Abstract:
A capacitive pressure sensor includes a stator which encircles a portion of a cylindrical diaphragm. This encircling forms a circumferential gap between the sidewalls of the stator and the diaphragm. Therefore, a greater area “A” can be achieved in smaller diameter sensor footprint than prior art designs and yet still detect relatively small changes in capacitance. Additionally, the width “g 1 ” of the gap can be wider than prior art designs without negatively affecting capacitance detection. A bonding agent which has a melting temperature of about half that of bonding agents used in prior art designs, secures the stator to the diaphragm and reduces oxidation issues during assembly, thereby decreasing manufacturing time and cost. To ensure proper side-to-side alignment of the stator relative to the diaphragm, a centering sleeve, which is removed after bonding, is placed over as stub at the upper end of the diaphragm.

Description:
BACKGROUND OR THE INVENTION 
       [0001]    This invention relates generally to gauges and sensors used to identify downhole pressure and temperature parameters in an oil and gas well. Specifically, the invention relates to downhole sensors which provide a variable capacitance effect in response to changes in pressure of a subterranean formation. 
         [0002]    Downhole capacitance sensors are well-known in the art and make use of the following relationship: 
         [0000]    
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       ɛ 
                        
                       
                           
                       
                        
                       A 
                     
                     g 
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where “C” is capacitance, “∈” is the dielectric constant of the medium in which the sensor is encapsulated, “A” is the area of the sensor (i.e., the area represented by the opposing wall surfaces of a diaphragm and stator of the sensor), and “g” is the distance or width of the gap between opposing wall surfaces of the stator and the diaphragm. The stator and diaphragm are arranged one above the other in a horizontal plane within a protective housing having a hollow interior and an open bottom. Essentially, the available area “A” is the top surface of the diaphragm less that of a spacer or washer placed between the stator and the diaphragm, that is, π(r 1   2 −r 2   2 ), where “r 1 ” is the radius of the diaphragm and “r 2 ” is the radius of the spacer or washer. Oil or gas enters the interior and applies pressure to an underside of the diaphragm. As the diaphragm flexes, the gap “g” between it and the stator increases (because the stator is connected to the post and moves up as the post moves) and capacitance “C” decreases, thereby indicating increased pressure. Additional explanation of the way in which this type of capacitance sensor works can be found in U.S. Pat. No. 4,125,0, to Clark, which is hereby incorporated herein by reference. A typical downhole capacitance sensor arranged in the above way exhibits a capacitance of between 25 and 40 pF. Therefore, controlling variation in the gap “g” during the assembly process is important. 
         [0003]    An emerging size requirement for the gauges which house these capacitance sensors is that the gauge have a maximum diameter of less than 1¼ inches (3.175 cm) and, preferably, less than ¾ inches (1.905 cm). However, shrinking the size of the sensor is challenging because as the size of the sensor decreases, the area “A” decreases and, therefore, so does the capacitance “C”. As capacitance decreases, the electronic circuit used to convert capacitance to frequency has difficulty isolating the difference between the sensor&#39;s capacitance and the stray capacitance. One way around this problem is to increase the capacitance “C” by decreasing the size of the gap “d” between the stator and the diaphragm. However, reducing the gap increases the likelihood of arcing. It also increases the overall cost of manufacturing the sensor because the gap is typically about 0.003 to 0.0035 in. (0.00762 to 0.00889 cm). Maintaining this gap size requires extreme precision machining much less trying to achieve an even smaller gap size. Therefore, reducing the gap “g” is extremely difficult to achieve. 
