Abstract:
Apparatus and method are provided for selectively firing apparatus in a well when the apparatus is fired by an electrical voltage and produces light upon firing. A photoresistor or other device changes in an electrical property when the light from firing of a section of the apparatus impinges on the photoresistor or other electronic device. The change in electrical property shifts the state of a relay such that the following section of the device can be fired with a voltage of the opposite polarity to that used in firing the preceding section. A test and resetting apparatus for the select-fire apparatus is also provided.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to well operations. More particularly, apparatus and method are provided for selective firing of explosive devices with a light-activated switch. 
         [0003]    2. Description of Related Art 
         [0004]    Casings in wells for producing or injecting fluids are cemented in a wellbore and holes are formed in the casing at selected locations opposite certain subterranean formations by a device called a “perforating gun.” The gun usually is made up of shaped charges that are detonated by a blasting cap. The cap is activated by an electrical current. In many wells it is desirable to perforate casing over larger distances in the wellbore than can be accommodated by one perforating gun. To avoid running perforating guns in the wellbore and withdrawing the spent charges repeatedly, it is advantageous to place a plurality of perforating charges or groups of charges in the well simultaneously and shooting the charges selectively when placed opposite the selected subterranean formation. This capability is called “select-fire,” and it is old in the art. 
         [0005]    Examples of apparatus for selectively firing perforating charges are disclosed in U.S. Pat. Nos. 5,531,164; 5,700,969; and 7,387,162. The electrical circuits in the devices are designed such that charges are fired sequentially by alternately applying a negative and a positive electrical voltage to the device. The circuits also include a mechanical device, referred to as a “dart.” The dart is disposed between chambers of a perforating charge or multiple charges that are to be fired selectively. The function of the dart is to electrically ground a blasting cap in the adjacent second chamber when the charges are fired in a first chamber. The electrical circuits are such that the perforating charges cannot be fired until the blasting cap for those charges is grounded. The dart moves in response to the shockwave pressure in the first chamber to place electrical conductors in contact, thus grounding the blasting cap. Darts may be made of aluminum or steel and may have rubber or other electrical insulation. A simplified drawing of a dart, to illustrate the principles of operation, is shown in  FIG. 1  (Prior Art). The explosive force of perforating charges puts electrical conductors in contact. 
         [0006]    One problem with darts is that about 1 in 120 devices now in use in industry fail and cause a misfire (lack of firing) of subsequent charges in a sequence of select-fire charges. This failure requires that the perforating apparatus be withdrawn from a well and another apparatus run into the well. This can be a very costly failure, particularly in deep wells, offshore wells and other wells in high-cost operating areas. Another limitation of mechanical darts is that there is no adequate test to predict the performance of a dart before it is used. 
         [0007]    Other explosive devices may be used in wells where firing at selected times and places is advantageous. For example, explosive devices may be used to cut casing or other tubulars, to obtain a sample of material surrounding a well or for other purposes. 
         [0008]    What is needed is a device to be used in an electrical circuit to replace the mechanical darts and an electrical circuit to be used with the device such that select-firing of devices can be achieved by alternating the electrical voltage applied to the device between positive and negative. Tests to predict the performance of the device before it is run into a well should be available. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    A select-fire device is provided employing a light-activated sensor to switch the position of a relay. Light to activate the device is produced by the ignition and burning of explosive materials in the perforating gun. A first perforating charge or charges in a first chamber can be fired by applying a DC voltage of selected polarity, for example, negative. A window disposed between the first chamber and the adjacent second chamber, each containing a perforating charge or multiple charges that are to be fired selectively, allows light from the first chamber to pass to a switch. The light passing into the switch decreases the resistance of a photoresistor in the switch. The decrease in resistance allows shifting of a relay to the position such that the charges above the switch (in the second chamber) can be fired by a voltage of opposite polarity—in the example positive. Successive switches between chambers containing perforating charges, each with a window, photoresistor and relay, allow the select-firing of an arbitrary number of charges or sets of multiple charges. Other explosive devices may be fired at selected places and times using the apparatus and method disclosed herein. A test device is provided that may be used to reset the respective switch relays for reuse (of the device) or to verify that the circuit is operable before deploying the select-fire device in a well. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0010]      FIGS. 1A and 1B  are simplified drawings illustrating operating principles of a prior art mechanical switch. 
