Underground Jet Perforating Using Resistive Blasting Caps

2. Field of the Invention

The present invention is in the field of a gun assembly used for underground jet perforating while exploring for oil and/or gas and extracting the same from underground. More particularly, the present invention is directed a gun assembly containing a plurality of shaped charges, the explosion of which is triggered by resistive blasting caps and which can be detonated in sequential order, and to an electronic module that renders the sequential detonation possible.

3. Background Art

It has been common practice for a long time in the oil and gas production industry to perforate the wall of the oil or gas well casing at locations where entry of oil or gas from the surrounding formation into the casing is desired. The prior art has created shaped explosive charges for this purpose. The charges are detonated by passing current through a blasting cap that ignites the charge through a detonating fuse. The term "shaped charge" in this regard is well understood in the art, and denotes an explosive charge specifically adapted for the purpose of creating certain desired size holes in the casing, and a desired amount of penetration into the surrounding formation.

In connection with the foregoing, a number of shaped charges are assembled in a "gun assembly", which is lowered into the well casing on a wireline including a logging cable. Spaces in the gun assembly which contain the individual charges are separated from one another by baffle plates that are usually not destroyed when the charge below the baffle plate is detonated. Sometimes, it is satisfactory to simultaneously detonate all shaped charges in the gun assembly. More frequently, however, it is desired to detonate the shaped charges sequentially, one by one, usually starting with the shaped charge that is located at the bottom of the gun assembly. This is commonly called "select fire" in the trade, and the prior art has developed several methods for accomplishing such "select fire"0 detonation. One such method utilizes a rotary switch operated at the surface with which the several charges can be detonated. This method, however, has its disadvantages, primarily in that the number of charges which can be detonated in this manner is limited.

Another prior art method, that is presently believed to be the most pertinent background to the present invention permits sequential "select fire" detonation of the charges starting at the bottom of the gun assembly, by sequentially applying direct current (d.c.) voltage of alternating polarity to the logging cable from the surface. In accordance with this method, the logging cable is electrically connected through a diode to the blasting cap attached to the charge on the bottom of the gun assembly, and this blasting cap is grounded. All other blasting caps attached to the other charges above the bottom charge are not grounded. Instead they are electrically connected to the diode and a dart which is mounted through an insulating gasket to the baffle plate. The diode is also connected to the logging cable. The dart is a device, well known in the trade, that seals the baffle from the portion of the gun assembly below, when the charge immediately below the dart has been detonated. In the process, by breaking through the insulating silicone gasket, the dart also becomes electrically grounded and thereby it grounds the blasting cap to which the dart is connected.

The diodes are mounted into an electronic module that also contains a small resistor (approximately 5 .OMEGA.) which is placed in series with the logging cable. Thus, each diode is connected to the logging cable and to the blasting cap, but except for the blasting cap on the bottom of the gun assembly, the rest of the caps are grounded only after the charge immediately below the dart has been detonated. The diodes are mounted with sequentially reversed polarity, so that for example, the diode on the bottom of the assembly permits current to pass through when negative voltage is applied on the surface, the diode above that passes current on positive voltage, the one above that again on negative voltage, and so on. Therefore, when negative voltage is applied to the logging cable on the surface, the diode on the bottom allows current to pass through the blasting cap which is grounded, and the charge on the bottom is detonated. The charges above are not detonated in this first application of negative voltage because the respective blasting caps are not grounded. Nevertheless, current can flow through to the bottom diode and blasting cap, because the logging cable, including the resistors built into the modules, represent a continuous electrical path. After the first charge has detonated, the dart in the baffle above breaks through its silicon gasket, seals the baffle into which it is mounted, and electrically grounds the blasting cap attached to it. This blasting cap receives current through the corresponding diode when positive voltage is applied on the surface. Thus, in accordance with this method, a series of explosive charges built into the gun assembly can be sequentially detonated, starting with the charge on the bottom. The resistor incorporated in each electronic module in series with the logging cable, serves to allow current to flow through to the successive caps on the bottom, even if the wire below such caps is grounded.

U.S. Pat. Nos. 5,105,742, 5,322,019, 5,355,802 and 5,359,935 comprise further background to the present invention.

