Patent Publication Number: US-6662875-B2

Title: Induction choke for power distribution in piping structure

Description:
This application claims the benefit of U.S. Provisional Application No. 60/178,000, filed Jan. 24, 2000, the entire disclosure of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the use of at least one unpowered induction choke to form an electrical circuit in a piping structure. In one aspect, it relates to providing power and/or communications to a device downhole in a borehole of a well using an electrical circuit formed in a piping structure by using at least one unpowered induction choke. 
     CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of the U.S. Provisional Applications in the following table, all of which are hereby incorporated by reference: 
     
       
         
           
               
            
               
                   
               
               
                 U.S. PROVISIONAL APPLICATIONS 
               
            
           
           
               
               
               
               
            
               
                   
                 Ser. 
                   
                   
               
               
                 T&amp;K # 
                 No. 
                 Title 
                 Filing Date 
               
               
                   
               
               
                 TH 1599 
                 60/177,999 
                 Toroidal Choke Inductor for 
                 Jan. 24, 2000 
               
               
                   
                   
                 Wireless Communication and 
               
               
                   
                   
                 Control 
               
               
                 TH 1599x 
                 60/186,376 
                 Toroidal Choke Inductor for 
                 Mar. 2, 2000 
               
               
                   
                   
                 Wireless Communication and 
               
               
                   
                   
                 Control 
               
               
                 TH 1600 
                 60/178,000 
                 Ferromagnetic Choke in 
                 Jan. 24, 2000 
               
               
                   
                   
                 Wellhead 
               
               
                 TH 1600x 
                 60/186,380 
                 Ferromagnetic Choke in 
                 Mar. 2, 2000 
               
               
                   
                   
                 Wellhead 
               
               
                 TH 1601 
                 60/186,505 
                 Reservoir Production Control 
                 Mar. 2, 2000 
               
               
                   
                   
                 from Intelligent Well Data 
               
               
                 TH 1602 
                 60/178,001 
                 Controllable Gas-Lift Well 
                 Jan. 24, 2000 
               
               
                   
                   
                 and Valve 
               
               
                 TH 1603 
                 60/177,883 
                 Permanent, Downhole, 
                 Jan. 24, 2000 
               
               
                   
                   
                 Wireless, Two-Way 
               
               
                   
                   
                 Telemetry Backbone 
               
               
                   
                   
                 Using Redundant 
               
               
                   
                   
                 Repeater, Spread Spectrum 
               
               
                   
                   
                 Arrays 
               
               
                 TH 1668 
                 60/177,998 
                 Petroleum Well Having 
                 Jan. 24 2000 
               
               
                   
                   
                 Downhole Sensors, 
               
               
                   
                   
                 Communication, and Power 
               
               
                 TH 1669 
                 60/177,997 
                 System and Method for Fluid 
                 Jan. 24, 2000 
               
               
                   
                   
                 Flow Optimization 
               
               
                 TS 6185 
                 60/181,322 
                 Optimal Predistortion in 
                 Feb. 9, 2000 
               
               
                   
                   
                 Downhole Communications 
               
               
                   
                   
                 System 
               
               
                 TH 1671 
                 60/186,504 
                 Tracer Injection in a 
                 Mar. 2, 2000 
               
               
                   
                   
                 Production Well 
               
               
                 TH 1672 
                 60/186,379 
                 Oilwell Casing Electrical 
                 Mar. 2, 2000 
               
               
                   
                   
                 Power Pick-Off Points 
               
               
                 TH 1673 
                 60/186,394 
                 Controllable Production Well 
                 Mar. 2, 2000 
               
               
                   
                   
                 Packer 
               
               
                 TH 1674 
                 60/186,382 
                 Use of Downhole High 
                 Mar. 2, 2000 
               
               
                   
                   
                 Pressure Gas in a 
               
               
                   
                   
                 Gas Lift Well 
               
               
                 TH 1675 
                 60/186,503 
                 Wireless Smart Well Casing 
                 Mar. 2, 2000 
               
               
                 TH 1677 
                 60/186,527 
                 Method for Downhole Power 
                 Mar. 2, 2000 
               
               
                   
                   
                 Management Using 
               
               
                   
                   
                 Energization from 
               
               
                   
                   
                 Distributed Batteries or 
               
               
                   
                   
                 Capacitors with 
               
               
                   
                   
                 Reconfigurable Discharge 
               
               
                 TH 1679 
                 60/186,393 
                 Wireless Downhole Well 
                 Mar. 2, 2000 
               
               
                   
                   
                 Interval Inflow and Injection 
               
               
                   