         [0004]    A final problem with existing downhole capacitance sensors, in addition to reducing size, is that the bonding agent between the stator and the diaphragm requires temperatures of about 900 to 1000° C. for bonding to occur. High temperatures such as this cause oxidation which then adds to the complexity and cost of manufacturing. Cleaning steps and equipment such as vacuum ovens are required. 
       SUMMARY OF THE INVENTION 
       [0005]    A capacitive pressure sensor made according to this invention has a cylindrical-shaped protective housing, stator and diaphragm, each being cylindrical-shaped with a hollow interior space and an open bottom end. The stator encircles a portion of the diaphragm and this encircling forms a circumferential gap between an inner sidewall surface of the stator and an outer sidewall surface of the diaphragm. The protective housing, stator, and diaphragm are each made of a material which is both thermally and electrically conductive. 
         [0006]    Because of this structural arrangement, a greater area “A” can be achieved in a smaller diameter sensor footprint than prior art designs and yet still detect relatively small changes in capacitance e.g., in a range of 25 to 40 pf) as well detect changes up to approximately 200 pF. Additionally, the width “g” of the gap may be in the range of about 0.0003 to 0.01 in. (0.00762 to 0.0254 cm) and could be wider than this depending on the amount of area “A” being required. Preferably, the housing diameter is no greater than 2 in. (5.08 cm) and, more preferably, no greater than 1.25 in. (3.175 cm). Housing diameters no greater than 0.75 in. (1.905 cm) can also be achieved. When in a final assembled state, the interior space of the protective housing contains a dielectric medium which fills the circumferential gap. 
         [0007]    A bonding agent, which also serves as an insulator and dielectric medium, bonds a lower planar surface of the stator to an upper planar surface of the diaphragm. Preferably, the bonding agent has a melting point no greater than 1832° F. (1000° C.) and, more preferably, no greater than about 842° F. (450° C.). The melting temperature of the stator and the diaphragm places an upper limit on the melting temperature of the bonding agent. 
         [0008]    To ensure proper alignment of the stator relative to the diaphragm so that a predetermined width of the circumferential gap can be achieved, the diaphragm has a stub located at its upper end which receives a thru-hole opening of the insulator and the stator. A centering, sleeve, is placed over the stub and aligns the bonding agent and stator side-to-side relative to the diaphragm. The inner height of the centering sleeve is shorter than the height of the stub. This height difference ensures that when the bonding agent is being cured at temperature, the bonding agent (flowing in its liquid state) does not interact with the centering sleeve. After the stator bonds with the diaphragm the centering sleeve can be removed from the final assembly and reused as required for another assembly process. The centering sleeve can be replaced with a bonding agent but this process will not be as accurate because the centering sleeve is machined to precise tolerance whereas the bonding agent is flowing in a liquid state, potentially causing the stator to shift relative to the diaphragm. 
         [0009]    The wall thickness at an upper end of the diaphragm is greater than the wall thickness of the sidewall of the diaphragm. The upper end wall thickness and sidewall thickness are selected such that within a predetermined pressure range the side of the diaphragm flexes or bends in response to the pressure changes but the upper end remains unaffected. The sensitivity (units of Hz/psi) of the sensor can be determined by the proper choice of the sidewall thickness to the maximum pressure the sensor is subjected to. By varying the sidewall thickness, the sensor could be used to measure pressures of up to 30,000 psi. The complete assembly is backfilled with an inert gas (e.g., helium or argon or a mixture of both) which forms a dielectric medium between the sidewalk of the stator and the diaphragm. 
         [0010]    Equation 1 is rewritten for this invention as follows 
         [0000]    
       