Blasting caps are usually manufactured to activate when approximately 0.25 to 0.8 amper current flows through them. More specifically, in accordance with practice in the art, blasting cap specifications usually state that the cap will not be activated by current less than approximately 0.25 A, but are certain to be activated with 0.8 A current. The blasting caps, which until relatively recently have been used in the prior art, had very low resistance so that the 0.3 to 0.8 A current could be accomplished by applying low voltage. Relatively recently, for safety reasons, however, blasting caps have been made with higher internal resistance, so that they can be activated only with higher voltage (approximately 25 to 100 Volts). Such blasting caps are called "resistive caps", and usually have internal resistance approximately in the 50 to 120 .OMEGA. range. The just described prior art method of select fire gun assembly is not well suitable for use with resistive caps. A primary reason for this is that for the cap of such resistance to draw enough current to be activated, the parallel disposed resistor would have to have much greater resistance. However, that would require very high voltage in order to send sufficient current through the combined resistance of the resistors which are disposed between the voltage source and the blasting to be activated. Also in accordance with this prior art method of select fire, if detonation of a charge left the wire below intact and grounded, then high resistance parallel with the cap would be needed for that cap to draw enough current for activation. Consequently, use of this method is not practical when the cap itself has resistance in approximately 50 to 120 .OMEGA. range, or higher.

In light of the foregoing, there is a need in the art for a gun assembly containing an electronic module that permits select fire detonation of charges for jet perforation which works well with resistive blasting caps. The present invention provides such a gun assembly and electronic module.

It is an object of the present invention to provide a select fire gun assembly to be used in connection with jet perforating of casings in underground oil and gas wells, which is capable of functioning with resistive blasting caps.

It is another object of the present invention to provide an electronic module for a select fire gun assembly that operates with resistive caps, which module can be assembled to and used in connection with otherwise substantially conventional gun assembly for jet perforation.

These and other objects and advantages are attained by a gun assembly that includes a plurality of shaped charges which are capable of being detonated by blasting caps activated by passage of current through the cap. The first electric terminal of the blasting cap on the bottom of the assembly is grounded. In one embodiment each of the remaining blasting caps of the assembly are electrically connected to a dart that grounds only after the charge below has been detonated. The other terminal of each blasting cap is connected to the logging cable through a first diode, and the polarity of the diodes are arranged in alternative sequence. The first terminal of the blasting cap is also connected electrically through a large resistor to the gate of an electronic switch which is closed either by positive or negative voltage depending on the nature of the switch. The switch is open when the gate is grounded. The electronic switch is incorporated in series with the logging cable. Positive and negative gated electronic switches, which are connected to the blasting caps, are arranged in alternative sequence. The resistance of the large resistor is orders of magnitude greater than the resistance of the blasting cap. A second diode is placed in series with the logging cable and parallel with the electronic switch that is associated with each blasting cap. The first and second diodes are arranged in opposite polarity to one another, and the electronic switch is in opposite polarity to the corresponding second diode. The diodes, electronic switch and the large resistor for each cap may be conveniently incorporated in an electronic module, having 4 lead wires, which are mounted to the logging cable, blasting cap and the dart, respectively.

In a second embodiment one terminal of the blasting cap is grounded and another terminal of the blasting cap is connected through an electronic switch to the output of a diode, with the input of the diode being connected to the logging cable. The polarity of the diodes are arranged in alternative sequence. The gate of the electronic switch is also connected to the diode through a large resistor and to the ground or a dart through another resistor. These electronic switches are closed when the gate is grounded. In this embodiment current can flow through the blasting cap only when the gate is grounded and when the diode associated with the blasting cap is of the appropriate polarity to allow current to pass through.

The features of the present invention can be best understood together with further objects and advantages by reference to the following description, taken in connection with the accompanying drawings, wherein like numerals indicate like parts.