                   
                 Control 
               
               
                 TH 1681 
                 60/186,394 
                 Focused Through-Casing 
                 Mar. 2, 2000 
               
               
                   
                   
                 Resistivity Measurement 
               
               
                 TH 1704 
                 60/186,531 
                 Downhole Rotary Hydraulic 
                 Mar. 2, 2000 
               
               
                   
                   
                 Pressure for Valve Actuation 
               
               
                 TH 1705 
                 60/186,377 
                 Wireless Downhole 
                 Mar. 2, 2000 
               
               
                   
                   
                 Measurement and Control For 
               
               
                   
                   
                 Optimizing Gas Lift Well 
               
               
                   
                   
                 and Field Performance 
               
               
                 TH 1722 
                 60/186,381 
                 Controlled Downhole 
                 Mar. 2, 2000 
               
               
                   
                   
                 Chemical Injection 
               
               
                 TH 1723 
                 60/186,378 
                 Wireless Power and 
                 Mar. 2, 2000 
               
               
                   
                   
                 Communications Cross-Bar 
               
               
                   
                   
                 Switch 
               
               
                   
               
            
           
         
       
     
     The current application shares some specification and figures with the following commonly owned and concurrently filed applications in the following table, all of which are hereby incorporated by reference: 
     
       
         
           
               
            
               
                   
               
               
                 COMMONLY OWNED AND CONCURRENTLY 
               
               
                 FILED U.S. PATENT APPLICATIONS 
               
            
           
           
               
               
               
               
            
               
                   
                 Ser. 
                   
                   
               
               
                 T&amp;K # 
                 No. 
                 Title 
                 Filing Date 
               
               
                   
               
               
                 TH 1599US 
                 09/769,047 
                 Toroidal Choke Inductor for 
                 Jan. 24, 2001 
               
               
                   
                   
                 Wireless Communications and 
               
               
                   
                   
                 Control 
               
               
                 TH 1602US 
                 09/768,705 
                 Controllable Gas-Lift Well 
                 Jan. 24, 2001 
               
               
                   
                   
                 and Valve 
               
               
                 TH 1603US 
                 09/768,655 
                 Permanent, Downhole, 
                 Jan. 24, 2001 
               
               
                   
                   
                 Wireless, Two-Way 
               
               
                   
                   
                 Telemetry Backbone Using 
               
               
                   
                   
                 Redundant Repeaters 
               
               
                 TH 1668US 
                 09/769,046 
                 Petroleum Well Having 
                 Jan. 24, 2001 
               
               
                   
                   
                 Downhole Sensors, 
               
               
                   
                   
                 Communication, and Power 
               
               
                 TH 1669US 
                 09/768,656 
                 System and Method for Fluid 
                 Jan. 24, 2001 
               
               
                   
                   
                 Flow Optimization 
               
               
                   
               
            
           
         
       
     
