Abstract:
A ruggedized pressure sensor is described for high pressure applications having a top part of hard material with a surface adapted to deform when exposed to pressure, transducers to transform deformation of the surface into a signal proportional to the pressure, and a base part of hard material, wherein the base part has one or more openings.

Description:
[0001]    The present invention relates to pressure sensors designed to withstand extreme environmental conditions, such as exposure to pressurized liquids as for example encountered in boreholes penetrating subterranean reservoirs. 
       CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0002]    This application claims the benefits of priority from Application Number 0618161.4, entitled “RUGGEDIZED PRESSURE SENSOR,” filed in the United Kingdom on Sep. 15, 2006, which is commonly assigned to assignee of the present invention and hereby incorporated by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0003]    There are many oil field applications which require downhole pressure and differential pressure measurements. For applications such as permanent monitoring of completion systems and logging/measurement in producing zones, where the deployment of the sensor may be done via a ¼″ hydraulic control line, reliable and compact sensors which occupy minimum space are much desired. 
         [0004]    There are also many applications where a differential pressure measurement is required. Typical examples include the determination of flow rate through flow control valves, down hole pump monitoring, Venturi based flow meters and density measurement based on manometer principle. In these applications, the measurement range varies from a few psi to a few thousand psi and for some applications accuracies better than +/− 0 . 5  psi are required. 
         [0005]    For permanent placement in a high pressure (above 1000 psi or 3000 psi or more) and possibly high temperature (above 70 or 90, or sometime even above 100 degrees Celsius) environment, it is important to provide sensors with a high reliability, measuring stability and a long lifetime. To date, there is no suitable differential pressure sensor available for such applications, at least in the field of monitoring of hydrocarbon reservoirs. Typically, two absolute pressure sensors are applied for such differential measurement. This approach has limited accuracy and increases the costs. 
         [0006]    Silicon-based differential pressure sensors have been used in downhole production logging (PL) tools. These sensors, having their maximum measurement range limited to just a few psi, are unsuitable for high range applications such as monitoring the downhole flow valves and downhole pumps. As the demand for permanent downhole differential pressure measurement increases, so does the demand for a high quality and high range downhole differential pressure sensor. 
         [0007]    Schlumberger has developed an absolute pressure sensor based on sapphire material. This is a very high performance sensor which has achieved an impressive track record during field applications. Typical performance data include excellent stability of 1 psi/year over a 10000 psi measurement range of 10000 psi, a resolution of 0.05 psi and accuracy to 0.01% of the full scale. 
         [0008]    There are some disadvantages associated with the known sensor. For example in its usual configuration it cannot be used to measure differential pressure. And as the pressure to be measured is applied to the side where the electrical connections to the strain gauges are made, isolation is required between the actual sensor and the pressurized borehole fluid. This limits the possible reduction of the sensor package size. 
         [0009]    It is therefore an object of the present invention to improve and enhance the capabilities of existing ruggedized pressure sensors. 
       SUMMARY OF THE INVENTION 
       [0010]    The invention includes as a first aspect a ruggedized pressure sensor for high pressure applications comprising a top part of hard material with a surface adapted to deform when exposed to pressure, transducers to transform deformation of said surface into a signal proportional to said pressure, and a base part of hard material, wherein the base part has one or more openings. The top part and the base part together enclose a small volume referred to herein as a cell. 
         [0011]    The cell should be sufficiently stable to allow the sensor to measure pressures of preferably more than 1000 psi, or more than 3000 psi or even more than 10 kpsi. 
         [0012]    In a second aspect the base plate extends into a tubular decoupling element to separate the base plate from a mounting surface. It is preferred to arrange for the decoupling element to have a smaller outer diameter that the base plate such that the decoupler has a higher compliancy against strains. In a particular preferred embodiment the arrangement of base plate and decoupling tube is mushroom-shaped. 
         [0013]    In another preferred embodiment the hard material has a Moh&#39;s hardness of at least 8, even more preferably it is made of gemstone material of this hardness, for example diamond, zirconia, ruby, or emerald, spinel or topaz. When cutting the parts of the sensor from a monolithic gemstone care should be taken to preserve the respective orientation of the top part, the bottom part and decoupling element in the gemstone within the assembled sensor so that no further anisotropies are created during assembly of the hard sensor parts. 
         [0014]    The opening in the base part can be used to pressurize the interior of the cell such that the cell can be operated as a pressure sensor with the pressurized side being the interior of the cell. Having two different variable pressures at the interior and exterior side, respectively, makes the membrane at the top of the cell responsive to the differential pressure of the two pressures. Alternatively, the opening can be used to guide probes, sensors, wires and the like into the interior of the cell. 