FIG. 1 is view, partly in cross section, of a jet perforating gun assembly constructed in accordance with the present invention;

FIG. 2 is a circuit diagram of the first embodiment of the electronic circuit used for the gun assembly shown in FIG. 1;

FIG. 3A is a circuit diagram of the positive electronic module of the first preferred embodiment of the present invention;

FIG. 3B is a circuit diagram of the negative electronic module of the first preferred embodiment of the present invention;

FIG. 4A is a perspective view of the negative electronic module of the first preferred embodiment;

FIG. 4B is a perspective view of the positive electronic module of the first preferred embodiment;

FIG. 5 is a schematic view of the components assembled in the first preferred embodiment of the gun assembly of the present invention;

FIG. 6 is a circuit diagram of the second embodiment of the electronic circuit used for the gun assembly shown in FIG. 1;

FIG. 7A is a circuit diagram of the positive electronic module of the second preferred embodiment of the present invention;

FIG. 7B is a circuit diagram of the negative electronic module of the second preferred embodiment of the present invention;

FIG. 8A is a perspective view of the negative electronic module of the second preferred embodiment;

FIG. 8B is a perspective view of the positive electronic module of the second preferred embodiment;

FIG. 9 is a schematic view of the components assembled in the second preferred embodiment of the gun assembly of the present invention;

The following specification taken in conjunction with the drawings sets forth the preferred embodiments of the present invention. The embodiments of the invention disclosed herein are the best modes contemplated by the inventor for carrying out his invention in a commercial environment, although it should be understood that various modifications can be accomplished within the parameters of the present invention.

Referring now to the drawing figures and particularly to FIG. 1, the select fire gun assembly of the present invention is disclosed in the environment in which it is used. As it was noted in the introductory section of the present application for patent, the gun assembly 14 is utilized for jet perforation of well casing. Inasmuch as several components of the gun assembly are conventional, the conventional parts or components are described here only briefly. It should also be understood in connection with the present description, and particularly in connection with FIGS. 1 and 2 that these figures show an example of the invention where 4 separate explosive charges can be detonated in sequential order. However, these figures serve only as examples, in that the number of explosive charges which can be detonated in "select fire" manner in accordance with the present invention is practically unlimited; for example as many as 50 charges can be incorporated in the gun assembly of the present invention.

FIG. 1 thus shows a well casing 10 which is disposed in a formation 12. The gun assembly 14 is held in the casing, and prevented from falling in further by a wireline (not shown) that includes a single conductor electrical cable (logging cable) 15. The single conductor of the electrical cable 15 is schematically shown as a conducting line in the circuit diagram drawings of FIGS. 2, 3 and 5. The gun assembly 14 includes a top adapter subassembly or "sub" 16, a plurality (in this example 4) of carriers 18, and a gun bottom 20. The foregoing components are assembled to one another by threaded connections, or are bolted together, in accordance with the state-of-the-art. The gun assembly 14 is of a tubular overall configuration and of a diameter which fits within the well casing. Well casings vary in diameter, and accordingly the gun assemblies which may be constructed in accordance with the present invention are limited in diameter only in the sense that they must fit into the well casing in which they are intended to be used. Presently contemplated diameter for the gun assemblies of the present invention is approximately in the 1.5 to 7"range. Instead of the gun bottom 20, a bottom decentralizer assembly which is not shown here but is well known in the art, could also be used. Still in accordance with the state of the art, each carrier 18 (section of the gun assembly) has a port hole 22 or equivalent, into which an explosive charge (shaped charge) 23 is mounted, and each carrier 18 is separated from the next carrier by a baffle plate 24. The baffle plate 24 is strong enough to usually withstand the detonation of the explosive charge below and to insulate the remainder of the gun assembly from entry of fluid from the formation 12. A metal dart 26 in an insulating gasket (not shown) is mounted in a hole in each baffle plate 24, and the logging cable 15 is passed through the hole in the baffle 24. Still in accordance with the state-of-the-art, when a charge is detonated immediately below the baffle plate 24 and dart 26, as a result of the impact the metal dart breaks through the insulating silicone (or like) rubber gasket (not shown) and plugs the hole in the baffle 24, thereby protecting the part of the gun assembly 14 which is above the detonated part. By contacting the baffle 24 the dart 26 also becomes electrically grounded. The logging cable/conductor 15 below the dart 26 may become grounded as a result of the detonation, or may be affected in such a manner that it has no electrical connection to ground or any other component of the gun assembly 14.