           [0011]      FIG. 2  is an electrical schematic of one embodiment of a circuit employing light-activated sensors in a select-fire device. 
           [0012]      FIG. 3  is an electrical schematic of a negative switch 
           [0013]      FIG. 4  is an electrical schematic of a positive switch 
           [0014]      FIG. 5  is an assembly drawing of a select-fire perforating apparatus. 
           [0015]      FIG. 6  is an isometric quarter-section view of one embodiment of the optical switch disclosed herein. 
           [0016]      FIG. 7  is an electrical schematic of a test and resetting circuit device showing the resetting of a negative switch. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Referring to  FIG. 2 , one embodiment of an electrical circuit for use with a light-activated switch as disclosed herein is shown. Variable voltage power supply  8 , preferably capable of supplying from 0 to about 65 VDC, is used in conjunction with voltage polarity switch  9  to send either positive or negative voltage down supply voltage line  11  to a perforating gun assembly such as assembly  37  ( FIG. 5 ) during a well casing perforation operation. Supply voltage line  11  has inherent line resistance  10 , which usually is in the range from 20 to about 200 ohms. Several gun sections or more could be used during any given perforating operation; however, in this embodiment, a single positive switch ( 21 ), a single negative switch ( 22 ), and a section without a switch ( 23 ) are discussed. These switches control the firing of charges in three sections of perforating gun assembly  37  of  FIG. 5 . To initiate a select-fire perforating operation, a negative voltage of approximately 30-50 VDC is applied to supply voltage line  11 . The negative voltage allows current to flow through blasting cap diode  13 ( a ) of section  37 ( a ), the lowest blasting section of perforating gun assembly  37 . The current causes blasting cap  14 ( a ) to ignite primacord  44 ( a ), firing all lower perforating guns  43 ( a ) in section  37 ( a ) of the tool. While perforating guns  43 ( a ) are firing, the negative voltage is applied across Zener diode  15 ( b ) of negative switch  22 , coils  24 ( b ) of relay  12 ( b ) and photoresistor  16 ( b ) to ground. The resistance of photoresistor  16 ( b ) is high, preferably in the range of about 200 k ohms, until light  17 ( a ) from perforating gun  43 ( a ) or blasting cap  14 ( a ) or primacord  44 ( a ) reaches photoresistor  16 ( b ), when the resistance drops to a low resistance—in the range of about 3 k ohms, for example. The low resistance of photoresistor  16 ( b ) allows sufficient current to flow through relay switching coil  24 ( b ) and Zener diode  15 ( b ) to shift the state of dual switch latching relay  12 ( b ). Referring to  FIG. 3 , relay switching wiper connection  27 ( b ), connected to input supply voltage line  18 , is switched from the normally closed switching wiper connection  28 ( b ) to the normally open switching wiper connection  29 ( b ). The switching within relay  12 ( b ) of negative switch  22  allows supply voltage line  18  to be in direct contact with next blasting cap wire  20 ( b ). Once the polarity of the supply voltage line  18  is switched through voltage polarity switch  9 , current is allowed to flow in a positive direction through blasting cap wire  20 ( b ) and blasting cap diode  13 ( b ) of negative switch  22  to blasting cap  14 ( b ), which ignites primacord  44 ( b ) and perforating guns  43 ( b ) section  37 ( b ) of perforating gun assembly  37 . While positive voltage is applied to the supply voltage line  11  and perforating guns  43 ( b ) of negative switch  22  are firing, the positive applied voltage is applied across Zener diode  15 ( c ) of positive switch  21 . Again, the high resistance of photoresistor  16 ( c ) embedded within positive switch  21  and behind a protective lens, as will be shown in detail below, prevents any significant current from passing through Zener diode  15 ( c ) and switching coil  24 ( c ) of dual switch latching relay  12 ( c ). Once light  17 ( b ) from the perforating gun  43 ( b ) or blasting cap  14 ( b ) or primacord  44 ( b ) reaches photoresistor  16 ( c ) of positive switch  21 , the resistance of photoresistor  16 ( c ) drops, allowing sufficient current to flow through relay switching coil  24 ( c ) and Zener diode  15 ( c ) to shift the relay from the normally closed wiper connection to the normally open switching wiper connection. The switching within relay  12 ( c ) of positive switch  21  allows supply voltage line  18  to be in direct contact with next Zener diode  13 ( c ) and blasting cap wire  14 ( c ). This process is continued until all of the switches are sequenced and the blasting caps fired. 
