A perforating gun assembly for use in a subterranean well incorporates therein an electromagnetic frequency receiver coupled to a motor section which, in turn, is coupled to the mechanically actuatable firing head portion of the perforating gun. The assembly is lowered into a cased wellbore to a subterranean formation location, and a surface-disposed transmitter generates encoded electromagnetic waves through the earth to the receiver. Upon sensing in the received waves a predetermined frequency and embedded firing code, the receiver electrically operates the motor which, in turn, mechanically actuates the firing head to initiate the firing of the perforating gun. While the assembly is illustratively lowered into the wellbore on a tubing structure, a variety of non-tubing structures may be alternatively utilized to lower the gun assembly into the wellbore and operatively support it therein.

BACKGROUND OF THE INVENTION 
     The present invention generally relates to control of downhole well tools and, in a preferred embodiment thereof, more particularly relates to an electromagnetic telemetry actuated firing system for a well perforating gun. 
     In a typical construction of a subterranean well, a metal-cased wellbore is extended downwardly through the earth and through a fluid-bearing formation beneath the earth's surface. To operatively communicate the formation with the interior of the casing for subsequent delivery of formation fluid to the surface, perforations are formed through the casing and outwardly into the formation using a perforating gun structure which is lowered through the casing, typically on a tubing string, to the level of the subterranean formation. 
     A firing head portion of the lowered perforating gun structure is subsequently actuated to fire the gun and create the desired casing perforations. Perforating gun firing heads are customarily of either a mechanically actuatable or electrically actuatable construction. A mechanical firing head is typically actuated by pressure, or a mechanical device dropped down the tubing to depress a plunger portion of the firing head and thereby initiate firing of the gun. An electrical firing head is typically actuated by an electrical current supplied to a blasting cap attached to the head to detonate the gun charges. Evolving wellbore technologies and completion techniques have surpassed the ability of current tubing conveyed perforating firing systems to fire their guns by the use of pressure or mechanical means. Moreover, due of such evolving wellbore technologies, a variety of wells simply cannot be perforated using conventional techniques. 
     For the foregoing reasons it can readily be seen that a need exists for improved apparatus and methods for firing perforating guns that eliminate or at least substantially reduce the above-noted problems, limitations and disadvantages typically associated with conventional perforating gun firing apparatus and methods. 
     SUMMARY OF THE INVENTION 
     In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, a specially designed well tool assembly is provided for operative placement in a subterranean wellbore, the well tool assembly representatively being a remotely actuatable mechanical perforating gun assembly operable to form perforations in a metal casing portion of the wellbore. 
     The perforating gun assembly, when disposed downhole, is selectively operable by an electromagnetic telemetry actuated firing system that includes a surface-disposed transmitter operable to propagate electromagnetic waves through a portion of the earth exteriorly adjacent the wellbore casing. Preferably, the electromagnetic waves are modulated square sine or cosine waves having a frequency in the range of from about 15 HZ or less, and have a predetermined firing address encoded therein. 
     The perforating assembly illustratively includes a perforating gun having a mechanically actuatable firing head, an actuating section connected to the firing head and having a motor portion operable to mechanically actuate the firing head, and a receiver operable to detect the electromagnetic waves and responsively operate the motor. The perforating gun assembly may also have a sensor portion for sensing a selected downhole parameter, and a transmitter for propagating through the earth electromagnetic waves indicative of the value of the sensed downhole parameter. These waves may be detected by a suitable surface-disposed receiver. 
     While the well tool assembly is representatively a perforating gun assembly, other types of well tool assemblies may be utilized if desired and actuated using the electromagnetic telemetry actuating system of the present invention. 
     According to one aspect of the invention, the tool assembly receiver has a control circuitry portion, and the tool assembly has first and second electrically conductive paths which are insulatively isolated from one another and are respectively operative to transmit an electromagnetic wave signal from a first casing portion to the receiver control circuitry portion with respect to a ground reference from a second casing portion, spaced apart a substantial distance in a downhole portion from the first casing portion, to the control circuitry portion. The receiver control circuitry portion representatively has programmed therein a wave frequency value and a firing address which must be matched with the frequency and firing address of the detected electromagnetic before the circuitry is operative to fire the perforating gun. 