         [0015]    This invention describes a new sensing structure which overcomes limitations of known device. The structure enables the measurement of differential pressure, the design of very compact absolute pressure sensor and the integration of P (absolute), ΔP and T (temperature) measurements in a single package. It also minimizes the effects of environmental variables, such as line pressure and temperature. 
         [0016]    By using a very hard material such as sapphire as structure material, a high measurement range, excellent resolution, high reliability, low drift as well as a compact sensor size can be achieved. These include up to ±5000 psi range and better than 0.05 psi resolution for differential pressure measurement, and a range up to 20000 psi for absolute pressure measurement. 
         [0017]    Devices in accordance with the present invention has the potential to satisfy most demanding downhole applications which require high dynamic range of measurement, good long term stability, long operating life, etc. Typical examples include monitoring flow through valves, inside pumps and pressure measurement behind packers in a well. This sensor also has the potential for low-range but high-sensitivity applications such as the Venturi-type flow meters. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein. 
           [0019]      FIG. 1  is a schematic drawing of a known pressure sensor; 
           [0020]    FIGS.  2 A,B illustrate elements of pressure sensors in accordance with examples of the invention; 
           [0021]      FIGS. 3A-3D  illustrate further variants and applications of sensors in accordance with the present invention; and 
           [0022]      FIG. 4  illustrates a protective hydraulic circuit including a pressure sensor in accordance with another aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In the following description, for the purposes of explanation, the background technologies, a basic example this invention and various preferred embodiments of the basic example are set forth in order to provide a thorough understanding of the invention. However, it will be apparent that the invention may be practiced without these specific details. 
         [0024]    A known variant of an absolute pressure sensor as developed by Schlumberger is shown in  FIG. 1 . In several versions such sensors are designed to cover ranges of 0 to 10000 psi and 0 to 20000 psi. The basic sensing cell of this sensor consists of a half capsule or cap  11  made of sapphire and a circular base plate  12  made of the same material. The two parts are joined by a glass sealing  111 . A vacuum is created at the internal space  13  between the two parts  11 ,  12 . The vacuum serves as a constant absolute pressure reference of effectively zero pressure. The sapphire base plate  12  is assembled to the metal body  10  of the sensor package. 
         [0025]    On the top face of the half capsule  11 , there are deposited several strain gauges  112 , which generate a signal representative of the deformation of the top section of the half capsule  11  and in turn represent the amount of pressure P causing the deformation. To insulate the strain gauges  112  and the electrical feed-throughs  113  from the environment, which for example may contain conductive borehole liquids, the pressure sensor is enclosed in a liquid-filled bellow-type housing  14 . The housing  14  though sufficiently compliant to be transparent to the outside pressure P is designed to prevent any direct exposure of the capsule  11  and the electrical connectors  113  to the pressurized medium outside the bellow  14 . The filling liquid is usually inert and non-conductive such as silicone oil. 
         [0026]    When a pressure is applied externally to the sensing half capsule  11 , the thin top section of the sapphire capsule, also herein referred to as membrane, deforms with its central area being pushed in the same direction as that of the applied pressure P (indicated by arrows). Such deformation is measured by the resistive strain gauges  112  deposited on the membrane using thin-film technology. The gauge&#39;s resistance change caused by pressure induced strain on the membrane is converted into a voltage signal via a resistivity measurement such as a Wheatstone bridge (not shown). 
         [0027]    It should be noted that to measure a differential pressure, two pressure sensors of the type illustrated in  FIG. 1  are required. This causes problems with respect to the mutual locations of the sensors, their calibration and other differences which make it difficult to precisely match the measurements. 
         [0028]    To avoid such problems, the invention provides an improvement of the known sensors. Some elements of the present invention are illustrated in  FIG. 2 . 
         [0029]    In  FIG. 2A , there is shown a sensor including a half capsule  21  made of sapphire and a circular base plate  22  made of the same material as in the known pressure sensor. Again the two parts are best joined by a glass sealing  211  though this can be replaced by other suitable joints such as metal brazing, and the like, provided that such seal can withstand the pressures for which such a ruggedized sensor is designed. 
         [0030]    However instead of providing a sealed vacuum in the half capsule  21 , it is an element of the present invention to provide one or more channels  221  which penetrate the base  22 . The channels  221  can be used for various purposes such as introducing a variable pressure P 2  to the inner face of the capsule  21 , or providing access for transducers or wiring to the inner face. Other parts of the device as far as already shown in  FIG. 1  are shown in this figure using the same numerals. 
         [0031]    Surprisingly it was found that the new capsule can withstand pressure which exerts tensile force on the capsule  21  and the sealing  211  even at high pressures differentials and in extreme temperature environments, even though the hole  221  in the base plate  22  could have lead to a significant weakening of the sensor&#39;s mechanical stability. 