Referring still to FIG. 1, a blasting cap 30 is attached to a detonating cord 31, which is in turn attached to the shaped charge 23. Although a gun assembly can be constructed in accordance with the present invention which would operate with blasting caps of the type that have low resistance, and therefore require only low voltage for activation, the present invention is specifically designed to operate with blasting caps of the type which have relatively high resistance (approximately 50 to 120 .OMEGA. or more) and which therefore require high voltage (approximately in the range of 25 to 100 V) to be activated. These type of blasting caps are commonly called resistive caps. However, as noted above, the invention will also work with blasting caps of practically no resistance. An electronic module 32 which contains components. in accordance with the present invention is connected to the logging cable 15 and to each blasting cap 30 and dart 26. The electronic module 32 is preferably placed into a cylindrical housing which is made from an electrically insulating material of the type that is capable of withstanding elevated temperatures which may be encountered in the underground formation. Preferably, the material of the housing, as well as all materials and components used in the gun assembly of the present invention, are capable of withstanding temperatures up to approximately 200.degree. C. For some applications the requirement for withstanding high temperature may be even more rigorous. The presently preferred material for the housing is high temperature resistance rubber. The outside dimensions of the electronic module housing are not critical. Nevertheless, it is noted that the housing of the module of the preferred embodiments is a cylinder of approximately 0.5" diameter and approximately 1.75" long. The components of the first preferred embodiment electronic modules 32 and the construction and operation of the gun assembly 14 in accordance with the present invention are further explained below, with primary reference to FIGS. 2, 3A and 3B.

FIG. 2 depicts the first preferred embodiment of the circuitry involved in the operation of the gun assembly 14 of FIG. 1. It shows four circuits connected with the logging cable 15. Each one of the four circuits is associated with an explosive charge 23 that is to be detonated in a "select fire" manner. On the surface the logging cable 15 is attached to a switch 34 and direct current source 35 which are capable of applying positive or negative voltage to the cable conductor 15, at the choice of an operator (not shown). In accordance with the present invention the explosive charge 23 on the bottom of the gun assembly is to be detonated first, without detonating the charges above. The first of the four circuits, shown in the bottom of FIG. 2, includes the blasting cap 30 which is shown as a resistor placed in an ellipsoid. As it was noted above, blasting caps conforming to modern requirements typically have approximately 50 to 120 .OMEGA. resistance. As it can be seen, a first electric terminal of the blasting cap 30 is grounded and a second terminal is connected through a first diode 36 to the conductor/logging cable 15. In the first preferred embodiment of the invention, the explosive charge 23 on the bottom of the gun assembly 14 is arbitrarily selected to be detonated when negative voltage is applied to the cable 15 by the switch 34. Therefore, the first diode 36 is mounted so that it allows current to pass only when negative voltage is applied to the cable 15. It should be understood however, that the invention can be practiced equally well with the opposite selection of voltage required to detonate the first (bottom most) charge.

The circuit (also shown in FIG. 3B) associated with the bottom explosive charge on the bottom of the gun assembly 14 has an electronic switch 38 which is placed in series with the conductor 15. The electronic switch 38 incorporated into this circuit is designed to permit detonation when negative voltage is applied. It has a gate that closes the switch 38 only when negative voltage is applied to the gate. The electronic switch 38 also remains open when the gate is grounded. The gate of the switch 38 in the bottom circuit is electrically connected to the first terminal of the blasting cap 30 (or to the ground) through a resistor 40 of relatively large resistance, and is, therefore grounded. Still in series with the conductor 15 and parallel with the electronic switch 38, a second diode 42 is interposed in the cable 15. The second diode 42 is mounted with a polarity that is reverse to the polarity of the first diode 36. Consequently, the second diode 42 of the herein described circuit (FIG. 3B) passes current when positive voltage is applied to it.

Several well known and commercially available devices can be used for the electronic switch in accordance with the present invention. For example bipolar transistors, silicone controlled rectifiers (SCR), silicon control switches, TRIACS, MOSET transistors, insulated gate bipolar transistors (IGBT), bipolar transistors, solid state relays, junction field effect transistors or optically coupled devices and similar solid state devices can be used. The primary requirement in this regard is that the electronic switch must be able to withstand and reliably operate at approximately 200.degree. C., and for some applications at even higher temperatures. In the herein described first preferred embodiment a MOSET transistor is used for the electronic switch 38. The resistor 40 is to be of a value which is substantially greater than the resistance of the blasting cap 30, preferably it is of several orders of magnitude greater than the resistance of the blasting cap 30. In the herein described first preferred embodiment the resistor 40 is of approximately 100K .OMEGA..

