US3010396A - Selective firing apparatus - Google Patents

Description

Nov. 28, 1961 H. R. COLEMAN SELECTIVE FIRING APPARATUS 5 Sheets-Sheet l' Filed Dec. 3l, 1957 Nov. 28, 1961 H. R'. COLEMAN SELECTIVE FIRING APPARATUS 3 Sheets-Shes?, 2

Filed Dec. 3l, 1957 Nov. 28, 1961 H. R. COLEMAN SELECTIVE FIRING APPARATUS 3 Sheets-Sheet 3 Filed Dec. 5l, 1957 United States Patent thee 3,010,396 `Patented Nov. 28, 1961 3,010,396 SELECTIVE FIRING APPARATUS Hubert R. Coleman, Midland, Tex., assignor to The Western Company of North America, a corporation o Delaware Filed Dec. 31, 1957, Ser. No. 706,339 11 Claims. (Cl. IGZ-21.6)

This invention relates to systems, apparatus and methods for tiling explosive charges and, more particularly, to improved remotely controlled systems and apparatus for selectively and separately detonating a number ot such charges. The new systems and apparatus are useful, among other purposes, for .firing a plurality of charges located at various points in or adjacent to an exposed earth formation and are especially well`suited for use in apparatus for perforating cased wells.

Although various systems for selectively firing a number of remote explosive charges have been developed practical applications remain for which previously available systems are not entirely satisfactory. An example of such an application is the detonation of a plurality of explosive charges in a well. lt is common practice in the completion of oil, gas or water, production or injection wells to lower a steel casing into a well bore hole and anchor it in the surrounding earth formation with cement. The cased well is then perforated at a preetermined depth or depths to provide communication between the earth formation and the inside or the casing to permit iiuid treatment of the formation through the casing or ow of ground fluids into the well. The perforating operation is generally carried out by lowering one or more elongated multi-shot perforating guns into the well on an electrically conductive cable and then simultaneously detonating the charges in the guns by causing an electrical impulse to flow down the cable. Perforating guns are commonly 7 or .ll feet in length and usually contain a maximum of 6 charges per foot. These charges are employed either to propel bullets through the well casing and into the surrounding earth formation or, when shaped explosive or jet charges are used, to penetrate the casing and formation by means of a high velocity metallic jet.

It is frequently desired to perforate an extended section of a cased well. This often presents problems, however, since the widely used carrier type jet perforating guns are generally designed for not more than about l() feet of charges due to the weight per foot of the carrier, which makes it ditlicult for men to handle longer guns in the ield. Therefore, when it was desired to perforate a section of a well more than 10 feet in length it was reviously the practice to lower a gun into the well repeatedly and retrieve it to be reloaded or replaced by a new gun after each lO-foot section of the well was perforated. The only alternative has been to connect severa guns together in a string, lower them into the well, and re them simultaneously. The latter method, while less time-consuming than the former, is not entirely satisfactory since the limitations imposed by the lowering cable, the gun coupling devices, and the well casing may not safely permit simultaneous detonation of the number of jet charges normally present in more than two ll-foot guns.

In one embodiment of the present invention there is provided a system for selectively and separately firing the individual jet perforating guns in a string of such guns. Such a system permits the perforation or" a continuous extended zone or two or more separated Zones on a single run into the well. This materially reduces the time required to complete a pertorating operation, as compared to that required by the procedure of repeatedly running a single gun in and out of the well, and

containing as many guns as necessary to perforate the desired zone or zones and selectively tiring the guns individually, repositioning the string in the well if necessary after firing each gun. For example, in order to eliminate unperforated intervals in an extended zone the string is repositioned after tiring each gun so that the next gun to be tired overlaps the unperforated section of the well previously adjacent to the coupling device between that gun and the gun fired previously. When perforating two or more widely separated Zones or increasing the perforation density in a' particular zone it is only necessary to raise or lower the string in the well after firing each gun to place an unlired gun at the desired level in the well. All of these operations are carried out in a minimum of time and without placing undue stress on the casing, bore hole, cable or gun coupling devices.

