Abstract:
A resonator clock suitable for use in downhole conditions is described. The resonator clock includes a resonator portion of piezoelectric material; two electrodes in electrical communication with the resonator portion such that the resonator portion resonates when voltage is applied between the two electrodes; and four supports to support the resonator portion. The supports are dimensioned and positioned to support the resonator portion under shock and vibration encountered in downhole use. The supports and the resonator portion are formed from the same continuous piece of piezoelectric material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This patent specification relates ruggedized quartz clocks. More particularly this patent specification relates to rugged quartz clocks particularly suited to downhole applications. 
         [0003]    2. Background of the Invention 
         [0004]    For many of down hole applications, a precision time reference clock is used. For example, a precision clock may be used downhole to provide a time reference for counting frequency outputs from resonator type pressure and/or temperature sensors. In another example, a precision clock can be used as a reference to provide synchronization between surface and downhole operations. 
         [0005]      FIG. 1  shows a precision time reference clock mechanically packaged in conventional metal can package. Note that the view shown in  FIG. 1  is of the interior of the clock. Normally, the quartz resonator  110  is hidden inside a metal can-type enclosure. The can is usually bonded to the metal base  112  by cold welding for the sake of secured seal. This package type has a relatively good track record for keeping the interior in vacuum, which is one of the important features to provide good performances of the clock. 
         [0006]    However, one significant drawback of the metal can type package is the mechanical robustness. As can be seen in  FIG. 1 , the quartz resonator  110  is supported by three legs  114 ,  116  and  118 . Other numbers of legs, such as 2 and 4 legs can be used depending on the size and kind of the package. The resonator  110  is normally glued to the legs  114 ,  116  and  118  by using conductive bonding agent in order to flow current while mechanically securing it in the package. Even with four legs, the resonator  110  can become dislodged from the legs due to a strong mechanical shock such as is incurred through downhole use. If the mechanical robustness is enhanced by increasing the bonding agent between the resonator and the legs, this will have a negative side effect to the electrical characteristics. 
         [0007]    Additionally, the resistance of the conductive bonding agent tends to slightly change long term under high temperatures, and therefore the resonance of the crystal also tends to change over time. Furthermore, the conductive bonding agent is typically organic in composition, such epoxy, and therefore degasses over time in the vacuum environment. 
         [0008]    Other packaging technologies are known in the industry, such as a ceramic package. But these alternative packages have been found in general to be less effective than the conventional metal enclosure for the sealing performances especially at elevated temperatures. 
         [0009]    Thus, there is a need for a more mechanically robust quartz clock design for downhole use under both high temperatures and high shock exposure. 
       SUMMARY OF THE INVENTION 
       [0010]    According to embodiments, a resonator clock for use in downhole conditions is provided. The resonator clock includes a resonator portion of piezoelectric material; two electrodes in electrical communication with the resonator portion such that the resonator portion resonates when voltage is applied between the two electrodes; and two or more supports to support the resonator portion. The supports are dimensioned and positioned to support the resonator portion under shock and vibration encountered in downhole use. The supports and the resonator portion are formed from the same continuous piece of piezoelectric material. 
         [0011]    Additionally, according to some embodiments downhole tool is provided which makes use of a resonator clock as described above. Furthermore, the invention is also embodied in a method for making measurements downhole using a resonator clock as described above. 
         [0012]    Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
           [0014]      FIG. 1  shows a precision time reference clock mechanically packaged in conventional metal can package; 
           [0015]      FIG. 2  illustrates a wellsite system in which the present invention can be employed; 
           [0016]      FIG. 3  shows another typical downhole setting for rugged quartz clock, according to embodiments; 
           [0017]      FIG. 4  shows an example of a quartz clock resonator, according to some embodiments; 
           [0018]      FIG. 5  shows further details of the example of a quartz clock resonator shown in  FIG. 4 ; and 
           [0019]      FIG. 6  is a cross sectional view along the line A-A′ of the quartz clock resonator shown in  FIGS. 4 and 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    In the following detailed description of the preferred embodiments, reference is made to accompanying drawings, which form a part hereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention. 
         [0021]    The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice. Further, like reference numbers and designations in the various drawings indicated like elements. 
         [0022]    For many of down hole applications, a precision time reference clock is used. For example, a precision clock may be used downhole to provide a time reference for counting frequency outputs from resonator type pressure and/or temperature sensors. In another example, a precision clock can be used as a reference to provide synchronization between surface and downhole operations. 