Referring still to the bottom circuit of FIG. 2 and to FIG. 3B, a voltage limiting device, such as a Zener diode 44 is disposed between the gate of the electronic switch 38 and the cable 15.

The purpose of the voltage limiting device is to protect the gate from having a greater than permissible voltage difference between itself and the conductor. This is usually described in the specification of the electronic switch as the maximum permissible voltage between the gate and the source of the switch. In the herein described preferred embodiment this maximum voltage is approximately 20 V.

A second resistor 46 having resistance which is approximately of the same order of magnitude as the resistance of the blasting cap 30, but may also be several orders of magnitude greater, is disposed in series with the cable 15 and parallel with the electronic switch 38. As it will be explained below, the voltage limiting device (Zener diode 44) and the second resistor 46 are not necessary for accomplishing "select fire" detonation of charges, but are of useful practicality to protect the electronic switch (Zener diode) and for testing the gun assembly 14 for electrical continuity without detonating any charge.

The conductor cable below the circuit associated with the explosive charge on the bottom of the gun assembly is not connected to any further circuits, nor is it necessarily grounded. Nevertheless, as its is described later, it would not affect the operation of the device if the conductor cable 15 was grounded below the last circuit. It has been found in practice, that the electronic switch 38 and the second diode 42 may be available commercially as one component. In the presently preferred embodiment a MOSET transistor and a diode is used in combination, purchased as transistor IRF830 or an IRF9620, respectively, for the positively and negatively gated switches. For this reason the electronic switch 38 and the second diode 42 are shown in FIGS. 2 and 3B together, in an ellipsoid. In the preferred embodiment the first diode 36 bears the IN4007 designation. All of the electronic components shown in FIG. 3B and in the bottom circuit of FIG. 2, except for the blasting cap 30, are preferably incorporated within the electronic module 32, and a module such as the one having the circuit of FIG. 3B is termed a "negative module". FIG. 4A shows the outside appearance of the preferred embodiment of the negative module. A similar module, designed to detonate a charge on application of positive voltage is termed a "positive module" and is shown by FIG. 3A and FIG. 4B. Each of these modules has 4 lead wires, which are preferably color coded for ease of mounting in the gun assembly 14. The use of the color coded leads is explained further below.

Referring now to the circuit shown second from the bottom up in FIG. 2, and shown (in part) on FIG. 3A, the basic components of this circuit, to be incorporated into a "positive module", are the same as described above, with the following differences. The first terminal of the blasting cap 30 and the gate of electronic switch are both connected to the dart 26, which, however is not grounded until the charge below has been detonated. The first diode 36 is mounted in reverse polarity to the diode of the negative module, the electronic switch 38 closes when positive voltage is applied to the gate and the voltage limiting Zener diode 44 is mounted in reverse polarity to the one described for a negative module. The second diode 42 is, again, mounted with reverse polarity to the first diode 36 of the same circuit. The circuit associated with the third explosive charge 23 from the bottom up, is again designed to detonate on application of negative voltage and has circuitry similar to the bottom circuit (negative module) with the difference that the blasting cap 30 and gate are electrically connected to the dart 26 which is not grounded. The fourth charge (from the bottom up) has positive circuitry, like the second charge. The number of explosive charges which may be incorporated in the gun assembly in a similar fashion is without limitation as far as the ability to cause their select fire detonation in accordance with the present invention, is concerned. The operation of the gun assembly that has the first preferred embodiment of the circuits in accordance with the invention is now described as follows.

As it was noted above, the explosive charge 23 on the bottom of the gun assembly is to be detonated first. Because in the example described here the bottom charge is designed to activate or set off on negative current, negative voltage (in the approximate range of 50 to 200 V) is applied on the surface by use of the switch 34. The path of the current, from the surface down to the bottom, is as follows. The logging cable 15 itself comprises a resistance in the range of approximately 50 to 200 .OMEGA., and this resistance is indicated on FIG. 2, as resistor 48. The negative current passes through the resistor 48, and with virtually no resistance through the second diode 42 of the positive electronic module 32 associated with the upper most charge 23. The first diode 36 of this circuit does not permit negative current, and neither does the electronic switch 38 because its gate does not receive voltage. No current flows through the blasting cap 30 associated with the first charge 23 on the top and consequently it is not set off. In the negative electronic module 32 associated with the charge 23 second from the top, the second diode 42 does not permit negative current, but the first diode 36 does and therefore negative voltage is applied to the gate of the electronic switch 38. Consequently this switch is closed, and negative current passes through the second module with virtually no resistance. The corresponding blasting cap 30, however, does not have sufficient current to set it off. The third circuit or module 32 from the top down is again a positive module and the negative current passes through it the same way as through the first module.