         [0018]    Referring to  FIGS. 3 ,  4 ,  5  and  6  one embodiment of the electrical schematics of a negative switch  22  ( FIG. 3 ) and a positive switch  21  ( FIG. 4 ) and the corresponding mechanical structures ( FIGS. 5 and 6 ) are shown in more detail. Incoming supply voltage wire  18  is connected to voltage pass-through wire  19  of negative switch  22  within perforating gun assembly  37 . Voltage pass-through wire  19  is connected to pass-through voltage connection  58  ( FIG. 6 ) on the nose of positive switch  21 . Dual switch latching relay  12  has two distinct sides, switching and resetting. Relay switching coil  24  is used to change the state of the switch and allow direct contact between supply voltage line  18  and the next blast cap wire  20 . The other side of the dual switch latching relay  12 , relay resetting coil  31 , is used to verify the state of the latch and reset the switch. Both relay switching wiper connection  27  and relay resetting wiper connection  34  are directly connected internally through switching and resetting wiper link  30 . One side of dual switch latching relay  12  cannot be activated without activating the other side. The same discussion applies to the electrical schematic of positive switch  21  referred to in  FIG. 4 . The main differences between the electrical schematics of negative switch  22  in  FIG. 3  and positive switch  21  in  FIG. 4  is the polarity of Zener diode  15 . 
         [0019]    Referring to  FIG. 5 , one embodiment of select-fire perforating gun assembly  37  is shown. From the top of perforating gun assembly  37 , supply voltage line  11  is connected to supply voltage wire  18  of positive switch  21  in section  37 ( c ). Voltage pass-through wire  19  is connected to the pass through voltage connection  58  at the end of positive switch  21 . Voltage pass-through wire  19  is connected to supply voltage wire  18  of negative switch  22  located in section  37 ( b ) of perforating gun assembly  37 . Voltage pass-through wire  19  of negative switch  22  is connected to blasting cap diode  13 ( a ) of the last gun section within the tool. Below the last negative switch  22  of perforating gun assembly  37  are perforating gun(s)  43 ( a ), perforating gun tube  38 ( a ), perforating gun holder  39 ( a ), primacord  44 ( a ), blasting cap  14 ( a ), and blasting cap diode  13 ( a ). Between the last negative switch  22  and the first positive switch  21  of perforating gun assembly  37  are perforating gun(s)  43 ( b ), perforating gun tube  38 ( b ), perforating gun holder  39 ( b ), primacord  44 ( b ), blasting cap  14 ( b ), and blasting cap diode  13 ( b ). Above the first positive switch  21  of perforating gun assembly  37  are perforating gun(s)  43 ( c ), perforating gun tube  38 ( c ), perforating gun holder  39 ( c ), primacord  44 ( c ), blasting cap  14 ( c ), and blasting cap diode  13 ( c ). Perforating gun assembly  37  body is made up of gun assembly nose  41 , perforating gun tubes  38 ( a )-( c ), and switch holder subs  40 ( a ) and ( b ). Positive and negative switches are held within respective switch holder subs by switch retaining nuts  42 . Perforating gun tubes  38  are sealed with o-rings and secured with bolts to the switch holder subs. Perforating gun assembly  37  can have as many alternating switches as necessary for the particular perforating operation. The supply voltage is dependent on perforating gun characteristics and perforating gun assembly  37  configuration. 
         [0020]    Referring to  FIG. 6 , an isometric quarter-section view of one embodiment of the optical switch disclosed herein is shown. Switch body  55  can be made of steel or aluminum. Switch body  55  has o-ring grooves  61  that allow the switch to seal explosive pressure to switch holder sub  40  and within perforating gun tube  38  ( FIG. 5 ) during perforating operations. The voltage passed down to the next sequential switch is made through pass-through connection component  58  on the end of the switch. Pass-through connection component  58  has wire wrap groove  62  for attaching a voltage pass-through wire for the next successive switch. Located within pass-through connection component  58  is photoresistor  16  that is connected to printed circuit board  60  within switch body  55 . Also attached to printed circuit board  60  are Zener diode  15 , dual switch latching relay  12 , and switch test connector  45 . Printed circuit board  60  and all of the electrical components are preferably held in place with non-conducting potting material  57 . Photoresistor  16  may be protected from explosive debris by protective window  59 . Window  59  may be made of high-strength glass or other optically transparent material. Electrical isolation between the pass-through connection component  58  and the switch body  55  is provided by voltage insulator  56 , which is typically made from polyetheretherketone (PEEK) or another similar non-conducting material. 
