during measurement, is to be found in a stress state that is close to buckling. For this purpose, it is possible to use heater means for heating the element and servo-controlled so as to keep the frequency of vibration thereof constant. The resonant element can be made of silicon. The invention is particularly applicable to oil wells.

Description: AMETHOD OF MEASURING PRESSUREBYMEANS OF APRESSURE GAUGEHAVINGARESONANTELEMENT The present invention relates to a method of measuring pressure by means of a pressure gauge having a resonant element, in particular for use in oil wells, and more particularly the invention relates to such a method in which an evacuated capsule containing a resonant element is placed in the fluid whose pressure is to be measured, a resonance characteristic of the element is measured, and the pressure is deduced from said characteristic. Document US — A — 4 547 691 discloses sensors of this type, known as crystal quartz gauges (CQGs) in which the evacuated capsule and the vibrating beam it contains are both made of quartz. Such sensors have the advantages of being capable of operating in a hostile environment, at very high pressure (up to 1500 kg/cm 2 ), and at high temperatures (up to 200°C), thereby making them particularly suitable for use in the field of oil exploration, particularly for continuous monitoring of deposits. Finally, they present very good accuracy, of the order of 0.01% of full scale. The quartz beam is set into resonance by the piezoelectric effect and the frequency of its vibration is measured accurately. The pressure to which the cell is subjected is deduced therefrom. Since the frequency of vibration is relatively insensitive to the effects of aging, of drift, of fatigue due to stress relaxation, of thermoelectric effects, or of instability of the electronics, such quartz sensors are therefore also very stable and of good resolution, unlike membrane or strain-gauge pressure sensors. Nevertheless, they have the drawback of being high in cost and of possessing dimensions that are relatively large. The present invention seeks to mitigate those drawbacks. More particularly, the invention seeks to provide a pressure gauge that is capable of operating under the same conditions as a quartz pressure gauge and of providing a measurement of the same accuracy, but which is much easier to industrialize and which is of much lower cost. Still more particularly, the invention seeks to provide both a method of measuring pressure and a pressure gauge which can be used on a large scale in the oil industry, particularly for continuous monitoring of deposits, but which could also be used while drilling. To this end, the invention firstly provides a method of measuring pressure by a pressure gauge having a resonant element, in particular for use in oil wells, in which an evacuated capsule containing a resonant element is placed in the fluid whose pressure is to be measured, a resonance characteristic of the element is measured, and the pressure is deduced from said characteristic, the method being characterized by the fact that a resonant element is used which is to be found, during measurement, in a stress state that is close to buckling. It will be observed that the invention relates to sensors having a resonant element, i.e. sensors of the same type as the above-mentioned quartz sensors, and not membrane or strain-gauge sensors. Advantage is thus taken of the fact that the magnitude being measured is associated with a frequency of vibration and not with a deformation. In addition, the sensitivity of the method of the invention is increased by the fact that the resonant element is in a stress state that is close to buckling. Under such circumstances, the stiffness of the beam tends towards zero. A very small variation in the compression exerted thereon therefore gives rise to a considerable variation in its frequency of vibration. This provides mechanical amplification of the sensitivity of the sensor. Advantageously, the resonant element is made of silicon. This makes it possible to benefit from the excellent mechanical properties of silicon, particularly when it is monocrystalline. In addition, technology associated with using monocrystalline silicon is thoroughly mastered and well adapted to mass production. It will be observed that, other things being equal, the sensitivity of a resonant element sensor made of silicon would normally be much less than that of an equivalent quartz sensor. The method of the invention makes it possible to remedy that drawback because measurement is performed while the resonant element is in a stress state that is close to buckling. Two implementations of the method can be envisaged. Firstly, it is possible merely to excite the element to resonance and measure its frequency of vibration. The sensor is then operating in an open loop mode. The stress state of the resonant element, and thus its frequency of vibration, depend on the pressure to be measured. In the other implementation, the stress state of the resonant element is adjusted to the limiting condition for buckling within the range of pressures to be measured. Its frequency thus remains constant, and pressure is measured by measuring the stress applied to the resonant element. The sensor is then operating in a servo- controlled mode. In a particular implementation, it is the temperature of the resonant element that is adjusted, e.g. using the Joule effect, so as to keep it in a stress state close to buckling. Under such circumstances, a thermal expansion stress is thus added to the mechanical stress exerted on the element by the pressure that is to be measured, with this being done in such a manner as to reach the desired proximity of the buckling state. The resonant element always vibrates at the same frequency and the electrical current used for adjusting its temperature is representative of the pressure to be measured. The invention