The bottom circuit or module 32 is negative. When negative current reaches this module 32, it does not pass through the second diode 42, nor through the open electronic switch which is grounded, but it passes through the first diode and the blasting cap 30 which then draws enough current to be activated, as intended. Moreover, this happens whether or not the conductor of the logging cable 15 below the bottom module is grounded.

In the event, through error or oversight, positive voltage is applied at the surface when it is intended to detonate the bottom charge in the herein described before embodiment, then no detonation occurs for the following reason. None of the first three blasting caps are grounded, and therefore no current passes through them to set them off. Although positive voltage will reach the module 32 associated with the bottom charge, if the conductor 15 of the logging cable is grounded then current passes through the second diode 42 to the ground, and does not flow through the blasting cap 30. If the conductor 15 of the logging cable is not grounded, then positive current cannot pass through the last module, and, again, the blasting cap 30 is not set off.

After explosion of the first charge 23, the dart 26 associated with the charge above the bottom charge becomes grounded. The dashed lines in the drawing figures represent conductors which become grounded only after explosion of the charge below. The charge second from the bottom is intended to be set off with positive current, regardless whether or not the conductor 15 of the logging cable below has been left grounded as a result of the earlier detonation. Positive current flows through the blasting cap 30 of this charge because the first diode 36 of the corresponding electronic module 32 allows current to flow through the blasting cap 30 to the ground. Inadvertent application of negative voltage on the surface would not set off this blasting cap 30, because the first diode 36 will prevent flow of current through the cap 30.

It should be readily understood by those skilled in the art from the foregoing description that a substantially unlimited number of shaped explosive charges can be assembled and detonated in a "select fire" manner in accordance with the present invention. When constructing a gun assembly in accordance with the present invention, the shaped charges 23, darts 26 and other hardware are assembled substantially as in the prior art. In contrast with the prior art, however, the electronic module 32 of the first preferred embodiment of the present invention is wired at one end thereof in series to the logging cable 15, and at the other end thereof with its respective lead wires to the two leads of the blasting cap 30, (or one wire to the dart 26) and logging cable 15. The first terminal of the blasting cap 30 on the bottom of the gun assembly is grounded. The blasting cap 30 on the bottom is arbitrarily assigned either a negative or positive module 30, and the remaining modules sequentially alternate in polarity. In the preferred embodiments the lead wires are color coded. FIG. 4A shows the first preferred embodiment of a negative module, and FIG. 4B shows a first preferred embodiment of a positive module. The yellow and blue lead wires in these embodiments are connected to the logging cable 15 in series, and the other two lead wires are connected to the two leads of the blasting cap (or one to the dart 26) respectively. FIG. 3B is the circuit diagram of the first preferred embodiment of a negative module and FIG. 3A is the circuit diagram of the first preferred embodiment of the positive module. In these figures the leads labeled T and B are connected to the logging cable 15, C.sub.1 is connected to the first terminal, and C.sub.2 is connected to the second terminal of the blasting cap 30. The first terminal of all blasting caps 30 other than the one on the bottom, is connected to the dart 26.

The Zener diodes 44 incorporated in the electronic modules of the present invention are an optional feature and serve to protect the gates of the electronic switches 38 from voltage in excess of approximately 20 V. The gates of the electronic switches of the first preferred embodiment do not to close the switch unless sufficient voltage (usually in excess of 1 V) of the right polarity is applied. The second resistors 46 provide a path parallel with the respective diodes and electronic switches when the gun assembly 14 is tested for continuity with low voltage that is insufficient to close the electronic switches. Low voltage must be applied for testing, in order to avoid setting off any of the blasting caps.