         [0021]    Referring to  FIG. 7 , an electrical schematic of test and resetting circuit device  54  is shown during the process of resetting negative switch  22  from the “fired” to the “armed” state. The left hand side of  FIG. 7  shows dual switch latching relay  12  of negative switch  22  in the “fired” state. After use in the perforating gun assembly  37 , negative switch  22  can be reset and reused. Test and resetting circuit device  54  can be connected through test circuit connector  46  to switch test connector  45  ( FIG. 6 ) mounted on printed circuit board  60  within the negative switch  22 , for example. The same applies for positive switch  22 . Once test and resetting circuit device  54  is connected to a “fired” negative switch  22 , red “fired” light emitting diode (LED)  47  is illuminated by current passing through a circuit made through DC power supply  53 , test circuit connector  46 , switch test connector  45  and relay resetting wiper connection  34  on the dual switch latching relay  12 , out to normally-open resetting wiper connection  36 , back to switch test connector  45 , test circuit connector  46 , through LED  47 , and 2 k ohm resistor  51 . Resetting of the negative switch  22  dual switch-latching relay  12  is accomplished by depressing the normally-open push button switch  52 . When push button switch  52  is depressed, it completes two circuits. The first circuit allows current to flow through push button switch  52 , the green “reset” light emitting diode (LED), test circuit connector  46 , switch test connector  45 , relay positive resetting coil connection  32 , relay resetting coil  31  on the dual switch latching relay  12  of negative switch  22 , through the relay negative resetting coil connection  33 , back to the switch test connector  45 , into the test circuit connector  46  and into the negative side of the 16-24 VDC power supply  53 . This circuit allows relay resetting coil  31  to switch the relay switching wiper connection  27  and relay resetting wiper connection  34  connected through the switching and resetting wiper link  30 , from the “fired” state to the “armed” state. Once relay resetting coil  31  is energized, the “armed” circuit is completed. The “armed” circuit is made when LED  48  is illuminated by current passing through a circuit made through DC power supply  53 , test circuit connector  46 , switch test connector  45 , relay resetting wiper connection  34  on the dual switch latching relay  12 , out to the normally-closed resetting wiper connection  35 , back to switch test connector  45 , test circuit connector  46 , through LED  48 , and 2 k ohm resistor  51 . Once dual switch latching relay  12  is in this final state, illustrated on the right-hand side of  FIG. 7 , negative switch  22  is ready for removal of the test and resetting circuit device  54  and loading within the perforating gun assembly  37 . 
         [0022]    A suitable relay for the disclosed apparatus is model 422H dual switch latching relay available from Teledyne, Inc. A suitable Zener diode is NTE5251A, 9.1 Zener Voltage, available from NTE Electronics, Inc. or 1N5262 51 Zener Voltage, available from Vishay Semiconductors. The range depends on the shooting voltage of the perforating assembly. A suitable photoresistor, having a resistance in darkness of 200 k ohm and 3 k ohm in light, is PVD-P8001, available from Advanced Photonix, Inc. 
         [0023]    The method and apparatus disclosed herein have been described primarily as activating perforating guns. It should be understood that the method and apparatus may also be employed to activate other devices by electrical current when light is produced. For example, selective firing of apparatus to cut pipe, recover a core sample or other material from a well using an explosive, or any other operating employing an explosive charge may be accomplished using the method and apparatus disclosed herein. 
         [0024]    Although a mechanical dual switch latching relay has been described above, it should be understood that a single switch, non-latching may be employed instead. Also, solid state electronic switching devices, well known in the art, may be used instead of a mechanical relay. Also, a decrease in resistance of a photoresistor is described in the apparatus and method disclosed herein, but a change in resistance or other electrical property of a material in response to light may also be employed in some embodiments of the method disclosed. A change in electrical resistance, both positive and negative, in response to light may be employed in the method disclosed herein. A change in electrical capacitance or inductance or electrostatic charge of an electrical circuit in response to light may be used to shift the position or state of a mechanical or electronic relay in the method disclosed herein. 
         [0025]    Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.