also provides a pressure gauge having a resonant element, suitable for performing measurements in oil wells, the gauge comprising a resonant element of crystalline material placed in an evacuated capsule organized to be subjected to a pressure that is to be measured, means for setting said element into resonance, and means for deducing the pressure from a vibration characteristic of the element, the gauge being characterized by the fact that stress control means are provided to ensure that during measurement the resonant element is to be found in a stress state close to buckling, thereby amplifying the sensitivity of the sensor. More particularly, the resonant element can be made out of silicon. In a first embodiment, said vibration characteristic of the resonant element is its frequency of vibration. In another embodiment, said vibration characteristic of the resonant element is an electrical magnitude to which said means need to be subjected in order to ensure that the element keeps a frequency of vibration that is constant in spite of pressure variations. It will also be observed that in prior art quartz sensors, vibration is excited by piezoelectric means. Other means need to be envisaged for a sensor having a resonant element made of silicon, since silicon is not piezoelectric. In the invention, the excitation means can be capacitive means or optical means. In a particular embodiment, said means for operating during measurement to cause the resonant element to be found in a stress state close to its limit condition for buckling can be means for heating the element, in particular by the Joule effect. A particular embodiment of the invention is described below by way of non- limiting example with reference to the accompanying diagrammatic drawing, in which: • Figure 1 shows a pressure gauge of the invention with its measurement cell in an exploded perspective view; and • Figure 2 is a diagram illustrating the general operation of the device. The measurement cell 1 of the Figure 1 pressure gauge is made in three parts, namely: a hollow package 2 and two covers 3a and 3b. The package 2 forms a rigid frame 4. A resonant element, in this case a vibrating beam 5, forms a bridge between two opposite sides of the frame 4. The cross-section of the beam 5 is small compared with that of the sides of the frame 4 such that the beam can be considered as being in compression between two fixed ends. Each cover 3a and 3b is constituted simply by means of a plane plate bonded to the package by any appropriate means so as to enclose the beam 5 in an evacuated enclosure. The package 2 including the vibrating beam 5, and the covers 3a and 3b are all made of silicon. The technique used can be the technique of anisotropic chemical machining, e.g. using potassium hydroxide, as is known in microelectronics. The faces of the covers 3a and 3b that come into contact with the package 2 are oxidized, e.g. thermally, so that the resulting layer of silica 6 provides electrical insulation. The covers 3a and 3b can be mounted on the package 2 by direct bonding or by anode soldering a thin layer of powdered glass. The electronics essentially comprises an oscillator circuit 7 and a servo-control circuit 8 operating on the limiting condition for beam buckling. The oscillator circuit 7 operates in conventional manner to provide capacitive excitation of the beam 5 and to measure its resonance frequency / The circuit 7 is shown here as being connected to the cover 3 a and to the frame 4 of the package 2. In Figure 2, it can be seen how interactions take place between the servo- control circuit 8, the beam 5, and the oscillator circuit 7. The pressure P of the external medium is added in the measurement cell 1 to the pressure P th due to the resistive heating of the beam 5 so as to give the pressure Pythat compresses the beam 5 longitudinally. The beam 5 is excited into vibration capacitively by the oscillator circuit 7 and it thus vibrates at its resonance frequency This frequency is compared in a comparator 9 with a reference f 0 that is suitably selected to correspond to the beam being in a state where it is in a limiting condition for buckling. The frequency difference is used to control a current generator 10. The current delivered by the generator 10 is injected into the beam 5 to generate the pressure P th which ensures the limiting condition in buckling. The frequency difference supplied by the comparator 9 (or the current supplied by the generator 10) is also representative of the pressure P. This difference (or current) is therefore transmitted to a circuit (not shown) which gives the pressure directly. Maximum sensitivity is thus made available since the resonance frequency of the beam 5 varies very quickly as a function of the applied compression, and thus as a function of the applied pressure P, when the beam is close to buckling. By way of example, the vibration modes of a silicon beam that is 0.25 mm long, 0.05 mm wide, and 4.8 μm thick have been computed. Under the conditions used for the computation, the buckling pressure was 1180 bars. In the absence of any applied pressure, the natural frequency of the first mode of vibration was 657 kHz. Its sensitivity to pressure in the range 0 to 800 bars in said first mode was about 360 Hz/bar. This sensitivity increased significantly for higher pressures and exceeded 2000 Hz/bar in the range 1100 to 1180 bars. The above description relates only to the embodiment in which frequency excitation/detection is of the capacitive type. In a variant it is possible to provide for excitation/detection to be of the optical type. Under such circumstances, vibration is generated by heating the beam 5 using light pulses delivered by an optical fiber. The optical embodiment makes it possible to take advantage of the fact that optical fiber is electrically insulating, of small diameter, flexible, and insensitive to electromagnet disturbances.

