Abstract:
A spark-gap tool includes a plurality of electrodes, a mandrel, transductive element(s), and a force transmission configuration. Upon relative movement between components a physical distortion of one or more transductive elements occurs, whereby an electrical potential is generated. A method for powering the spark-gap tool is by physically distorting one or more transductive elements by moving components axially and/or rotationally. A method for treating a borehole is by physically distorting one or more transductive elements thereby creating sufficient voltage potential to cause an arc of selected magnitude across a spark-gap in the tool. A downhole power generation arrangement includes a first member and a second member that are movable and a piezoelectric element on one of the first member and the second member and in force transmissive communication with the other of the first member and the second member.

Description:
BACKGROUND  
       [0001]     Spark-gap tools are known in the hydrocarbon industry. These tools have not, however, gained strong acceptance in permanent completions primarily because they require a large voltage to function acceptably. Such voltage is often delivered to the spark-gap tool in a downhole environment through electrical conductors from a surface supply system. As one of ordinary skill in the art clearly recognizes, the longer the electrical conductor, the greater the voltage drop. For this reason the voltage at the surface supply needs to be even greater than that required to produce an acceptable arc at the spark-gap tool. Since many rig operators are uncomfortable with utilizing systems employing greater than 200 volts from a surface supply, the spark-gap tools&#39; functionality has been limited. Moreover, because of the electrical requirements, other compromises are also made throughout the wellbore to accommodate power at the site of the spark-gap tool. Each of the above issues creates a lack of interest in the industry in using the spark-gap tools.  
       SUMMARY  
       [0002]     Disclosed herein is a spark-gap tool which includes a housing, a plurality of electrodes at the housing, a mandrel nested with the housing, transductive element(s) located at one of the housing and the mandrel, and a force transmission configuration located at the other of the housing, and the mandrel, the initiator, upon relative movement between the housing and the mandrel, causing a physical distortion of one or more transductive elements, whereby an electrical potential is generated by the one or more transductive elements.  
         [0003]     Further disclosed herein is a method for powering the spark-gap tool by physically distorting one or more transductive elements cyclically by moving the mandrel within its housing axially and rotationally thereby creating sufficient voltage potential to cause an arc of selected magnitude across a spark-gap in the tool.  
         [0004]     Further disclosed herein is a method for treating a borehole by physically distorting one or more transductive elements thereby creating sufficient voltage potential to cause an arc of selected magnitude across a spark-gap in the tool.  
         [0005]     Further disclosed herein is a downhole power generation arrangement including a first member, a second member, at least one of the first member and second member being movable relative to the other of the first member and the second member; and a piezoelectric element of one of the first member and the second member and in force transmissive communication with the other of the first member and the second member, at least one of the first member and the second member being mechanically movable from a surface location. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     Referring now to the drawings wherein like elements are numbered alike in the several Figures:  
         [0007]      FIGS. 1A and 1B  are an extended schematic elevation view of a wellbore with the spark-gap tool deployed therein;  
         [0008]      FIG. 2  is an expanded view of the circumscribed Section  2 - 2  in  FIG. 1B ;  
         [0009]      FIG. 3  is an expanded view of the circumscribed view Section  3 - 3  in  FIG. 1B ;  
         [0010]      FIG. 4  is a schematic elevation view of an alternate voltage operation arrangement.  
         [0011]      FIG. 5  is a schematic elevation view of another alternate voltage generation arrangement. 
     
    
     DETAILED DESCRIPTION  
       [0012]     Referring to  FIGS. 1A and 1B , an overview is provided of a wellbore  10 , a pump jack  12  and a spark-gap tool  14 . As illustrated, the spark-gap tool includes a pair of electrodes  16   a  and  16   b  located within a section of pipe  18  having a plurality of openings  20 . Further illustrated, generally, is a voltage generation arrangement  22 . With arrangement  22  utilizing mechanical function in conjunction with one or more transducers, the problem in the prior art of supplying high voltage from surface and carrying that voltage to the spark tool has been eliminated. Because the voltage generation arrangement can be located proximate the spark-gap electrodes  16   a  and  16   b , voltage loss (due to distance) is not a factor.  
         [0013]     Referring to  FIG. 2 , one embodiment of a mechanical voltage generation arrangement  22  is depicted in more detail. Central to this embodiment is a piezoelectric element  24  (transductive element). A piezoelectric element is a transducer and thereby capable of creating a voltage potential when subjected to a mechanical energy input in any selected direction or combination of directions causing physical distortion of the element.  
         [0014]     In this embodiment, mechanical energy input is provided through a configuration described hereunder, to the piezoelectric element(s)  24  to produce the desired voltage. In specific embodiments hereof, the mechanical energy may be imparted to the element(s)  24  any number of times from one to infinity in order to produce a buildup of charges or a continuous charge or some combination of these. In one embodiment, the mechanical energy is provided by set down weight of an inner mandrel  26  of the spark-gap tool  14 . Set down weight is operative when a tool housing  28  of the spark-gap tool  14  is anchored such that the mandrel  26  is moveable relative to the tool housing  28 . The housing  28  may be anchored within casing  10  in any of a number of conventional ways and not shown. Because of the anchoring of the housing  28 , that housing will no longer move downhole when further set down weight from the pump rig  12  is applied to the mandrel  26 . Such application of mechanical energy is transmitted to a compression piston  30  (embodiment of force transmission configuration), which in turn is force transmissive communication with the piezoelectric element(s)  24 . Mechanical energy (more generically deformative energy, which may include hydraulic, pneumatic, and even optic energy could be used. The phrase “mechanical energy” as used herein is intended to also encompass these other ways of physically distorting the element(s)  24 .) applied to the compression piston causes a compression of the piezoelectric element  24  thereby creating the desired voltage potential in that element. It should be noted in passing that the piezoelectric element contemplated may be of a single crystalline variety or a polycrystalline variety, such as a ceramic material. Single crystalline varieties are more efficient but also are more costly to procure. Some ceramic materials operable as piezoelectric materials include barium titanate, lead zirconate, lead titanate, and lead zirconate titanate, etc. Since most ceramic materials are composed of random crystalline structure, in order to reliably produce the desired voltage potential upon mechanical energy input, the ceramic material must be polarized thereby aligning the individual crystals therein prior to use to generate a voltage potential. Polarization allows the structure to act more like a single crystalline piezoelectric material. Axiomatically, single crystalline varieties of piezoelectric elements do not require poling prior to use. The voltage potential generated is proportional to the thickness of the material in element  24  and the amount of physical distortion of the element, in turn related to the applied force thereon. In this particular embodiment the compression piston  30  is configured, at an internal dimension thereof, with a profile  32 . The profile  32  includes specific features allowing it to engage and then release a collet mechanism or series of collet mechanisms  34 . The specific features are rounded ridge type projections known in the art. Such ridges transfer a load until a predetermined maximum load is reached whereafter the ridge yields and drops the load.  