     Illustratively, the well tool assembly has an elongated, electrically conductive tubular outer body portion and a generally coaxially extending electrically conductive tubular inner body portion, each of the outer and inner body portions having insulative gaps formed therein between adjacent longitudinal sections thereof. Preferably, the adjacent longitudinal sections of the tubular outer body portion has axially spaced apart threaded end portions threadedly connected to an annular collar member at thread joints containing an electrically insulative material defining spaced apart insulation gaps between the longitudinal sections of the outer body portions and electrically isolating them from one another. 
     According to another feature of the invention, the receiver has a circuit board portion with a main CPU portion adapted to receive an electromagnetic wave detection signal and a ground reference and responsively generate an actuation request signal, and an auxiliary fail-safe CPU portion operative to receive the actuation request signal, monitor selected parameters of the well tool assembly to detect whether system errors exist, and responsively generate a final actuation signal, to actuate the tool portion of the assembly, only in the absence of sensed system errors. 
     The perforating gun assembly may be operatively supported in the wellbore on a variety of support structures including a tubing string, coil tubing, wire line, slick line or a casing hanger. The electromagnetic telemetry actuated firing system of the present invention provides a variety of advantages over conventional perforating gun firing systems. For example, the system is essentially wireless, with no downhole cabling required. 
     The motor section of the well tool may have an output member which is translatable in a selectively variable direction through a selectively adjustable stroke. Additionally, the overall well tool assembly may comprise a plurality of separately actuatable well tools which may be actuated in any desired sequence. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view through a portion of a subterranean well having disposed therein a perforating gun assembly with which is operatively associated a specially designed electromagnetic telemetry actuated firing system embodying principles of the present invention; 
     FIG. 2 is a schematic depiction of a preferred electromagnetic wave pattern transmitted through the earth to a receiver portion of the perforating gun assembly; 
     FIGS. 3A and 3B are enlarged scale schematic cross-sectional views, partly in elevation, through vertically successive portions of the overall perforating gun assembly; 
     FIG. 4 is a schematic block diagram of a portion of a dual processor circuit board used in an electromagnetic frequency receiver portion of the perforating gun assembly; 
     FIG. 5 is a schematic side elevational view of an alternate embodiment of the perforating gun assembly; and 
     FIG. 6 is a schematic side elevational view of a multiple perforating gun assembly. 
    
    
     DETAILED DESCRIPTION 
     Schematically depicted in cross-sectional form in FIG. 1 is a portion of a well  10  including a wellbore  12  extending downwardly from the surface  14  of the earth  16  through a subterranean hydrocarbon fluid-containing formation  18 . Wellbore  12  is lined with a tubular metal casing  20  which is cemented into the wellbore  12 , as at  22 , and is associated at its upper end with a wellhead portion  24  of a drilling rig  26  at the surface  14 . A tubing string  28  extends downwardly from the wellhead  24  centrally through the casing  20  and forms with the casing  20  an annulus  32  that circumscribes the tubing string  28 . 
     Supported on a lower end portion of the tubing string  28  is a well tool assembly that embodies principles of the present invention and is representatively a perforating gun assembly  34 . From top to bottom as viewed in FIG. 1, the perforating gun assembly  34  includes an electromagnetic frequency receiver  36 , an electrically operable motor control section  38 , a mechanically actuatable firing head  40 , and a perforating gun  42 , each of which has a generally tubular configuration. The firing head  40  and the perforation gun  42  together form an actuatable well tool. 
     The perforating gun assembly  34  is operatively positioned within the casing by lowering the assembly  34  through the casing  20  on the tubing string  28  until, as shown in FIG. 1, the perforating gun  42  is positioned in the subterranean formation  18 . An optional packer  44  is then set in the annulus  32  above the positioned assembly  34  to seal off a portion of the annulus  32  below the packer  44  from the portion of the annulus  32  above the packer  44 . 
     Still referring to FIG. 1, well  10  also includes a surface-disposed electromagnetic wave transmitter  46  having a positive electrical lead  48  connected to an upper end portion of the metal casing  20 , and a negative or grounding electrical lead  50  coupled to the earth  16 , representatively via a metal grounding stake  52 . When it is desired to fire the perforating gun  42 , the transmitter  46  is operated to transmit through the earth  16  electromagnetic waves  54  which are received by the receiver  36 . In a manner subsequently described in greater detail herein, in response to detecting the waves  54  the receiver  36  transmits an electrical firing signal to the electric motor control section  38 . Motor section  38 , in response to the receipt of the electrical firing signal from the receiver  36 , then mechanically actuates the mechanically actuatable firing head  40  which, in turn, fires the perforating gun  42  to create casing perforations  56  that extend outwardly through the casing  20  and the cement  22  and communicate the formation  18  with the interior of the casing  20 . 