Systems for selectively firing a number of perforating guns or charges have been suggested previously. For example, it has been proposed to employ several electrical elector switches and multiple electrical lead wires. Other systems comprising complex networks of electrical resistances are adapted to selectively tire perforating. guns when different critical amounts of electrical current are applied. `None of these systems has been found entirely practicable for use in wells, however, and most have serious limitations. For example, the multiplicity of electrical lead wires required by some adds materially to the time required to assemble a string of perforating guns in the eld and inevitably multiplies the chances for equipment failure or malfunctioning. SystemsV employing networks of resistances for establishing critical amounts of current are also disadvantageous lin many cases due to the possibility of firing all of the guns simultaneously when too larf'e a current is accidentally applied to the system.

lt is therefore, an object of the present invention to provide a reliable, mistake proof system for selectively and separately tiring a plurality of remote explosive vcharges in a predetermined sequence.

It is yet another object of the invention to provide a system for selectively firing a plurality of well casing perforating guns which permits the use of a self-generating magnetic casing collar locator in conjunction with the tiring system without affecting the selectivity of-.the latter.

lt is another object of the invention to provide a' selec`- tive firing system for a plurality of perforating guns which may be embodied in an easily assembled, ruggedly constructed, substantially mistake proof well casing perforating apparatus capable of providing safe and reliable operation under service conditions in the field.

In one illustrative embodiment ofthe invention there are provided one or more rectiers associated with the individual detonators or blasting caps. lf, for example, three guns are to be fired, one by one, the first and third guns each may include, in circuit with its detonator, a rectiiier arranged -to be selectively responsive to voltage of one polarity, and the intermediate, second gun may include a rectier selectively responsive to voltage of the opposite polarity. There are also provided circuit means, including for example a switch, for applying to the rectitiers a voltage, the positive or negative polarity of which is controlled, step by step, in such a wayras to actuate the detonators selectively in corresponding steps.

Thus there may be provided, in one embodiment, a source of voltage; control means connected tothe voltage source and adapted to be switched between a first anda second condition for producing alternately an output volt- Lpresent in the well.

tively responsive to output voltage having the first characteristic for individually firing one or a group of explosive charges associated with these individual devices; and one or more devices selectively responsive to output voltage having the second characteristic for firing other explosive charges. The voltage responsive devices are arranged in the system with those responsive to voltage of one characteristic alternating with those responsive to voltage of the other characteristic. Actuation of the control means repeatedly to its rst land second conditions causes the voltage responsive devices, one-by one, to detonate their'respective charges, as desired.

In addition, there are described herein certain highly advantageous circuits for protecting the rectifiers referred to above against excessive inverse vol-tages and also oircuits and a method useful for checking the apparatus to make certain that a given gun is properly armed before attempts are made to fire it. The apparatus also includes means for normally preventing the firing of any of the explosive charges except one or a group of them, for example, rthe one most remote from the voltage source.

Other features of the invention will appear in the course of the following detailed description of a typical embodiment thereof taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagrammatic elevational view in section of a cased well bore hole containing a string of perforating guns;

FIG. 2 is an axial cross-sectional fragmentary view showing portions of Itwo adjacent carrier type jet perforating guns .and an intermediate coupling device;

FIG. 3 is a diagrammatic representation of a system of the invention and the electrical circuit included therein.

FIG. 4 is a diagrammatic representation of another systern` of the invention and the electrical circuit included therein..

In FIG. 1, a string of perforating guns is shown positioned in a well penetrating an earth formation 11. The well is lined with steel well casing 12. The string of guns is made up of two relatively long perforating guns 13 and 14 and a shorter gun 15 joined at their ends by means of coupling devices 16. The lower end of gun 14 is sealed against well fluids by means of bull plug 17. The string of guns is supported in the well on cable 18 which may be a conventional single conductor, reverse concentric cable or any other suitable electrical conductor. The cable 18 is connected to a cable head 19 which serves to retain the cable and insulate its central conductor from any uid A self generating magnetic casing collar locator 20 is connected between cable head 19 and a kcontact sub -21 which is connected in turn to the upper gun of the string through top sub 22. Although three perforating guns are shown in the drawing any convenient number may be employed in the present invention.