     BACKGROUND OF THE INVENTION 
     Several methods have been devised to place controllable valves and other devices and sensors downhole on a tubing string in a well, but all such known devices typically use an electrical cable along the tubing string to power and communicate with the devices and sensors. It is undesirable and in practice difficult to use a cable along the tubing string either integral with the tubing string or spaced in the annulus between the tubing and the casing because of the number of failure mechanisms are present in such a system. Other methods of communicating within a borehole are described in U.S. Pat. Nos. 5,493,288; 5,576,703; 5,574,374; 5,467,083; and 5,130,706. 
     U.S. Pat. No. 6,070,608 describes a surface controlled gas lift valve for use in oil wells. Methods of actuating the valve include electro-hydraulic, hydraulic, and pneumo-hydraulic. Sensors relay the position of the variable orifice and critical fluid pressures to a panel on the surface. However, when describing how electricity is provided to the downhole sensors and valves, the means of getting the electric power/signal to the valves/sensors is described as an electrical conduit that connects between the valve/sensor downhole and a control panel at the surface. U.S. Pat. No. 6,070,608 does not specifically describe or show the current path from the device downhole to the surface. The electrical conduit is shown in the figures as a standard electrical conduit, i.e., an extended pipe with individual wires protected therein, such that the pipe provides physical protection and the wires therein provide the current path. But such standard electrical conduits can be difficult to route at great depths, around turns for deviated wells, along multiple branches for a well having multiple lateral branches, and/or in parallel with coiled production tubing. Hence, there is a need for a system and method of providing power and communications signals to downhole devices without the need for a separate electrical conduit filled with wires and strung along side of production tubing. 
     U.S. Pat. No. 4,839,644 describes a method and system for wireless two-way communications in a cased borehole having a tubing string. However, this system describes a downhole toroid antenna for coupling electromagnetic energy in a waveguide TEM mode using the annulus between the casing and the tubing. This toroid antenna uses an electromagnetic wave coupling that requires a substantially nonconductive fluid (such as refined, heavy oil) in the annulus between the casing and the tubing as a transmission medium, as well as a toroidal cavity and wellhead insulators. Therefore, the method and system described in U.S. Pat. No. 4,839,644 is expensive, has problems with brine leakage into the casing, and is difficult to use for downhole two-way communication. Thus, a need exists for a better system and method of providing power and communications signals to downhole devices without the need for a nonconductive fluid to be present in the annulus between the casing and tubing. 
     Other downhole communication concepts, such as mud pulse telemetry (U.S. Pat. Nos. 4,648,471 and 5,887,657), have shown successful communication at low data rates but are of limited usefulness as a communication scheme where high data rates are required or it is undesirable to have complex, mud pulse telemetry equipment downhole. Still other downhole communication methods have been attempted, see U.S. Pat. Nos. 5,467,083; 4,739,325; 4,578,675; 5,883,516; and 4,468,665. Hence, there is a need for a system and method of providing power and communications signals to downhole devices at higher data rates and with available power to operate a downhole device. 
     It would, therefore, be a significant advance in the operation of petroleum wells if tubing, casing, liners, and/or other conductors installed in wells could be used for the communication and power conductors to control and operate devices and sensors downhole in a petroleum well. 
     Induction chokes have been used in connection with sensitive instrumentation to protect against surges and stray voltage. For example, most personal computers have some sort of choke incorporated into its AC power cord for such protection. Such protection chokes work well for their intended purpose, but do not operate to define a power or communication circuit. 
     All references cited herein are incorporated by reference to the maximum extent allowable by law. To the extent a reference may not be fully incorporated herein it is incorporated by reference for background purposes, and indicative of the knowledge of one of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The problems and needs outlined above are largely solved and met by the present invention. In accordance with a first aspect of the present invention, a current impedance device for routing a time-varying electrical current in a piping structure is provided. The current impedance device comprises an induction choke that is generally concentric about a portion of the piping structure, such that during operation a voltage potential forms between the piping structure and an electrical return when the time-varying electrical current is transmitted through and along the portion of the piping structure, and such that during operation part of the current can be routed through a device electrically connected to the piping structure due to the voltage potential formed. The induction choke may be unpowered and may comprise a ferromagnetic material. The induction choke can be generally cylindrical shaped with a generally cylindrical shaped borehole formed therethrough. The choke may be enclosed within an insulating shell. 
     In accordance with a second aspect of the present invention, a system for defining an electrical circuit is provided. The system comprises a piping structure, a source of time-varying current, an induction choke, a device, and an electrical return. The piping structure comprises a first location, a second location, and an electrically conductive portion extending between the first and second locations. The first and second locations are distally spaced along the piping structure. The source of time-varying current is electrically connected to the electrically conductive portion of the piping structure at a location along the first location. The induction choke is located about part of the electrically conductive portion of the piping structure. The device comprises two terminals. A first of the device terminals is electrically connected to the electrically conductive portion of the piping structure. The electrical return electrically connects between a second of the device terminals and the source to complete the electrical circuit. When applying the system in a petroleum well for example, the first location is near the surface and the second location is downhole in a borehole of the well. 
     In an embodiment of the system in accordance with the second aspect of the present invention, the choke can be located along the second location, and the electrical connection location for the first device terminal can be between the choke and the electrical connection location for the source. 