         [0023]      FIG. 2  illustrates a wellsite system in which the present invention can be employed. The wellsite can be onshore or offshore. In this exemplary system, a borehole  11  is formed in subsurface formations by rotary drilling in a manner that is well known. Embodiments of the invention can also use directional drilling, as will be described hereinafter. 
         [0024]    A drill string  12  is suspended within the borehole  11  and has a bottom hole assembly  200  which includes a drill bit  205  at its lower end. The surface system includes platform and derrick assembly  10  positioned over the borehole  11 , the assembly  10  including a rotary table  16 , kelly  17 , hook  18  and rotary swivel  19 . The drill string  12  is rotated by the rotary table  16 , energized by means not shown, which engages the kelly  17  at the upper end of the drill string. The drill string  12  is suspended from a hook  18 , attached to a traveling block (also not shown), through the kelly  17  and a rotary swivel  19  which permits rotation of the drill string relative to the hook. As is well known, a top drive system could alternatively be used. 
         [0025]    In the example of this embodiment, the surface system further includes drilling fluid or mud  26  stored in a pit  27  formed at the well site. A pump  29  delivers the drilling fluid  26  to the interior of the drill string  12  via a port in the swivel  19 , causing the drilling fluid to flow downwardly through the drill string  12  as indicated by the directional arrow  8 . The drilling fluid exits the drill string  12  via ports in the drill bit  205 , and then circulates upwardly through the annulus region between the outside of the drill string and the wall of the borehole, as indicated by the directional arrows  9 . In this well known manner, the drilling fluid lubricates the drill bit  205  and carries formation cuttings up to the surface as it is returned to the pit  27  for recirculation. 
         [0026]    The bottom hole assembly  200  of the illustrated embodiment a logging-while-drilling (LWD) module  220 , a measuring-while-drilling (MWD) module  230 , a roto-steerable system and motor, and drill bit  205 . 
         [0027]    The LWD module  220  is housed in a special type of drill collar, as is known in the art, and can contain one or a plurality of known types of logging tools. It will also be understood that more than one LWD and/or MWD module can be employed, e.g. as represented at  220 A. (References, throughout, to a module at the position of  220  can alternatively mean a module at the position of  220 A as well.) The LWD module includes capabilities for measuring, processing, and storing information, as well as for communicating with the surface equipment. In the present embodiments, the LWD module includes an acoustic measuring device, which includes a rugged quartz clock as described herein. 
         [0028]    The MWD module  230  is also housed in a special type of drill collar, as is known in the art, and can contain one or more devices for measuring characteristics of the drill string and drill bit. The MWD tool further includes an apparatus (not shown) for generating electrical power to the downhole system. This may typically include a mud turbine generator powered by the flow of the drilling fluid, it being understood that other power and/or battery systems may be employed. In the present embodiment, the MWD module includes one or more of the following types of measuring devices: a weight-on-bit measuring device, a torque measuring device, a vibration measuring device, a shock measuring device, a stick slip measuring device, a direction measuring device, and an inclination measuring device. In the present embodiments, the LWD module can also include a rugged quartz clock as described herein. 
         [0029]      FIG. 3  shows another typical downhole setting for rugged quartz clock, according to embodiments. Shown in  FIG. 3  is wireline truck  310  deploying wireline cable  312  into well  330  via well head  320 . Wireline tool  340  is disposed on the end of the cable  312 . According to one example, wireline tool  340  is a downhole sampling tool such as the Modular Formation Dynamics Tester tool from Schlumberger. Within tool  340  are one or more downhole pressure transducers each housed in a sealed container. The harsh downhole environments such as shown in  FIGS. 2 and 3 , typically expose the downhole quartz clock to various extreme conditions such as shock and large temperature and pressure fluctuations. 
         [0030]      FIG. 4  shows an example of a quartz clock resonator, according to some embodiments. Resonator portion  410  is circular in shape and is part of quartz plate  412  which is a single piece of crystalline quartz. The circular center portion of quartz plate  412  forms circular resonator portion  410  due to four circular openings  414 ,  416 ,  418  and  420 . The openings  414 ,  416 ,  418  and  420  thereby create four support members  440 ,  442 ,  444  and  446 , which support resonator portion  410  and are formed from the same single continuous piece of quartz plate  412 . Further the resonator portion  410  is of convex shape which can be seen more clearly in  FIG. 6 , described below. According to some embodiments, the quartz plate  412  is made from stress-compensated cut (“SC-cut”) quartz, which has the advantage of being relatively force insensitive in this application. The SC-cut has been found to be suitable for dual-mode clocks. 