Claims: CLAIMS 1/ A method of measuring pressure by a pressure gauge having a resonant element, in particular for use in oil wells, in which an evacuated capsule containing a resonant element (5) is placed in the fluid whose pressure is to be measured, a resonance characteristic of the element is measured, and the pressure is deduced from said characteristic, the method being characterized by the fact that a resonant element is used which is to be found, during measurement, in a stress state that is close to buckling. 2/ A method according to claim 1, in which said resonant element is made of silicon. 3/ A method according to claim 1 or 2, in which the vibration frequency of the resonant element is measured. 4/ A method according to claim 1 or 2, in which the stress state of the resonant element is adjusted onto its limiting condition for buckling in the range of pressures to be measured. 5/ A method according to any one of claims 1 to 4, in which the temperature of said resonant element is adjusted so as to maintain it in a stress state close to buckling. 6/ A method according to claim 4, in which the temperature of said resonant element is adjusted by heating it using the Joule effect. 7/ A pressure gauge having a resonant element, suitable for performing measurements in oil wells, the gauge comprising a resonant element (5) of crystalline material placed in an evacuated capsule (1) organized to be subjected to a pressure that is to be measured, means (7) for setting said element into resonance, and means (9) for deducing the pressure from a vibration characteristic of the element, the gauge being characterized by the fact that stress control means (10) are provided to ensure that during measurement the resonant element is to be found in a stress state close to buckling. 8/ A pressure gauge according to claim 7, in which said resonant element is made of silicon. 9/ A pressure gauge according to claim 7 or 8, in which said vibration characteristic of the element is its frequency of vibration. 10/ A pressure gauge according to claim 7 or 8, in which said vibration characteristic of the element is an electrical magnitude to which said stress control means (10) need to be subjected to ensure that the element maintains a constant frequency of vibration in spite of variations in pressure. 11/ A pressure gauge according to any one of claims 7 to 10, in which said means (7) for setting the element into resonance are capacitive means. 12/ A pressure gauge according to any one of claims 7 to 10, in which said means (7) for setting the element into resonance are optical means. 13/ A pressure gauge according to any one of claims 7 to 12, in which said stress control means (10) for ensuring that the resonant element is to be found, during measurement, in a stress state close to buckling, are means for heating the element.

Inventors: DONZIER ERIC, PERMUY ALFRED, REZGUI FADHEL, DONZIER, ERIC, PERMUY, ALFRED, REZGUI, FADHEL
Applicants: DONZIER ERIC, PERMUY ALFRED, REZGUI FADHEL, SCHLUMBERGER CA LTD, SCHLUMBERGER HOLDINGS, SCHLUMBERGER SERVICES PETROL, SCHLUMBERGER TECHNOLOGY BV, DONZIER, ERIC, PERMUY, ALFRED, REZGUI, FADHEL, SCHLUMBERGER CANADA LIMITED, SCHLUMBERGER HOLDINGS LIMITED, SCHLUMBERGER TECHNOLOGY B.V., SERVICES PETROLIERS SCHLUMBERGER
Application year: 2000
Main classification: G01L9
IPC codes: G01L9