     Another embodiment of the system in accordance with the second aspect can further comprise a second induction choke located about a portion of the piping structure along the first location, such that the source is connected to the piping structure between the chokes. Yet another embodiment of the system further comprises an electric power transformer located about a portion of the piping structure between the electrical connection location for the source and the second choke. 
     Still another embodiment of the system in accordance with the second aspect can further comprise an electric power transformer located about a portion of the piping structure between the electrical connection location for the source and the choke. The electric power transformer may comprise a ferromagnetic toroid wound by wire such that the wire is generally parallel to a central axis of the toroid when wound about the toroid. 
     In a further embodiment of the system in accordance with the second aspect of the present invention, the choke is located along the first location, the electrical connection location for the first device terminal is along the second location, and the electrical connection location for the source is between the choke and the electrical connection location for the first device terminal. A still further embodiment can further comprise a second induction choke located about a portion of the piping structure along the second location, such that the electrical connection location for the source is between the chokes, and such that the electrical connection location for the first device terminal is between the second choke and the electrical connection location for the source. 
     Another embodiment of the system in accordance with the second aspect further comprises an electric power transformer located about a portion of the piping structure, such that the electrical connection location for the source is between the choke and the transformer. 
     In accordance with a third aspect of the present invention, a system for providing power or communications to a remote device is provided. The system comprises a piping structure, an induction choke, an electric power transformer, a source of time-varying current, a device, and an electrical return. The piping structure comprises a first location, a second location, and an electrically conductive portion extending between the first and second locations. The first and second locations are distally spaced along the piping structure. The induction choke is located about a portion of the piping structure. The source of time-varying current is electrically connected to the electrically conductive portion of the piping structure for supplying primary electrical current. The transformer is located about a portion of the piping structure and adapted to form a secondary coil for supplying secondary electrical current corresponding to the primary electrical current when the primary electrical current is flowing in the electrically conductive portion of the piping structure, wherein the electrically conductive portion of the piping structure acts as a primary coil. The electrical return electrically connects between the electrically conductive portion of the piping structure and the source to complete an electrical circuit, such that the transformer is located between the electrical connection location for the source and the electrical connection location for the electrical return along the piping structure. The device is electrically connected to the transformer for receiving the secondary electrical current. Hence, when the system is operable, the device can receive power, and/or send or receive communication signals, via the transformer and the electrical circuit formed. 
     In an embodiment of the system in accordance with a third aspect of the present invention, the choke can be located along the first location, and the electrical connection location for the source can be located between the choke and the transformer. Also, the transformer can be located along the first location or the second location. The embodiment can further comprise a second induction choke located about a portion of the piping structure along the second location, such that the transformer is located between the chokes. In another embodiment of the system in accordance with the third aspect, the choke can be located along the second location, and the electrical connection location for the source can be located along the first location. The electrical connection location for the electrical return can be located between the transformer and the choke, or the choke can be located between the transformer and the electrical connection location for the electrical return. The embodiment can further comprise an electrical insulator along the first location, such that the electrical connection location for the source is between the insulator and the transformer. The insulator can comprise an insulated hanger. 
     In accordance with a fourth aspect of the present invention, a petroleum well for producing petroleum products is provided. The petroleum well comprises a piping structure and an electrical circuit. The piping structure comprises a first location, a second location, and an electrically conductive portion extending between the first and second locations. The first and second locations are distally spaced along the piping structure. The electrical circuit comprises the electrically conductive portion of the piping structure, a source of time-varying current, an induction choke, a device, and an electrical return. The source of time-varying current is electrically connected to the electrically conductive portion of the piping structure at a location along the first location. The induction choke is located about part of the electrically conductive portion of the piping structure. The device comprises two terminals, a first of the device terminals being electrically connected to the electrically conductive portion of the piping structure. The electrical return electrically connects between a second of the device terminals and the source to complete the electrical circuit. 
     The piping structure can comprise at least a portion of a production tubing string, at least a portion of a pumping rod, at least a portion of a well casing, at least a portion of at least one branch forming a lateral extension of a well, at least a portion of an oil refinery piping network, at least a portion of above surface refinery production pipes, or any combination thereof. The electrical return can comprise at least a portion of a well casing, at least a portion of an earthen ground, a conductive fluid, a packer, at least a portion of another piping structure of a same well, at least a portion of another piping structure of another well, or any combination thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon referencing the accompanying drawings, in which: 