         [0015]     In the particular embodiment illustrated in  FIG. 3 , collet mechanisms  34  are depicted. As illustrated, this embodiment provides for voltage buildup in a capacitor  36  by creating multiple compressive and release cycles on the piezoelectric element  24 . As the mandrel  26  moves in the direction of arrow  38 , profile  32  of compression piston  30  is picked up on collet ridge  40  and released, then picked up on collet ridge  42  and released, and then picked up on collet ridge  44 . As illustrated, collet ridge  42  is at the release position with the collet  34  deforming to allow the ridge  42  to release the piston  30 . During each compression cycle, the piezoelectric element generates a voltage which is sent for storage to the capacitor  36 . As the collet mechanism  34  deflects, the compression piston  30  is released thereby removing mechanical energy from the piezoelectric element  24 . This will, in turn, eliminate the production of voltage from the piezoelectric element  24  and reset it to its natural position. Upon further motion of the mandrel  26 , the next ridge  42  picks up profile  32 , transmitting mechanical energy once again to the piezoelectric element  24 . Upon release of each ridge  40 ,  42 ,  44 , the collet mechanism  34  is deflected regularly inwardly relative to the mandrel  26 . This can be seen in  FIG. 2  with respect to the collet mechanism ridge  42 . Although three collet mechanisms  34  are illustrated, more or fewer can be utilized as desired. Limitation in the number of collet mechanisms employable relates only to stroke possibilities for the mandrel  26 . This may be limited by the pump jack  12  on the surface or may be limited by available open space within the wellbore or within the tool. In the illustrated embodiment, in order to generate additional voltage, one need merely move the mandrel  26  uphole resetting the collet mechanism(s) for a further movement in the downhole direction and thereby create three more pulsed electrical signals to be stored in the capacitor. Depending upon exactly how much voltage a particular application requires, the above-stated procedure may be repeated indefinitely to fully charge the capacitor prior to creating an arc across the electrodes  16   a  and  16   b.    
         [0016]     Referring to  FIG. 3 , the spark-gap portion  46  is illustrated very schematically. The device comprises a rectifier diode  48 , the capacitor identified previously as  36 , and a switch  50  which completes the circuit to either side of the spark-gap  52 . Once the circuit is completed, electrodes  16   a  and  16   b  function together to generate an arc that jumps over the spark-gap. Upon the formation of the arc, fluid located in the spark-gap  52  is vaporized and a shockwave is initiated. Referring back to  FIG. 1 , and still referring to  FIG. 3 , this embodiment illustrates that the tool housing  28  includes perforated interval  54  located adjacent to spark-gap  52 . The perforated interval may be a slotted pipe, a holed pipe, or other construction configured to allow propagation of the shockwave generated at spark-gap  52  through the tool housing  28 . Since it may be desirable to propagate the shockwave into the formation itself, a casing segment radially outwardly disposed of the spark-gap tool would also have a perforated interval, schematically illustrated as  56 .  
         [0017]     Mechanical energy may also be imparted utilizing rotational initiation. Referring to  FIG. 4 , a rotary mandrel  60  may be provided with one or more actuator bumps  62 . In a tool housing  64  surrounding the mandrel  60 , one or more piezoelectric elements  66  are installed. In this embodiment, one or more compression pistons  68  are located between the piezoelectric elements  66  and the bump or bumps  62 . It is noted that in some applications the pistons  68  may be omitted and contact between bump or bumps  62  directly with element or elements  66  may be had. Upon rotation of mandrel  60 , sequential elements  66  will be compressed and released. This will generate a voltage potential which may then be stored in a capacitor similar to that depicted in  FIG. 3  or may simply be used without storage if appropriate for the application. This arrangement will then be connected to the spark-gap electrodes.  
         [0018]     In yet another embodiment of the mechanical energy arrangement, referring to  FIG. 5 , a mandrel  70  is configured with a shoulder  72  that has an offset profile such that a portion of shoulder  72  will be in contact with a relatively small portion of a counter shoulder  74  located within the spark-gap tool housing  76 . Located at  78 , around the periphery of housing  76 , is one or more piezoelectric elements which can be mechanically compressed one after the other as mandrel  70  rotates. It should also be noted that a compression piston arrangement such as, for example, a metal disk may be placed atop the element  78  to protect them from direct frictional degradation due to rotation of mandrel  70  but still allow the compressive force of shoulder  72  to cause the desired voltage potential in element(s)  78 . As is clear from the drawing, however, such disk is not required but merely is optional.  
         [0019]     While preferred embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.