         [0032]    In  FIG. 2B  there is shown a further improved variant of the invention. In this variant the base plate  22  is mounted onto a strain decoupling element  222 . The decoupling element is made of the same sapphire material as the capsule  21  and the base plate  22 . It separates the metal housing  20  from the sensor  21 . In the example shown the decoupling element  222  has an outer diameter of 5 mm. This outer diameter is smaller than the diameter of the base plate  22 . Depending on the application, the total length of the decoupling element  222  exceeds the thickness of the base plate  22  by at least a factor of 2. A more effective decoupling can be achieved by increasing the length to at least 3 times the thickness of the base plate. 
         [0033]    In the example shown the base plate  22  and the decoupling element  222  are actually one monolithic piece of sapphire, onto which the capsule  21  is glass bonded using a glass seal  211 . To increase the resistance against failure on tensile strain, a step has been milled between the capsule  21  and the base in the region of the seal  211  such that a bond between the capsule  21  and the base plate  22  exist between both vertically and horizontally oriented surfaces of both parts. 
         [0034]    At the distal end the decoupling tube  222  is assembled to the packaging/housing  20  of the device using metal brazing using an intermediate metallization step to increase the bonding strength at this joint. 
         [0035]    It is an important function of the decoupling tube  222  to isolate the sensing element from the overall sensor package, so that stresses in the packaging body (normally metal) caused by pressure and temperature will have little effect on the sensing element. The stresses will be absorbed by the body of the tube  222  and will not be transferred to the base  22  and the capsule  21 . For this reason, the tube is referred to as decoupling element. 
         [0036]    Numerical simulation show that the sensor zero shift due to a line pressure change from 0 to 20000 psi is less than 2 psi, and maximum sensitivity change is around 4% from a normal value of 24 μV/psi. The zero drift for a temperature change from 0 to 150° C. is about 6 psi (before compensation). 
         [0037]    Preferably all three parts of the sensor are milled or machined from the same crystal and more preferably mounted such as to preserve the original orientation of the parts. This way of orienting the parts avoids that the anisotropy of the crystalline material is oriented differently in different parts. Effectively, the sensor capsule  21 , the base plate  22  and the decoupler  222  are manufactured and assembled such that all three parts are as close as possible to resemble a monolithic device made from a single piece of sapphire. 
         [0038]    As shown the device can be described as a ‘mushroom’ like structure with a sensing element in the form of a half capsule  21  with a thin membrane, and a ‘T’ shaped base plate  22  with a decoupling guide tube  222 , both made of the same material. 
         [0039]    Sapphire is chosen as the preferred material for the sensing capsule and the decoupling tube because it has a rare combination of excellent mechanical, thermal, electrical and chemical properties. Mechanically, sapphire has high strength, very low hysteresis and a very linear elastic region over a large stress range, which is ideal for use as pressure sensing membrane. It is also micro-machinable, allowing to be made into desired shapes. It has very low thermal expansion (and hence low temperature sensitivity) and high thermal diffusivity to reach thermal equilibrium quickly. It is a very good electrical insulator which is required for strain gauge based measurements. Sapphire has high resistance to chemical attack and to sand erosion when directly exposed to well bore fluids. 
         [0040]    As described above, when assembling the two sapphire parts together to form the mushroom structure, the anisotropy of the capsule and that of the tube are aligned to the same direction, so that stresses generated by mismatch of physical properties (e.g. thermal expansion coefficient) of the two parts are minimized. 
         [0041]    Other natural or synthetic gemstone materials can be used to at least partially match the above advantages of sapphire. These materials include for example diamond, zirconia, ruby, or emerald, spinel or topaz. As far as hardness is concerned, suitable material has a Moh&#39;s hardness of 8 to 10. 
         [0042]    Possible applications of pressures sensors in accordance with examples of the invention are described in the following. 
         [0043]    When the sensor is for example used as differential pressure sensor, the channel  221  serves to introduce the second pressure, P 2 , via the decoupling guide tube  222 , into the inner space of the sensor cell, i.e., the cavity between the capsule  21  and the base plate  22 . Thus the pressure sensing membrane of the capsule  21  deforms according to the difference between the external and the internal pressure, i.e. the central area deflects inward if the external pressure, P 1 , is larger than the internal pressure, P 2 , or outward vice versa. 