         
           
             
               
                 
                   C 
                   = 
                   
                     
                       
                         
                           
                             ɛ 
                              
                             
                                 
                             
                           
                           1 
                         
                          
                         
                           A 
                           1 
                         
                       
                       
                         g 
                         1 
                       
                     
                     + 
                     
                       
                         
                           ɛ 
                           2 
                         
                          
                         
                           A 
                           2 
                         
                       
                       
                         g 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
         [0011]    where “C” is the capacitance of the sensor, “∈ 1 ” is the dielectric constant of the inert gas, “A 1 ” is the area of the sidewall of the stator relative to the diaphragm, g 1  is the gap between the inner sidewall surface of stator and the outer sidewall surface of diaphragm, “∈ 2 ” is the dielectric constant of the bonding agent. “A 2 ” is the area on the lower planar surface of the stator, and “g 2 ” is the gap between the lower planar surface of the stator and the upper planar surface of the diaphragm. 
         [0012]    Objects of this invention include but are not limited to: (1) improving the manufacturability and ease of assembly of a downhole capacitive pressure sensor; (2) decreasing the overall diameter of the sensor relative to prior art designs while maintaining or increasing the effective area “A” available to increase capacitance (“C”) and also measure capacitance changes (“ΔC”) relative to those same designs; and (3) accommodating an increased width “g” of the gap between the stator and the diaphragm relative to prior art designs while maintaining or increasing the ability of the sensor to detect capacitance changes in a wider range than prior art designs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is an isometric view of a capacitance sensor made according to this invention. The diameter of the protective housing is 2 inches or less and, preferably, no greater than about 1.25 in, (3.175 cm). 
           [0014]      FIG. 2  is an exploded isometric view of the capacitance sensor of  FIG. 1 . A cylindrical-shaped stator is received by a cylindrical-shaped diaphragm, thereby forming a circumferential gap having a width “g” between the inner sidewalls of the stator and the outer sidewall surface of the diaphragm. 
           [0015]      FIG. 3  is front view of the diaphragm of the capacitance sensor of  FIG. 1 . 
           [0016]      FIG. 4  is a cross-section view of the diaphragm taken along section line  4 - 4  of  FIG. 3 . 
           [0017]      FIG. 5  is a front view of the stator of the capacitance sensor of  FIG. 1 . 
           [0018]      FIG. 6  is a cross-section view of the stator taken along section line  6 - 6  of  FIG. 5 . 
           [0019]      FIG. 7  is a front view of the housing of the capacitance sensor of  FIG. 1 . 
           [0020]      FIG. 8  is a cross-section view of the housing taken along section line  8 - 8  of FIG. 
           [0021]      FIG. 9  is a cross-section view of the stator and diaphragm of the capacitance sensor of  FIG. 1 . 
           [0022]      FIG. 10  is a cross-section view of the assembled capacitance sensor of  FIG. 1 . The protective housing provides an interior space for a dielectric medium which fills the circumferential gap. 
           [0023]      FIG. 11  is an enlarged view of the section  11  of  FIG. 9 . A bonding agent in the form of a spacer or washer bonds the stator to the upper end of the diaphragm but does not affect the area defined by the sidewalls. 
           [0024]      FIG. 12  is an isometric view of the assembled capacitance sensor of  FIG. 1 . 
           [0025]      FIG. 13  is a cross-section view taken along section line  13 - 13  of  FIG. 12 . A conductive wire passes through an evacuation tube and is welded to the upper planar surface of the stator. 
       
    
    
       [0026]      
         [0000]    
       
         
               
             
               
               
             
           
               
                   
               
               
                 List of Element Numbers Used in the Drawings and 
               
               
                 Detailed Description 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 Capacitance sensor 
               
               
                 20 
                 Protective housing 
               
               
                 21 
                 Interior space 
               
               
                 22 
                 Upper planar surface 
               
               
                 23 
                 Open bottom end 
               
               
                 24 
                 Protective housing stub 
               
               
                 25 
                 Thru-hole opening 
               
               
                 27 
                 Upper end 
               
               
                 28 
                 Tapped hole 
               
               
                 29 
                 Evacuation tube bonding agent 
               
               
                 31 
                 Evacuation tube 
               
               
                 33 
                 Upper end of 31 
               
               
                 35 
                 Conductive wire 
               
               
                 37 
                 Lower end of 35 
               
               
                 39 
                 V-groove 
               
               
                 40 
                 Stator 
               
               
                 41 
                 Interior space 
               
               
                 43 
                 Open bottom end 
               
               
                 45 
                 Sidewall 
               
               
                 47 
                 Inner sidewall surface 
               
               
                 49 
                 Upper end 
               
               
                 51 
                 Upper planar surface 
               
               
                 53 
                 Lower planar surface 
               
               
                 55 
                 Thru-hole opening 
               
               
                 59 
                 Lower end 
               
               
                 60 
                 Diaphragm 
               
               
                 61 
                 Interior space 
               
               
                 63 
                 Open bottom end 
               
               
                 65 
                 Base portion 
               
               
                 67 
                 Column portion 
               
               
                 69 
                 Un-encircled portion of 67 
               
               
                 71 
                 Stator-encircled portion of 67 
               
               
                 73 
                 Upper planar surface 
               
               
                 75 
                 Sidewall 
               
               
                 77 
                 Outer sidewall surface 
               
               
                 79 
                 Upper end 
               
               
                 85 
                 Stub 
               
               
                 87 
                 Circumferential gap 
               
               
                 90 
                 Bonding agent 
               
               
                 91 
                 Top planar surface of 90 
               
               
                 95 
                 Thru-hole opening 
               
               
                 100 
                 Centering sleeve/fixture 
               
               
                 101 
                 Lower end of 100 
               
               
                 103 
                 Gap between 91 and 101 
               
               
                   
               
             
          
         
       
     