FIGS. 6 through 9 disclose a second preferred embodiment of the circuits and electronic modules of the gun assembly of the present invention. An overall view of the gun assembly in accordance with the second preferred embodiment still looks substantially the same as the view shown in FIG. 1, except that each electronic module of the second preferred embodiment has only 3 rather than 4 lead wires. Those features, components and principles of operation employed in the second preferred embodiment which are identical or substantially identical with the analogous features, components or principles in the first preferred embodiment are not described below, or are described only to the extent necessary to understand the difference between the structure and operation of the first and second embodiments.

Thus referring now primarily to the circuits shown in FIGS. 6, 7A and 7B, the herein described gun assembly of the second embodiment also has 4 explosive charges, which are to be exploded sequentially with the first charge on the bottom of the gun assembly to be exploded first. As in the first preferred embodiment, in this embodiment also the circuitry is arbitrarily selected such that the first charge on the bottom of the gun assembly is detonated when negative voltage is applied at the surface. FIG. 6 does not show the explosive charges but shows the blasting cap 30 which, as in the description of the first preferred embodiment, is depicted as a resistor placed in an ellipsoid. The second embodiment of the circuitry used in the gun assembly of the present invention is also designed to function advantageously when resistive blasting caps (having a resistance in the range of approximately 50 to 120 .OMEGA.) are employed. However, this embodiment too, can be used with blasting caps of much lower resistance. As in the first preferred embodiment, all components of the circuit associated with each blasting cap are included in a module contained in a housing. A module associated with the second preferred embodiment and which is wired to a blasting cap 30 to detonate when negative voltage is applied is a "negative module" 50. A module associated with the second preferred embodiment and which is wired to a blasting cap 30 to detonate when positive voltage is applied is a "positive module" 52. Outside appearance and dimensions of these modules are substantially the same as of the corresponding modules in the first preferred embodiment except that each of the modules 50 and 52 have only three lead wires extending from the housing. FIG. 8A is a perspective view of the negative electronic module 50 and FIG. 8B is a perspective view of the positive electronic module 52.

The structure and operation of the circuitry of the second embodiment is explained starting with the description of the negative module 50 associated with the first blasting cap 30 disposed on the bottom of the gun assembly. The logging cable 15 is connected to a diode 54 which is positioned so that it permits passage of current only when the applied voltage is negative. The diode 54 is then connected through a resistor 56 of relatively large resistance (approximately 500 .OMEGA.) to the gate of an electronic switch 58 and thereafter through another resistor 60 of approximately 100 .OMEGA. to the ground. In a negative module 50 that is disposed in the association with the third (from the bottom up) explosive charge the logging cable 15 is connected through the same resistors 56 and 60, and gate, to a dart 26 which becomes grounded only after the explosive charge below has been detonated. The output of the diode 54 is also connected to the source of the electronic switch 58. The drain of the electronic switch 58 is connected to the blasting cap 30, the other terminal of which is connected to the ground. Generally speaking, the same descriptions and qualifications apply to the electronic switches utilized in the second embodiment as in the first preferred embodiment. However wiring of the switches in the second preferred embodiment is reversed relative to the first preferred embodiment, in that in the second preferred embodiment in both negative and positive modules 50 and 52 the source of the switch is connected to the logging cable 15 and the drain of the switch is connected to the non-grounded terminal of the blasting cap 15. Moreover, the electronic switch which serves in a positive module in the first preferred embodiment serves in the negative module 50 of the second preferred embodiment. The electronic switch which serves in a negative module of the first preferred embodiment serves in the positive module 52 of the second preferred embodiment. The result of this arrangement is that the electronic switch 58 of the negative module 50 of the second preferred embodiment is closed when its gate is grounded, and is also closed when positive voltage is applied to its gate. The electronic switch 58 of the positive module 52 of the second preferred embodiment is closed when its gate is grounded, and is also closed when negative voltage is applied to its gate. The workable range of the resistors 56 and 60 of the second preferred embodiment is approximately 100 K.OMEGA. to several mega.OMEGA. for resistor 56 and approximately 10 K.OMEGA. to 500 K.OMEGA. for resistor 60.