In FIG. 2 a cylindrical steel coupling device 23 having an laxial bore 24 terminating in a machined recess 25 at the lower end of the coupling device serves to connect Vtwo carrier type jet perforating gunst26 and 27 (only the parent that the coupling device 23 in the string of guns in FIG. 2 serves the same function as the coupling devices 16 in the string of guns in FIG. l. A shaped explosive or jet charge 28 is shown positioned to re through a port 29 in the lower end of gun 26. Port 29 is closed with a soft metal plug 30 which serves to seal the port to prevent entry of desensitizing well fluids but does not, however, seriously interfere with the jet when the charge is fired. A voltage polarity selective detonator assembly 31 is positionedin the lower end'of gun 26 by means of spring wire retainer 32. Explosively actuated switch assembly 33 4 is press fitted into the recess 25 in the lower end of coupling device 23.

The voltage polarity selective detonator assembly 31 comprises an electrically insulating casing 34 containing an electrical blasting cap 35 or other suitable detonator, a rectifier 36, a primacord initiating booster charge 37 and a length of primacord or other suitable fuse 38 which runs through the detonator assembly and the length of gun 26 adjacent to the bases of the plurality of jet charges 2S therein. Silicon rectifiers are preferred for use in well treating systems or apparatus since they are relatively small and have superior high temperature characteristics. Other types of'rectiers are, however, also suitable for use in the systems of the invention. A passageway 39 in insulating casing 34 ensures contact of the explosive wave front from the detonator 35 with the booster charge 37. A main electrical conductor 40 in electrical contact with the central conductor of the cable 18 runs through the gun 26 and into the detonator assembly 311 where it is electrically connected to rectifier 36. An electrical lead wire 41 is connected to main conductor 40 ahead of rectifier 36 and is led through and out of the casing 34. Electrical lead wire 42 electrically connects the detonator or blasting cap 35 with the rectiiier 36. Electrical leadwire 43 connected to blasting cap 35 is led through and out of the casing 34.

Explosively actuated switch assembly 33 comprises an insulating body member 44 which is press fitted into the recess 25 in coupling device 23. Body member 44 is provided with an axial bore 45 opening into an enlarged recess y46. An insulating plug 47 having a small axial hole therein is snugly fittedV into the upper end of bore 45 of body member `44. A stationary metal'electrical contact member 48 is press fitted into the remainder of bore 45 of body member 44. An electrical lead wire 49 is led through the axial hole in insulated plug 47 and one end is electrically connected to lead wire 43 which is connected to the main conductor -40 throughV blasting cap 35 in the detonator assembly 31. The other end of electrical lead 49 makes electrical contact with stationary Contact member 48. A movable steel electrical contact `member 50 having a shaft portion 56 adapted to slide into the enlarged recess 46 in insulating body member 44 is shown in an initial position. The end of the shaft portion 56 of movable contact member 50, which is positioned Within the recess 46, is adapted to engage an insulating and wire shearing plug 51 having an axial bore therein to receive a spring wire electrical contact' 52 connected at one end to movable contact member 50. One end of an electrical lead wire 53 is connected to lead 41 which is in turn connected to the main conductor 40 ahead of the rectifier 36 in the detonator assembly 31. insulating sleeves 54 and 55 are provided toV protect the connections between leads 43 and y49 and 41 and 53 respectively. Movable contact member 50 is provided with a iiat base 59 having an area substantially larger than the area of the open end of enlarged recess 46 4in body member 44. A screw 57 is threaded into a hole in base 59 to provide electrical contact between the movable contact member 50 and a second main electrical conductor 58 which is led into-and through the next lower gun in the string (not shown) Where it would be connected to a rectifier in a selective detonator assembly in the same manner as main conductor 40 is connected to rectifier 36 in detonator assembly 31 of gun 26. The other end of electrical lead wire 53 is run through insulating body member 44 to a point where it emerges into the recess 46 and is then inserted into insulating and wire shearing plug 51 through which it runs to make electrical contact with movable contact member 50.

The schematic diagram of a system of the present invention shown in FIG. 3 includes a source of D.-C. current 60, a single pole single throw switch 61 and a double pole double throw reversing switch 62 for applying positive or negative voltage to a main conductor 63 as desired. The voltage of the source 60 may, for example, be about 200 to 320 volts. The .main conductor 63 in practice would be, for example, the central conductor of a reverse concentric cable, the insulating sheath of which is indicated at 64. The system also includes a selective detonator assembly 65 adapted to be detonated by a positive voltage, an explosively actuated switch assembly 66, a second selective detonator assembly 67 adapted to be initiated by a negative Voltage, an explosively actuated switch assembly 68 and a terminal detonator assembly 69 adapted to be initiated by a positive voltage.