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Referring to  FIGS. 1-10 , a preferred embodiment of a capacitance sensor  10  made according to this invention includes a protective housing  20 , a stator  40 , and a diaphragm  60 . The protective housing  20 , stator  40  and diaphragm  60  each have a cylindrical shape and an open bottom end  3 ,  43 , and  63  respectively. The protective housing  20  has a hollow interior space  21  to accommodate the stator  40  and diaphragm  60  combination, the stator  40  has a hollow interior space  41  to accommodate the diaphragm  60 , and the diaphragm  60  has a hollow interior space  61  to receive fluid or gas pressure from the downhole well. A base portion  65  of the diaphragm is received by the protective housing  20  and is in sealed relationship to the housing  20 . This sealed relationship is achieved by welding the protective housing  20  to the base  65  of diaphragm  60 . Welding is performed in the “V” grooves  39  where the weld bead flows and solidifies, thereby forming a seal. 
         [0028]    The stator  40  and the diaphragm  60  are sized relative to each other such that when they are in an assembled and bonded state, a circumferential gap  87  is created and maintained between an inner side wall surface  47  of the stator  40  and an outer sidewall surface  77  of the diaphragm  60 . The width “g 1 ” of the circumferential gap  87  can be in a range of 0.003 to 0.01 in. (0.00762 to 0.0254 cm), and preferably no to 0.005 in, (0.0127 cm) but, because of the above structural arrangement, can be wider than this range, depending on the amount of additional area “A 1 ” being provided or required (see Eqs. 1 &amp; 2). 
         [0029]    During the assembly process, a bonding agent  90 , which is preferably a glass seal or washer, is placed over a stub  85  located at the upper end  79  of the diaphragm  60  (see  FIGS. 9 &amp; 11 ). The stator  40  is then placed over the diaphragm  60 , with a thru-hole opening  55  located at the upper end  49  of the stator  40  being received by the stub  85 . A centering sleeve or fixture  100  is then placed over the stub  85  and through a thru-hole  95  of the bonding agent  90  in order to ensure the correct width “g 1 ” of the circumferential gap  87  (see  FIG. 11 ). As the width “g 1 ” of the gap  87  decreases the capacitance “C” increases (see Eq 2). Similarly, as the width “g 1 ” of the gap  87  increases, the capacitance “C” decreases. 
         [0030]    The inner height of the centering sleeve  100  is shorter than the height of the stub  85 . This height difference provides a gap  103  between the lower end  101  of the sleeve  100  and the top planar surface  91  of the bonding agent  90 . The gap  103  helps ensure that when the bonding agent  90  is being cured at temperature, the bonding agent  90 , when flown in its liquid state, does not interact with the lower end  101  of the centering sleeve  100 . After the stator  40  bonds with the diaphragm  60 , the centering sleeve  100  can be removed from the final assembly and reused as required for other assembly processes. The centering sleeve  100  could be replaced with bonding agent to accomplish a similar function as that of the sleeve  100 , but this process will not be as accurate because the centering sleeve  100  is machined to precise tolerance and does not change its state during the process. A bonding agent, in contrast, would be flowing in a liquid state during the process and could potentially cause the stator  40  to shift relative to the diaphragm  60 . 
         [0031]    The fixture  100  is removed after the bonding agent  90  bonds to the lower planar surface  53  of the stator  40  to the upper planar surface  73  of the diaphragm  60 . Bonding agent  90  provides two additional functions in the final assembly. First, the bonding agent  90  acts as an insulator, eliminating an electrical short between the stator  40  and diaphragm  60 . Second, the bonding agent  90  acts as a dielectric material, aiding in providing a capacitance to the final sensor assembly. 
         [0032]    The bonding agent  90  preferably has a melting point below 1832° F. (1000° C.) and preferably below 842° F. (450° C.). Because bonding agent  90  operates at about half the temperature used in existing capacitance sensor designs, the agent  90  reduces issues with oxidation, thereby reducing the overall manufacturing time and cost. Bonding agent  90  may also have a melting point in a range of prior art bonding agents (e.g. 1652 to 1832° F. (900 to 1000° C.)). The melting point of the bonding agent and the housing determines the maximum operating condition of the developed sensor. For example, using Inconel  718  along with a glass whose melting point is up to 1832° F. (1000° C.) allows the usage of the sensor to temperatures of up to 1472° F. (800° C.). 
         [0033]    In an assembled and bonded state, the capacitor sensor  10  has an area “A 1 ” (see Eq. 2) which is now the surface area of the stator-encircled portion  71  of the outer sidewall surface  77  of the diaphragm  60 , that is, 2πr d (h d −h u ), where “r d ” is the outer radius of the diaphragm  60 , “h d ” is the height of the column portion  67  of the diaphragm  60 , and “h u ” is the height of an un-encircled portion  69  of the column  63  (see  FIGS. 9 &amp; 10 ). The un-encircled portion  69  is that sidewall portion of the diaphragm  60  which is not encircled by the sidewalls  45  of the stator  40 . Note that bonding agent  90  also helps control the vertical position of stator  40  relative to diaphragm  60  and, therefore the width of the gap “g 2 ” (see Eq. 2 and  FIG. 11 ). This helps ensure that “h u ” is sufficient to prevent the lower end  59  of stator  40  from arcing or shorting against the base portion  65  of diaphragm  60 . Preferably, the height “h d ” of the column portion  67  of the diaphragm  60  is in a range of 1.5 to 3 times the diameter “d d ” of the column portion  67 . 