     At this point it should be noted that the present invention permits a mechanically actuatable downhole well tool assembly (representatively the gun assembly  34 ) to be selectively actuated using electromagnetic waves transmitted through the earth. Accordingly, the portion of the tubing string  28  above the receiver is used only to lower and support the assembly  34 —this portion of the tubing  28  is not needed to receive and guide a dropped mechanical firing member to the firing head  40  to transmit a pressure signal to the firing head  40 , or to receive and guide a lowered electrical line to electrically actuate the firing head  40 . This feature of the invention permits the gun assembly  34  to be lowered through the casing  20 , and operatively supported therein, in a variety of other manners not utilizing a tubing string extending to the surface  14 . Examples of alternate lowering and support structures include, for example, wire line, slick line, coil tubing, drill pipe, or a casing hanger structure for supporting the lowered assembly. 
     AS previously mentioned, principles of the present invention are not limited to the illustrated perforating gun assembly  34 —such principles could also be advantageously employed with a variety of other types of actuatable downhole well tools. Also, while the illustrated perforating gun  42  is mechanically actuatable via its firing head  40  as later described herein, principles of the present invention could also be advantageously utilized in conjunction with electrically actuatable downhole well tools. 
     With reference now to FIGS. 1 and 2, the electromagnetic waves  54  propagated through the earth  16  by the transmitter  46  are preferably modulated square sine or cosine waves (see FIG. 2) of the QPSK (quadrature phase shift keying) pulse type which desirably increases the power of the waves and correspondingly increases the maximum earth depth through which they may be effectively transmitted. For purposes later described herein, a predetermined firing address A is suitably encoded in the electromagnetic waves  54  as schematically indicated in FIG.  2 . Preferably, the frequency of the electromagnetic waves  54  propagated through the earth  16  by the transmitter  46  is variable within the ULF/ELF frequency range of about 15 Hz or less. 
     Turning now to FIGS. 3A and 3B, the mechanically actuatable firing head  40  and the perforating gun  42  are of metal, electrically conductive constructions as are tubular outer metal body portions  58 , 60  of the assembly  36 . These body portions  58 , 60  are representatively defined by lower sections of the metal tubing string  28 . As illustrated in FIG. 3A, a lower end  61  of an upper section of the body portion  58  is upwardly spaced apart from the upper end  62  of a lower section of the body portion  58 . These spaced apart end portions  61  and  62  are externally threaded and are threaded into an internally threaded annular metal connection collar  64 . For purposes later described herein, a suitable electrically insulative material  66  is disposed in the mated thread areas of the collar  64  and the spaced apart body end portions  61 , 62  and serves to form dual insulating gaps  66 — 66  between the body end portions  61 , 62  and thereby prevent electrical conductance therebetween. 
     As schematically depicted in FIG. 3A, the specially designed receiver  36  has a cylindrical, electrically conductive interior portion centrally extending through the outer housing  58  and extending upwardly into the lower end of the tubing string  28 , such interior portion including an upper battery section  68  and a lower receiver control section  70  having a circuit board  72  operatively disposed within its interior. Sections  68 , 70  are electrically coupled by a connector structure  74  interposed therebetween. The upper end of the battery section  68  has secured thereto an electrically conductive centralizer structure  76  with flexible metal arm portions  78  that slidably engage an interior side surface of an outer body portion  58   a  horizontally facing a corresponding section  20   a  of the casing. 
     An upper end portion of the circuit board  72  is electrically coupled to an outer wall portion of the receiver control section  70  by an electrical lead  80 , and a lower end portion of the circuit board  72  is coupled to an electrical connector  82  by electrical leads  84  and  86 , lead  84  being a ground lead and lead  86  being a firing signal lead. Electrical leads  88 , 90  extend downwardly from the connector  82  through a central passage portion  92  of the receiver control section  70 , with leads  88 , 90  being respectively coupled to the leads  84 , 86  through the connector  82 . 