Selective detonator assembly 65 consists of a detonator 79 which may suitably be an electrical blasting cap, and a rectifier 71 or other suitable device designed to pass current in only one direction. The detonator assembly 31 of FIG. 2 shows a suitable mechanical arrangement which may be used in the construction of a typical detonator assembly such as 65 or 67 of FIG. 3. Explosively actuated switch assembly 66 includes a normally closed switch 72 and a normally open switch 73. The explosively actuated switch assembly 33 of FIG. 2 shows a suitable mechanical arrangement which may be used in the construction of a typical explosively actuated switch assembly such as 66 or 63 of FIG. 3. The rectier 7l in assembly 65 is connected in parallel with the main conductor 63 so as to otter a relatively low resistance to current flow when a positive voltage is applied to the system and the explosively actuated normally open switch 73 has been closed to connect the detonator 7i) with the circuit ground.

Switch assembly 68 contains a normally open switch 76 and a normally closed switch 77. The circuit branch including selective detonator assembly 67 contains a detonator 74 and a rectitier 75 or equivalent device poled to offer a relatively low resistance to current liow when a negative voltage is applied to the main conductor 63 and the explosively actuated normally open switch 76 is closed so as to connect the detonator 74 to circuit ground. The circuit branch including the terminal detonator assembly 69 contains a grounded detonator 78 and a rectifier 79 arranged to ofer a relatively low resistance to current ilow when a positive voltage is applied to the main conductor. v

There is also provided a magnetic casing collar locator assembly 80. This includes a high resistance col 81, connected between the main conductor 63 and ground, and also includes a pair of back-to-back rectiers, the pair of them being in parallel with each other and in series with the conductor 63. i

Operation it may be assumed that initially the switch 62 is in the position where its arms are toward the left. The switch 6i is now closed. With the switch 62 in the left position, negative voltage is applied to the main conductor 63 and thereby to the rectiiier 79. The back-to- back parallelconnected rectiers 82 and 83 permit the passage of voltage of either polarity (with the exception that for very small voltages of the magnitude generated by the magnetic casing collar locator they provide a high resistance, as will be explained later). Hence for voltages of the magnitude to be used for tiring, and also for smaller voltages to be referred to later in connection with the testing of the circuit of FIG. 4, the rectiiiers 82 and 83, provide, in efect, a low-resistance connection between the upper part of the conductor 63 and that part of it below these rectiers. From the previous description it will be understood that the rectier 79 is arranged to offer high resistance when negative voltage is applied to it. For this reason not enough current can now ow through this rectifier to detonate blasting cap 78. Also, none of the blasting caps in the string of guns can now be detonated for the reason that the switches such as 76 and 73 are open.

To tire the detonator of the bottom gun the switch 62 is now thrown to the right. This has the effect of applying positive voltage through the conductor 63 to the rectifier 79, which now passes suicient current to initiate the detonator 73 and thereby re the bullets or shaped charges of the bottom gun.

When the bottom gun is tired, the explosive waves set up by the detonation open the switch 77 and close the switch 76. The way in which this is accomplished may be understood by referring to the typical construction of an explosively actuated switch shown at 33 in FIG. 2. The explosive waves force the movable contact member 5b into recess 46. As the movable contact member 50 moves into recess 46, insulating and shearing plug 51 shears the electrical lead 53. This has the eiect of opening the normally closed switch 77 in theschematic representation of this portion of the apparatus, as seen in FIG. 3. As a second effect, the upward movement of the movable contact member causes the spring wire electrical contact 52 to engage the stationary metal electrical contact member 48. This completes a circuit from the main conductor through the rectifier and detonator of the next-tobottom gun to ground. As shown in FlG. 2, a path may be traced through a portion 46 of the main conductor, rectitier 36, the lead 42, detonator 35, the leads 43 and 49, the stationary contact member 48, spring wire contact52, the movable contact member 50, and thence to its flat base portion 59 which comes into contact with the body of metallic coupling 23, when it has been forced upwardly as now assumed. As seen in the schematic diagram, FIG. 3, this same path may be traced from the main conductor 63 through one or the other of the rectiliers of the magnetic casing collar locator, and downwardly through the lower part of the main conductor 63 and through the rectitier 75, the detonator 74 and through switch 76, to the ground. Thus, in summary when the member 50 is forced upwardly, it has two effects. t brings the spring wire Contact 52 into engagement with the stationary contact 48, and this, as seen in FIG. 3, is represented by theV closing of the switch 76. The other enect of the upward movement of the member 50 is to shear ol the lead 53, and this is represented in FIG. 3 corresponding to the opening of the switch 77. The closing of the switch 76 and opening of the switch 77 has the elfect of arming the detonator assembly of the next-to-bottom gun. So long as the switch 62 remains in the right-hand position, however, the detonator of this assembly will not be initiated. This is true because the rectifier is arranged to provide high resistance to the passage of current, so long as it receives positive voltage from the main conductor 63. It may again be noted at this point that no detonator assembly above 67 can beV tired at this time regardless of the polarity of the voltage in the conductor 63 because all other energizing current paths are open, so long as the switches corresponding to the switch 73 are open.