In light of the foregoing, when negative voltage (in the range of approximately 50 to 200 V) is applied to the logging cable 15 at the switch 34, current passes through the diode 54 of the negative module associated with the first (bottom-most) blasting cap 30. Because the end of the logging cable 15 is grounded, the gate of the switch 58 is grounded and the switch 58 is closed. Consequently current passes through the switch 58 and through the blasting cap 30 to the ground triggering detonation of the explosive charge on the bottom of the gun assembly. FIGS. 6, 7A and 7B also show another diode 62 within a circle, associated with and parallel to the electronic switch 58. The second diode 62 is present in the herein described preferred embodiment because, as in the first preferred embodiment, the electronic switch 58 and the diode 62 of the negative module are preferably obtained as a single commercially available component. For the negative module 50 of the second preferred embodiment an IRF830 MOSET transistor and diode combination is used. For the positive module 52 of the second embodiment an IRF9620 MOSET transistor and diode combination is used. The diode 54 is preferably of the IN4007 designation, as in the first preferred embodiment. The second diode 62 in the second preferred embodiment, however, has no function, and is included in the preferred embodiment only because of its presence in the above-noted commercially available component.

A Zener diode 64 mounted with a polarity opposite to the polarity of the diode 54 is interposed between the source and gate of the electronic switch 58. Although this Zener diode 64 is not essential for the operation of the embodiment, it is advantageous, because just like in the above-described first embodiment it serves to limit the voltage applied to the gate and thereby protects the electronic switch 58 from damage.

Referring now to the situation where through inadvertence or oversight positive voltage were applied to the logging cable 15 before the first charge has been detonated, current will not pass through the diode 54 of the negative module 50, and therefore the bottom-most charge will not detonate. A blasting cap 30 associated with the second charge (from the bottom up) is wired with a positive module 52, the blasting cap 30 above that with a negative module 50, and so on. In each of these instances the blasting cap 30 itself is wired between the ground and a lead wire of the respective module, the diodes 54 are of sequentially alternating polarity and positive and negative gated electronic switches 58 alternate sequentially in the modules. Another lead wire of each module, except for the one on the bottom of the gun assembly which is grounded, is connected to the dart 26. However, each dart 26 becomes grounded only after the charge below it has detonated. Therefore, as it can be seen in the circuit diagram of FIG. 6, first application of power detonates only the blasting cap 30 on the bottom of the gun assembly. Specifically, the second and fourth (from the bottom up) blasting caps 30 will not be set off with negative voltage because the diodes 54 in the positive modules 52 associated with these positions do not allow passage of current. They will set off by positive voltage only when their respective dart 26 is grounded, because their electronic switches require either ground or negative voltage to dose the switch 58. The drawings next to the right margin of the sheet on FIG. 6 comprise a schematically simplified depiction of the circuit and indicate the open or closed status of the electronic switches when negative voltage is first applied to the gun assembly having four explosive charges and four modules in accordance with the second preferred embodiment.

In the negative or positive electronic modules 50 and 52 of the second preferred embodiment a yellow colored lead wire of the module is connected to the logging cable 15, and a blue lead wire is connected to the dart 26, or in case of the module in the bottom of the assembly, to the ground. Black lead wire of the negative module 50, or red lead wire of the positive module 52 is connected to the non-grounded terminal of the blasting cap 30. FIG. 9 of the appended drawings schematically shows how the blasting caps 30, negative and positive electronic modules 50 and 52 and darts 26 are assembled in the herein-described example of the second preferred embodiment.

It should be readily apparent from the foregoing that the number of explosive charges which may be assembled and sequentially detonated in accordance with the second embodiment of the invention is practically unlimited, or limited only by considerations unrelated to the present invention.

What has been described above is a select fire gun assembly containing multiple explosive charges and resistive blasting caps which can be detonated in a select fire manner by sequentially applying direct current voltage of alternating polarity from the surface, and an electronic module that is incorporated in the gun assembly to render the select fire detonations possible. An important advantage of the above-described gun assembly and electronic module of the invention is that it functions well with charges set off by resistive blasting caps.

Several modifications of the present invention may become readily apparent to those skilled in the art in light of the foregoing disclosure. For example, instead of diodes a functional equivalent can be used, that is a device which passes current in only one direction. Still further by way of example, it is noted that a transistor could be used for this purpose, and in such case the transistor would function as a diode. Therefore the terminology "diode" in this application should be interpreted meaning a diode, transistor or other devices used in the invention to function as a diode for passing current only in one direction. In light of the foregoing, the scope of the present invention should be interpreted solely from the following claims, as such claims are read in light of the disclosure.