     Turning now to FIG. 3B, the motor control section  38  has a cylindrical, electrically conductive interior portion centrally extending through the outer housing  60  and extending upwardly into the lower end of the outer housing  58 , such interior portion including, from top to bottom as viewed in FIG. 3B, a battery section  94 , a motor control section  96  and an electric motor  98 . For purposes later described herein, a suitable electrically insulative material  100  is suitably interposed between adjacent end portions of the receiver control section  70  and the battery section  94  to form an insulating gap therebetween and preclude electrical conduction between these sections. 
     Electrical leads  88  and  90  are appropriately routed through the battery section  94 , through a central passage  102  therein, and coupled to a connector  104  disposed at a bottom end portion of the battery section  94 . The motor control section  96  has a circuit board  106  disposed therein. The upper end of the circuit board  106  has a ground lead  108  which, via the connector  104 , is coupled to the lead  88 . The upper end of the circuit board  106  also has an electrical lead  112  which is coupled to the electrical lead  90  via the connector  104 . At the bottom end of the circuit board  106  are motor control leads  114  and  116  operatively coupling the circuit board  106  to the electric motor  98 . A lower end portion of circuit board  106  is grounded to the housing of motor control section  96  via a suitable grounding path  113 . 
     As schematically depicted in FIG. 3B, the perforating gun  42  contacts a portion  20   b  of the casing which is in a downwardly spaced apart relationship with the casing portion  20   a  (see FIG. 3A) adjacent the outer body portion  58   a  conductively contacted by the centralizer arms  78 . Accordingly, during propagation through the earth  16  of the electromagnetic waves  54  by the transmitter  46  (see FIG. 1) the electrical potential at the upper casing section  20   a  is appreciably higher than at the lower casing section  20   b . The previously described dual insulating gaps  66 — 66  in the outer body portion  58  (see FIG. 3A) and the insulating gap  100  between the receiver control section  70  and the motor control section  96  advantageously permit the simultaneous communication to the receiver circuit board  72  of received, relatively high potential electromagnetic wave signals from the upper casing portion  20   a  with respect to a relatively low potential ground reference from the lower casing portion  20   b  through first and second conductive paths which are electrically isolated from one another. 
     When it is desired to fire the in-place perforating gun  42 , the transmitter  46  is activated to propagate the electromagnetic waves  54  through the earth  16 , with the waves  54  being propagated at a predetermined frequency, and with the preselected firing address A encoded therein, the frequency and encoded firing address matching a corresponding firing frequency and address pre-programmed into the electronic circuitry of the receiver circuit board  72 . Propagated electromagnetic wave signals received at the upper casing section  20   a  (see FIG. 3A) are transmitted across the casing annulus  32  to the outer body portion  58   a  and from the outer body portion  58   a  to the receiver circuit board  72 , sequentially via the centralizer  76 , outer wall portions of the battery and control sections  68  and  70 , and the lead  80 , in the form of a wave input signal  118  (see FIG.  4 ). If desired, a second electrically conductive resilient centralizer (not shown) may be placed between and in conductive contact with the casing section  20   a  and the outer body portion  58   a  to facilitate the transmission of electromagnetic wave signals therebetween. 
     While the electromagnetic waves  54  are being propagated through the earth  16 , the lower casing section  20   b  (see FIG. 3B) is at an appreciably lower electrical potential than the electrical potential of the upper casing section  20   a  (see FIG. 3A) from which the lower casing section  20   b  is conductively isolated by the dual insulation gaps  66 — 66  (see FIG.  3 A). This lower (or “ground”) potential of the lower casing section  20   b  is connected to the receiver circuit board  72  (see FIG. 4) as a ground reference  120  (through a conductive path isolated from the conductive path through which the wave input signal  118  reaches the circuit board  72 ) sequentially via the perforating gun  42  (see FIG.  3 B), the firing head  40 , body portions of the motor  98 , the outer housing of the motor control section  96 , the grounding path  113 , the motor control circuit board  106 , the lead  108 , the connector  104 , the lead  88 , the connector  82  (see FIG.  3 A), and the lead  84 . 
     As schematically shown in FIG. 4, the receiver circuit board  72 , according to a feature of the present invention, is preferably provided with a main CPU portion  122 , which receives the wave input and ground signals  118  and  120 , and an auxiliary fail-safe CPU portion  124 . If the wave input signal  118  has a frequency and encoded firing address respectively matching the corresponding frequency and firing address programmed into the main CPU  122 , the main CPU  122  transmits a firing request signal  126  to the auxiliary fail-safe CPU  124  which verifies the absence of various preselected malfunctions in the overall firing system before responsively transmitting a final electrical firing signal  128  to the motor controller section  96  (see FIG.  3 B). 