To tire the next-to-bottom gun, the switch 62 is next thrown to the left. This applies negative voltage to the main conductor 63, and thence to the rectifier 75. This rectier now allows sufficient current to pass through the detonator 74 to cause it to be initiated, thereby firing the charges in the next-to-bottom gun. This has the effect of closing the switch 73 and opening the switch 72, thereby arming the next higher gun. The switch 62 is next thrown to the right, thereby applying positive voltage to the main conductor 63 and the rectifier 71 and thus initiating detonator 70.

The action of the magnetic casing collar locator assembly will now be described. It is often important to locate a perforating gun at an exactly measured depth in a cased oil well, for example, in order to perforate only the particular section of casing encompassed by the oilproducing earth formation to be treated or drained. Measurement of the length of cable payed out is an indication of the depth to which the gun has been lowered. The depth may be checked by detecting and identifying a well casing collar located at a known depth and using that collar as a reference point for accurately positioning the 'netic casing collar locator is used or not.

gun. To detect such collars, as the apparatus passes them, the magnetic casing collar locator includes a magnetic circuit, including permanent magnets, arranged so that when they pass a collar, there is a change in the magnetic flux in the magnetic circuit, and the coil 81 is arranged in this path so that when this flux changes a very small voltage is induced in the coil. This voltage appears in the upper part of the lead 63, and is detected by suitable voltage responsive means connected to the lead, but not illustrated in the drawing. Such means may, for example, include an amplier and a recording or indicating voltmeter.

A characteristic of Ithe rectifiers 82 and 83, assumed here to be of the silicon type, is that even when a voltage is applied to such a rectiiier in the forward direction, if

this voltage is small enough the rectifier provides a relatively high resistance. The voltage generated by the magnetic casing collar locator is small enough'to result in this type action. It may thus be seen that these rectifiers 82 and 83 tend to isolate the lower part of the circuit from the eifects of the coil 81.

It is desirable that magnetic casing collar assemblies be capable of use, interchangeably with circuits of the type shown in FIGS. 3 and 4, on the one hand, or with other firing circuits, for example circuits which themselves omit the rectiiiers such as 71, 75 and 79. The latter type circuit would fire a plurality of charges at once. The rectiers 82 and 83 are useful in such circuits for making sure that the voltage generated by the coil 81 does not accidentally fire the charges.

It may be understood that the circuit of the present invention is highly useful irrespective of whether a mag- If it is to be omitted, it would be replaced by a `continuous connection of t-he upper part of the conductor 63 with the part of that conductor which in the drawings appears immediately below the assembly 80.

Also, with circuits of the types shown in FIGS. 3 and 4,

Ythe rectifiers in series with the detonators, such as 71, 75

and 79, although they readily permit the passage of large voltages in the forward direction, tend to isolate the detonators from voltages of the small magnitude generated by the magnetic casing collar locator coil 81, and for that reason, even if a magnetic casing collar locator is to be used, `the rectiiiers 82 and 83 can, in some embodiments, be omitted and replaced by a continuous portion of the main conductor 63.

It will be understood that the individual detonators which have been referred to, such as 78, 74 and 70 may each fire a single charge, or, in other arrangements, each detonator may be arranged to fire a group of charges.