         [0044]    As a preferred implementation, the differential pressure value is determined through strain measurement using resistive gauges  212  deposited on the external surface of the membrane (e.g. by thin film technology). The membrane thickness and the internal shape of the sapphire capsule are optimized according to the required differential pressure measurement range, burst pressure rating and symmetry requirement for response to positive and negative pressure differences. For example a thickness 200 micron is considered to be sufficient for pressures of up to 3000 psi. A thickness of 400 mm covers most application up to 10 kpsi or even 20 Kpsi. The internal cutting of the top membrane inside the capsule can be performed using a small transition radius, between the cylindrical bore and the flat membrane, for example r1=0.5 mm at the edge of the membrane and a much larger diameter, for example r2=100 mm for the central flat region. Thus the top membrane of the capsule  22  is very flat which facilitates the differential pressure measurement. 
         [0045]    The overall packaging of a differential pressure sensor provides the passages of the two pressures to the external and internal of the Sapphire sensing cell, as well as the isolation between the two passages. The packaging also isolates the pressure ports from the measurement transducers as well as associated electrical or optical connections. 
         [0046]    For different applications the basic design of the sensor can be altered. Some examples are shown in  FIGS. 3A-D . Only differentiating elements are highlighted by the drawings whilst other elements already described above are omitted for the sake of greater clarity. 
         [0047]    In  FIG. 3A , the sensing element consists of two Sapphire capsules  31   a,b  which are back to back assembled to a Sapphire base plate  32 , thus forming the sensing cells. The plate  32  has a central bore  321  and a second bore  322  from one side to the center, in order to provide a passage for introducing pressure P 2  to the internal spaces of the two sensing cells. A sapphire tube  33  links the dual-cell assembly to the packaging body  30 . The tube  33  acts as a decoupler between the sensing elements  31   a,b  and the housing  30 . The strain gauges, their respective pick-up circuits and protective bellows are similar to those shown in the example of  FIG. 1  and are therefore not shown in detail. This variant of the invention achieves double sensitivity by adding the outputs of two Wheatstone bridges, one on each capsule, together while maintaining the same membrane thickness and burst pressure rating. 
         [0048]      FIG. 3B  shows a similar structure which combines a differential pressure cell  31   c  with an absolute pressure cell  31   d . In this case the hole  323  in the center of the base plate  32  is drilled half way through to guide pressure P 2  into only one of the cells  31   c , whereas vacuum is maintained behind the membrane of the other cell  31   d . Such integration allows a measurement differential pressure and an absolute pressure from one sensor package. 
         [0049]    Using the temperature dependence of deposited material on the pressure sensor, it is possible to add a temperature measurement to the package without further increasing its size. These three measurements are of significance in any type of flow measurements where the flow is measured though a pressure drop with a flow constriction, e.g., Venturi-type flowmeters as often encountered in the hydrocarbon production monitoring. 
         [0050]    The invention can also be used to improve the performance and applicability of the sensor as absolute pressure sensor. For example  FIG. 3C  shows an absolute pressure sensor. In contrast to the known device however, the well bore fluid at pressure P is introduced into the internal space of the sensor cell  31   e  through the decoupling guide tube  33  with the strain gauges  312  and their electrical connections  313  on the constant or reference pressure side. Compared to the known absolute pressure sensor of  FIG. 1 , which requires bellows filled with clean liquid to protect the electrical connections, this configuration eliminates the need for such protective components because the well bore fluid will be only in contact with sapphire material, which has high corrosion resistance. As a result, the package of such a bellows-free device can be made much smaller. 
         [0051]    Another variation of an improved absolute pressure sensor is shown in  FIG. 3D , where the absolute pressure is applied externally to the sensing cell  31   f  but the strain gauges usually applied to the outside of the pressure cell are removed. The pressure measurement is done by measuring the displacement of the top part or membrane of the cell  31   f  by using an internal displacement sensor or transducer  314  which can be based on optical methods as shown, or capacitance or acoustic methods. 
         [0052]    The internal of the sensor cell is kept as vacuum or at another reference pressure and the external of the cell  31   f  can be directly exposed to the environment without additional packaging. The end of the decoupling tube  33  is assembled to the packaging shell tube  301  that protects the signal cables  315 . The total outer diameter of the sensor can be made reduced to less 10 mm or even less than 5 mm. Such a sensor on the tip of for example a cable can be used for deployment and logging through a thin guide tube such as hydraulic control lines. 
         [0053]    Important issues associated with down-hole differential pressure sensors include over range protection or pressure shock protection. In  FIG. 4  there is shown a pressure sensor  41  in accordance with an example of the present invention embedded into a network of pipes  40  with a three-valve configuration. The valves  42 ,  43 ,  44  protect the sensor from excessive pressure load by using for example valve  43  to short-circuit the pressurized fluids before an excessive pressure can built up at the sensor  41 . 
         [0054]    The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. It will, however, be evident that additions, subtractions, deletions, and other modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the claims. Although specific embodiments of the invention have been described, various modifications, alterations, alternative constructions, and equivalents are also encompassed within the scope of the invention.