         [0034]    Prior art designs, in which the stator is arranged above the diaphragm and does not encircle the diaphragm, have, an area “A” equal to π(r d   2 −r w   2 ), where r d  is the radius of the diaphragm  60  and “r w ” is the radius of opening  55  in stator  40 . A capacitance sensor  10  made according to this invention increases the area “A” and permits the sensor  10  to have a smaller diameter than prior art capacitance sensors while capable of providing sensing in the range of prior art sensors as well as sensing in an increased capacitance range (e.g., above 40 pF and up to about 200 pF). In one preferred embodiment, capacitance sensor  10  can detect capacitance of about 120 pF. The diameter of the protective housing  20  can be 2 in. (5.08 cm) or less, and is preferably in a range of about 0.75 to 1.25 in. (1.905 to 3.175 cm). Diameters of less than 0.75 in. (1.905 cm) can also be accomplished by extending the height of the diaphragm  60 . 
         [0035]    The sidewalls  75  of the column portion  67  of the diaphragm  60  have a thickness “t s ”, but the upper end  79  of the diaphragm  60  has a wall thickness “t t ”. The thickness “t t ” is sized so that the upper end  79  does not flex or bend in response to variations in pressure. Note that if the upper end  79  bends or deforms, then the bonding agent  90  may not return to its original shape, thereby potentially causing repeatability issues. The thickness “t s ” is sized so that the sidewalls  75  flex or bend in response to variations in pressure, thereby permitting the sidewalls  75  to move toward for away from) the inner sidewall surface  47  of the stator  40  and reduce the width “g 1 ” of the circumferential gap  87 . By varying the wall thickness “t s ”, sensor  10  can be used for high pressures (e.g., up to 30,000 psi (206.8 Mpa)). 
         [0036]    Referring to  FIGS. 1 ,  2  &amp;  13 , an evacuation tube bonding agent  29  and an evacuation tube  31  are placed in the respective positions. The lower end  37  of the conductive wire  35  is welded on the upper planar surface  51  of stator  40  and conductive wire  35  is brought out through the evacuation tube  31 . The bonding agent  29  ensures that the evacuation tube  31 , which is conductive, is electrically insulated from the protective (metal) housing  20 . The protective housing  20  is welded to diaphragm  60  as previously described. Bonding agent  29  is cured at the required temperature in order to form a seal, which could be the same temperature as that of bonding agent  90  or a different temperature. The complete assembly is evacuated and backfilled with the required dielectric medium (e.g., Argon, Helium, or a mixture of both) and the upper end  33  of the evacuation tube  31  is crimped and welded. The evacuation tube  31  and the protective housing  20  forms the two leads required for the electronic circuits to convert capacitance into frequency. 
         [0037]    When in the assembled state, capacitance sensor  10  includes an oscillator board and other electronics (not shown) of a type known in the art and located above the upper planar surface  22  of protective housing  20 . The oscillator board (not shown) contains an induct and is used to convert capacitance into frequency, which is achieved using Equation 3 as shown below: 
         [0000]    
       
         
           
             
               
                 
                   f 
                   = 
                   
                     1 
                     
                       2 
                       * 
                       Π 
                       * 
                       
                         
                           L 
                           * 
                           C 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    Where “f” is the frequency, “L” is the inductance, “C” is the capacitance of the sensor, and “π” takes the value of 3.14. The advantage of using such a device is its capability to amplify small capacitance changes into large frequency changes. The oscillator board includes two atm-hole openings, one for receiving stub  24  (and providing a ground), the other for receiving evacuation tube  31 . The oscillator board is in communication with means for receiving and processing the frequency signal which results from the measured capacitance “C”. Tapped holes  28  (see  FIG. 8 ) receive threaded screws (not shown) to attach other electronic components (not shown and which are used to transmit the generated frequency to the surface for further processing) and subsequent housing for those components to the sensor  10 . 
         [0038]    A person of ordinary skill the art would recognize that the preferred embodiments described above are not all possible embodiments of a capacitance sensor made according to this invention, and that changes could be made in its design and construction without departing from the scope of the following claims.