Thus the apparatus is applicable to firing a plurality of charges, one by one, or in groups, in a single gun, and it is also applicable to iiring respective guns, one by one, each gun including a plurality of charges.

Although three detonator assemblies are shown in FIG. 3, two, three, four or more may be actuated by the present system. There have been described in these various assemblies, devices, here illustrated as rectiers, selectively responsive to voltage of one polarity, at least one of these devices being arranged to be responsive to voltage of one polarity and at least another of these devices being arranged to be responsive to voltage of the opposite polarity. In an illustrative arrangement, the apparatus may be regardedY as including two sets of rectiiiers, one set being responsive to positive voltage and one set to negative voltage, the rectiiers being spaced along the length of the well with the irst set alternating with the second set. By a switching arrangement, the polarity of the voltage applied to the rectiiiers is reversed in a step by step manner so as to initiate the corresponding detonators controlled by the rectiers in a corresponding step by step manner.

vIn FIG. 4 there is diagrammatically shown another circuit which is in many respects similar Vto that of FIG. 3, but includes additional features. Certain elements in FIG. 4 which correspond to those of FIG. 3 are designated in FIG. 4 by reference numerals higher by 100 than the corresponding numeral in FIG. 3. Thus, element 171 in FIG. 4 corresponds to element 71 in FIG. 3, etc.

The selective detonator assemblies 165, 167 and 169 in FIG. 4 include, respectively, in addition to the rectifiers 171, 175 and 179, other rectiers 193, 187 and 189, connected in parallel 'branch paths, as shown. These other rectiers like the ones mentioned previously, are preferably of the silicon type. Resistor 188 is connected in series with the rectifier 179 and its detonator 178. Resistors 186 and 192 are similarly connected with respect to their rectifiers 175 and 171, and to the associated detonators, respectively.

The rectiiier 189 is in a branch path connected in parallel with the rectier 179, its detonator 178 and its resistor 188. The rectifier 189 is turned in a direction opposite to that of the rectifier 179. The rectiiiers 187 and 193 are respectively similarly connected.

Thus the rectiers which are, at some time in the operation of the system, conductive in response to positive voltage applied to the conductor 163 are 179 and 171, and 187 and 182. Those responsive to negative voltage are 175, 189 and 193, and 183.

Whenever the Voltage applied to any of the rectiliers 1.87, 189 and 1,93 is in its forward direction, it provides in effect a low-resistance shunt path around whatever circuit elements are in parallel with it, including the associated detonator. This shunt path being of much lower resistance than the path including the detonator, is the preferred one for current How,

An advantage of the additional rectifiers 193, 187 and 189 is to limit the inverse voltage applied to their associated rectiiiers 171, 175 and 179. Thus, before the bottom gun is fired, when a negative voltage is applied through the conductor 163 to the detonator assembly 169, the rectifier 179 does not conduct, but the rectifier 189 does. The voltage across the branch circuit including rectifier 179, detonator 178 and` resistor 188 is limited to the forward voltage across therrectier 189 which is of the order of a volt or two under typical operating conditions. In a similar manner, after the bottom gun has been iired and the switch 176 has -been closed, and the switch 177 has been opened, the rectifier 187 protects the rectifier 175, when positive voltageis'applied through the main conductor.

With the protection described above, the system eliminates any tendency for inverse voltages to burn out the rectiiiers 171, 175 and 179,Y and to cause misfiring.

The resistors 192, 186 and 188 are advantageous in providing a readable diiference in the resistance of the branch circuits in` which they are located andthe branch circuits in parallel with them. This may be better eX- plained as follows: In the preferred method of operating the system, before any gun is fired, the system is tested to determine that the desired gun is properly armed. For this purpose the effective input resistance of the system, as seen through the upper end of the main conductor 163, is measured, making use of a very low applied D.C. voltage, this resistance being measured when the voltage is positive and also when it is negative. Thus, for this testing operation there is provided a source of known low D.C. voltage, a reversing switch for applying posi- -tive or negative voltage from that source to the main conductor 163, and a meter responsive to the current owing from that source into the main conductor 163. Except for that meter and the size ofthe voltage source, this circuit for the testing operation has the same arrangement as that shown in FIG. 4. The meter may conveniently be calibrated to read the resistance in ohms.