     For example, before outputting the final firing signal  128 , the auxiliary fail-safe CPU  124  verifies (via power inputs  130 , 132 , 134  thereto) that the various voltages in the overall receiver circuitry are at correct levels, and (via reset signals  136 , 138  transmitted between the two CPU's  122 , 124 ) that no defects are present in the various system reset functions. If a system parameter error is detected by the auxiliary fail-safe CPU  124  it will not generate the final firing signal  128 , even if the main CPU  122  generates the firing request signal  126 . 
     If the final firing signal  128  is generated by the auxiliary fail-safe CPU  124 , the signal  128  is delivered to the motor  98  (see FIG. 3B) sequentially via the lead  86  (see FIG.  3 A), the connector  82 , the lead  90 , the connector  104  (see FIG.  3 B), the lead  112 , the motor controller circuit board  106 , and the leads  114  and  116 . Receipt of the final firing signal  128  by the motor  98  causes the motor  98  to upwardly extend a movable rod portion  140  of the motor, as indicated by the arrow  142  in FIG. 3B, in a manner causing the rod  140  to disengage and release an underlying plunger portion  144  of the firing head  40 , at the same time allowing wellbore pressure to drive the plunger. This mechanically actuates the firing head  40  which, in turn and in a conventional manner, fires the perforating gun  42 . The motor  98  may be operative to translate the rod  140  in selectively variable directions through a selectively adjustable stroke if desired. 
     A variety of modifications can be made to the representatively illustrated perforating gun assembly  34  (see FIG.  1 ), if desired, without departing from general principles of the present invention. For example, the receiver  36 , motor control  38  and firing head  40  could be positioned on the bottom end of the perforating gun  42  instead of its top end as schematically depicted in FIG.  1 . Further, one or more additional perforating gun assemblies  34  could be utilized within the casing  20  instead of the single perforating gun assembly  34  illustratively shown in FIG.  1 . Additionally, the specially designed perforating gun assembly  34  could also be advantageously utilized in conjunction with the transmitter in a subsea well application. 
     While the depicted perforating gun assembly  34  is representatively designed to operate on a “receive only” basis, it can be easily modified to additionally transmit selected data to the surface if desired. For example, an alternate embodiment  34   a  of the previously described perforating gun assembly  34  is schematically shown in FIG.  5 . For ease in comparing the assembly embodiment  34   a  to the previously described assembly embodiment  34 , elements in the assembly embodiment  34   a  similar to those in the assembly embodiment  34  have been given the same reference numerals to which the suffixes “a” have been appended. 
     In the alternate perforating gun assembly embodiment  34   a , an electromagnetic frequency transmitter  146  is added to the assembly  34   a , representatively between the receiver  36   a  and the motor section  38   a , and is associated with a suitable sensor  148  operative to sense a predetermined downhole parameter, such as temperature or pressure. The transmitter  146  may be utilized to propagate electromagnetic waves  150  through the earth  16  to a suitable surface receiver  152 , the waves  150  having suitable characteristics imparted thereto which are indicative of the sensed downhole parameter. 
     While a single well tool assembly  34  (representatively a perforating gun assembly) has been illustratively depicted as being operatively positioned within the wellbore  12  (see FIG.  1 ), a plurality of well tool assemblies, such as the well tool assemblies  34 ′ and  34 ″ schematically depicted in FIG. 6, may alternatively be supported in the wellbore  12  on, for example, the tubing  28 . These well tool assemblies  34 ′ and  34 ″ may be sequentially actuated, in any predetermined order, in response to their receipt of actuating signals  128 ′, 128 ″ generated by their receiver sections in response to their detections of corresponding electromagnetic waves being propagated through the earth by the transmitter  46 . The electromagnetic waves that create these actuating signals  128 ′, 128 ″ have different actuating addresses encoded therein, and may also have different frequencies. 
     The electromagnetic telemetry actuated firing system representatively described above provides a variety of advantages over conventional perforating gun firing systems. For example, the system is essentially wireless, with no downhole cabling required. 
     The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.