         [0031]    It is noted that the structures described herein are applicable to both single mode (i.e single frequency) and dual-mode (i.e. dual frequency) clocks. For embodiments using single-mode clocks the following crystalline orientations have been found to be suitable AT-cut and BT-cut. For embodiments using dual-mode clocks, the following orientations have been found to be suitable, SC-Cut, RT-cut, X+30o-cut, and SBTC-cut. 
         [0032]      FIG. 5  shows further details of the example of a quartz clock resonator shown in  FIG. 4 . The electrodes  530  and  532  (which shown more clearly in  FIG. 6 ) are deposited directly on the surface of the resonator portion  410  using for example, vacuum deposition techniques. According to some embodiments, a chromium or titanium substrate is first deposited, upon which a gold layer is then deposited. Spacer  524  is hermetically bonded to the surface of quartz plate  412  in the area outside of the four openings  414 ,  416 ,  418  and  420 . Spacer  524  is bonded to the surface of crystal quartz plate  412 , for example, using a non conductive, non organic bonding agent such as silicon dioxide. According to some embodiments, spacer  524  is made of silicon dioxide as well. According to some other embodiments, the thickness of the boding agent used, such as silicon dioxide, is thicker than the convex portion of resonator portion  410  and therefore the spacer  524  may consist solely of the boding agent and no additional spacer is required. Note that since there is no organic conductive bonding agent used such as is common in the prior art, the problem of degassing by such agents is greatly decreased. Furthermore, since there is no organic conductive bonding agent used to support the resonator portion  410 , the problem of long term thermal effects causing a change in resistivity and associated resonator characteristics, is greatly diminished. 
         [0033]    Note that although four circular openings or slits are shown in  FIGS. 4 and 5 , according to some embodiments, other number of circular slots are used. For example, according to some embodiments, there are only two slots (and therefore two support members). Such designs may be used, for example, in applications where the expected shock exposure is lower. However, it has been found that providing four slots and therefore four support points minimize the stress effect to the C-mode frequency. Thus, the four-slot, four support design tends to minimize the frequency variation of the C-mode over a relatively wide temperature range. The preferred use of four slots and four supports to decrease C-mode variations for a downhole clock is in stark contrast to a downhole pressure gauge design, which benefits from just the opposite: large C-mode variations. 
         [0034]    According to other embodiments, greater numbers of slots are provided. However it has been found that having a maximum of four support members allows for decreased force sensitivity, particularly when using SC-cut quartz. While having more than four support members (and therefore, greater than four slots) will slightly increase the robustness of the design, it has been found that this will significantly increase the force sensitivity of the clock. The result of increased force sensitivity, is that the clock will be generally more susceptible to changes and error due to temperature changes causing thermal expansion. 
         [0035]      FIG. 6  is a cross sectional view along the line A-A′ of the quartz clock resonator shown in  FIGS. 4 and 5 . Note that spacer  524  is bonded on one side of the quartz quartz plate  412  and spacer  526  is bonded on the other side of quartz plate  412 . Also shown are plates  610  and  612  which are solid and bonded to spacers  526  and  524  respectively such that an interior cavity  620  is formed. Plates  610  and  612 , spacers  524  and  526 , and quartz plate  412  are all hermetically bonded using a bonding agent such as silicon dioxide such that cavity  620  is maintained in as a vacuum. The material for plates  610  and  612  are preferably the same material and the same crystalline orientation as quartz plate  412 , in this case crystalline quartz, so as to minimize stress due to thermal expansion. As mentioned above, according to some embodiments, the bonding agent, such as silicon dioxide, after curing is substantially thicker than the convex portion of resonator portion  410 , such that no separate spacer is required. In such embodiments, the reference numbers  524  and  526  refer to the bonding agent  524  and bonding agent  526 . 
         [0036]    Thus, the described embodiments eliminate the use of a bonding agent to support the resonator by making the resonator and its peripheral support from one piece of quartz plate, two additional plates for the enclosure. These three pieces of quartz are hermetically sealed with a non-conductive bonding agent. By integrating the resonator supports into the same crystalline plate as the resonator itself, the structure is robust enough to withstand the demanding shock and vibration environments of downhole use, especially with LWD/MWD applications. 
         [0037]    According to some embodiments, the entire assembly shown in  FIG. 6  is mounted and packaged in a standard electrical package as is known in the art. According to some embodiments, a metal can package such as is used with prior art downhole quartz clocks is used to surround the assembly. 
         [0038]    According to some embodiments, alternative piezo-electric materials are used instead of quartz, such as Langasite and/or Langatite 
         [0039]    Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Further, the invention has been described with reference to particular preferred embodiments, but variations within the spirit and scope of the invention will occur to those skilled in the art. It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words, which have been used herein, are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.