The voltage source used for this testing operation is smallrenough in magnitude that there is no danger, under any conditions, of its actuating any of the detonators. It is, on the other hand, large enough to pass in the forward direction through one of the rectiers in the magnetic casing collar locator 180.

When the bottom gun is properly armed, application of this low voltage to the system in a positive direction should produce an indication of relatively high resistance, because now conduction of the rectifier 179 and nonconduction of the rectifier 189 will cause the measurement to be iuiiuenced materially by resistor 188. r[his resistor like the resistors 186 and 192 may, for example, be about 100 ohms.

On the other hand when the low voltage is applied to the conductor 163 with negative polarity, the conduction will be through the rectier 189, and a low resistance will be measured.

Such measurements indicate that the bottom gun is properly armed.

The bottom gun is next tired, by applying a high positive voltage to the conductor 163, as previously described.

Next the system is checked to determine whether the next-to-bottom gun is properly armed. Now the application of a low negative voltage will result in a measurement of high resistance (through the rectifier 17S), and the application of a low positive voltage will result in the measurement of a low resistance (through the rectifier 187), provided the system is properly armed. Other types of indications show that something is wrong with the system. if, for example, the switch 176 did not close after the detonation of the bottom gun, as above, an extremely high resistance or open circuit condition will be measured for both positive and negative applied voltage. This is an indication that the system is not properly armed.

After the next-to-bottom gun is fired, the system is again checked, and if a low positive voltage gives an indication of relatively high resistance and the low negative voltage gives an indication of low resistance, this indicates that the top gun is properly armed. It is then tired with the use of the high positive voltage.

The optimum value of the resistors 188, 186 and 192 is the value which gives the maximum measurable difference in resistance during the above testing steps, for small positive and negative applied voltages, while still allowing enough current flow through the detonator to permit reliable detonation when the high voltage is applied.

It is apparent from the foregoing description that the present invention provides extremely useful methods, systems and apparatus for the selective firing of a plurality of remote explosive charges under a wide variety of conditions and that the invention is especially suitable for use in the perforation of cased wells since it provides a safe and reliable means for selectively firing any number of perforating guns in a well on a single run without subjecting the casing to prohibitive stresses.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the lfeatures shown and described or portions thereof, but it is recognized that various modifications are possible Within the scope of the invention claimed.

I claim:

l. ln apparatus for selectively firing charges in a Well, in combination, a plurality of branch circuits, a first device connected in one of said branch circuits and selectively responsive to voltage of only one polarity for firing one of said charges, a second device connected in another of said branch circuits and selectively responsive to voltage of only the opposite polarity for tiring another of said charges, means connected to each of said branch circuits for applying voltage -to said devices of the polarity 4to which said first polarity-selective device is selectively responsive, to actuate said first device to lire a iirst of said charges, and means for later reversing the polarity of said applied voltage, to actuate said second polarity-selective device -to iire another of said charges, said apparatus including circuit means in interconnected relationship 10 with saidl one branch circuit and responsive to the tiring of the lirst said charge for thereafter preventing current ilow in said one branch circuit.

2. In combination, a plurality of branch circuits, each of said circuits including a rectiiier and a current responsive detonator in series; means including a D.C. voltage source, a switch and a conductor in interconnecting relationship with each other, said conductor being connected to each of said branch circuits, for applying D.-C. voltage to said rectiers and for reversing its polarity, at least one of said rectiers being oriented to pass current readily in response to positive voltage received from said conductor and the remainder being oriented to pass current readily in response to negative voltage received from said conductor, and pressure sensitive means operatively associated with the rectiiier and the detonator in each of said branch circuits for preventing the actuation at any given moment of all but one of said detonators, whereby operation of said switch to alternate positions in steps actuates said detonators in corresponding steps.

3. In combination, a plurality of branch circuits, each of said circuits including a rectiiier and a current responsive detonator in series; means including a D.-C. voltage source, a switch and a conductor in interconnecting relationship with each other, said conductor beingconnected to each of said branch circuits, for applying D.-C. voltage to said rectifiers and for reversing its polarity, at least one of said rectiiiers being oriented to pass current readily in response to positive Voltage received from said conductor and the remainder being oriented to pass current readily in response to negative voltage received from said conductor, pressure sensitive means respectively connected to the said individual branch circuits for preventing the actuation at any given moment of all but one of said detonators, whereby operation of said switch to alternate positions in steps actuates said detonators in corresponding steps, and, connected in parallel with each of said branch circuits, another rectier orient-ed in a direction opposite to the one in its parallel branch circuit, for limiting the inverse voltage appearing across said lirst-mentioned rectiers.

4. Apparatus according to claim 3 including a resistance in each of said branch circuits, in series with the said rectifier and detonator therein.

5. ln apparatus yfor selectively tiring a plurality of charges in a well, in combination, a plurality of detonators, a source of unidirectional voltage, control means connected to said source, said control means being switchable to a first condition and to a second condition, for producing alternatively an output voltage having a first polarity or a second polan'ty, a first set of devices selectively responsive only to said output voltage having said lirst polarity for actuating individual ones of said detonators respectively, at least one other device selectively responsive only to said output voltage having said second polarity for actuating another of said detonators, said devices being spaced in said well with said last-mentioned device being interposed between the devices of said iirst set, means for normally preventing the actuation of more than one of said detonators at a time, and means for applying said output voltage to said devices, whereby actuation of said control means repeatedly to its said first and second conditions actuates said devices one at a time to cause said detonators one by one to ire said charges selectively.

6. Apparatus according to claim 5 in which said devices comprise rcctiiiers- 7. In apparatus for selectively firing a plurality of charges in a well, in combination, a plurality of voltagecontrolled actuating circuits spaced along said well, one for each of said charges, each of said actuating circuits including a device having a preferential direction of current liow, said devices being poled in opposite directions, alternately, in successive ones of said actuating circuits, means including a main conductor for applying D.-C.

1 1 voltage to said actuating circuits, a control for shifting the polarity of said applied D.-C. voltage, at least some of said actuating circuits including a pressure responsive switch connected thereto for normally maintaining its said actuating circuit in a disarmed condition to prevent the firing of its said charge until an adjacent charge has been tired, whereby the ring of said charges may be controlled by operation of said control to repeatedly reverse the polarity of said voltage applied to said actuating circuits.

8. Apparatus according to claim 7 including a magnetic casing collar locator comprising a coil connected to said main conductor for generating a voltage when said collar locator'V passes a casing collar, and a pair of parallel reversely connected rectiers connected in series with said conductor at a point below the junction of said coil with the same.

9. In apparatus for selectively tiring a plurality of charges spaced along the length of a well, in combination, a plurality of devices selectively responsive to voltage of one polarity, said devices comprising a rst group and a second group, each of said groups comprising at least one said device, said tirst group being selectively responsive to voltage of only a first polarity for firing individual ones of said charges respectively, and said second group being selectively responsive to voltage of only the opposite polarity for tiring other individual ones of said charges respectively, said devices being spaced along the length of said well, with the devices of said rst and second groups alternating, means for normally preventing the ring of all of said charges except the lowermost unred one, means for applying voltage to said devices of the polarity to which said rst group is selectively responsive, to actuate the lowermost one of said devices in said iirst group to fire its said charge, and means for repeatedly reversing the polarity of said applied voltage to actuate successively higher ones of said polarity-selective devices, to re successive ones of said charges.

10. A voltage selective detonator assembly comprising, in combination, a rectifier and an electrical blasting cap connected in a series circuit whereby said series circuit oiers a high resistance to the passage of direct current in response to voltage of one polarity, and offers a substantially lower resistance to the passage of direct current in response to voltage of the opposite polarity, so that said blasting cap may be actuated selectively by voltage of controlled polarity, and another rectifier connected in parallel with said series circuit and poled in a direction opposite to that of said first mentioned rectifier, said other rectifier serving to prevent inverse voltagesV from causing misiring of said blasting cap.

11. Apparatus according to claim 2 in which said pressure sensitive means includes a plurality of explosively actuated switch assemblies, respectively interposed between successive junctions of said branch circuits and said conductor, each comprising a pair of contacts in series with said conductor, a pair of contacts in series lwith an adjacent one of said branch circuits and an eX- plosively actuated movable member for opening said first pair of contacts and closing said second pair of contacts.

References Cited in the le of this patent UNITED STATES PATENTS 1,787,550 Sorensen Jan. 6, 1931 2,543,823 Barry Mar. 6, 1951 2,703,053 Castel Mar. 1, 1955 2,732,518 Bricaud Jan. 24, 1956

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