     FIG. 1 is a schematic of a petroleum well illustrating the general disposition of the major elements of the present invention in relation to the major elements of a conventional well; 
     FIG. 2 is related to FIG.  1  and shows in more detail a petroleum well incorporating electrical chokes and associated communication, measurement and control equipment in accordance with the methods of the present invention; 
     FIG. 3 is related to FIG. 2, and shows the electrical equivalent circuit of that well; 
     FIG. 4 a  is related to FIG. 2, and shows the overall assembly of one of the chokes of FIG. 1; 
     FIG. 4 b  is related to FIG. 4 a,  and shows in detail the components used in the construction of the choke assembly of FIG. 4 a;    
     FIG. 5 a  is a longitudinal cross-section in partial section of a choke showing variables used in the design analysis of a choke disposed between tubing and casing; 
     FIG. 5 b  is a radial cross-section of a choke showing variables used in the design analysis of a choke disposed between tubing and casing; 
     FIG. 5 c  is a longitudinal cross-section in partial section of a choke showing variables used in the design analysis of a choke external to both tubing and casing; 
     FIG. 5 d  is a radial cross-section of a choke showing variables used in the design analysis of a choke external to both tubing and casing; 
     FIG. 6 is a schematic showing a possible application of the first embodiment of FIG. 1; 
     FIG. 7 is a schematic illustrating a method for unloading a gas lift well using the embodiment of FIG. 2; 
     FIG. 8 is a schematic showing a disposition of chokes and downhole modules providing an electrical parallel configuration; 
     FIG. 9 a  is a simplified electrical equivalent circuit representation of the embodiment of FIG. 8; 
     FIG. 9 b  is a simplified electrical equivalent circuit representation of the embodiment of FIG. 9 a  but without using inductive chokes, for comparison; 
     FIG. 10 is a schematic showing a system with an embodiment of the present invention using a current transformer; 
     FIG. 11 is related to FIG.  10  and shows in more detail the current transformer of that embodiment; and 
     FIG. 12 is a schematic showing a system with an embodiment of the present invention using chokes external to the casing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein like reference numbers are used to designate like elements throughout the various views, several embodiments of the present invention are further described. The figures are not necessarily drawn to scale, and in some instances the drawings have been exaggerated or simplified for illustrative purposes only. One of ordinary skill in the art will appreciate the many possible applications and variations of the present invention based on the following examples of possible embodiments of the present invention and cited patents and articles incorporated by reference. 
     The terms “first end” and “second end” as used herein are defined generally to call out a side or portion of a piping structure, which may or may not encompass the most proximate locations, as well as intermediate locations along a called out side or portion of the piping structure. Similarly, in accordance with conventional terminology of oilfield practice, the descriptors “upper”, “lower”, “uphole” and “downhole” refer to distance along the borehole from the surface, which in deviated wells may or may not accord with relative vertical placement measured with reference to the ground surface. 
     Also, the term “wireless” as used in this application means the absence of a conventional, insulated wire conductor e.g. extending from a downhole device to the surface. Using the tubing and/or casing as a conductor is considered “wireless.” 
     Also, the term “modem” as used herein is not limited to conventional computer modems that convert digital signals to analog signals and vice versa (e.g., to send digital data signals over the analog Public Switched Telephone Network). For example, if a sensor puts out measurements in an analog format, then such measurements may only need to be used to modulate a carrier frequency and be transmitted—hence no analog/digital conversion needed. As another example, a relay/slave modem or communication device may only need to identify, filter, amplify, and/or retransmit a signal received. 
     As used in the present application, a “valve” is any device that functions to regulate the flow of a fluid. Examples of valves include, but are not limited to, bellows-type gas-lift valves and controllable gas-lift valves, each of which may be used to regulate the flow of lift gas into a tubing string of a well. The internal workings of valves can vary greatly, and in the present application, it is not intended to limit the valves described to any particular configuration, so long as the valve functions to regulate flow. Some of the various types of flow regulating mechanisms include, but are not limited to, ball valve configurations, needle valve configurations, gate valve configurations, and cage valve configurations. The methods of installation for valves discussed in the present application can vary widely. Valves can be mounted downhole in a well in many different ways, some of which include tubing conveyed mounting configurations, side-pocket mandrel configurations, or permanent mounting configurations such as mounting the valve in an enlarged tubing pod. 
     The term “sensor” as used herein refers to any device that detects, determines, monitors, records, or otherwise senses the absolute value of or a change in a physical quantity. Sensors as described in the present application can be used to measure temperature, pressure (both absolute and differential), flow rate, seismic data, acoustic data, pH level, salinity levels, valve positions, or almost any other physical data. 
     In the first embodiment shown in FIG. 1, the piping structure comprises a production tubing string  34  for a well, which is typically steel tubing. The system has an electrical impeding device  146  located about the tubing  34  along a first end  41  near the surface. Device  146  may consist of an electrically insulating joint as shown in FIG. 1, or an unpowered choke of the present invention. A lower choke  32  is located about the tubing along a second end  42  downhole within the well. The source of time-varying current  38  is electrically connected to the tubing  34  between the impeding devices  146 ,  32 . The time-varying current can be alternating current (AC) or a varying direct current (DC), but AC is typically more practical in use. AC power and communications signals from the source  38  are connected to the tubing  34  via an insulating feedthrough  76 . The device  40  comprises two terminals  51 ,  52 . A device terminal is defined generally as an electrical connection point for a device, which may include but is not limited to: a wire, a device enclosure, a prong, a pin, a contact pad, a solder point, a female receptacle, a shaft, or any combination thereof. A first device terminal  51  is electrically connected to the tubing  34  downhole between the connection location for the source of current  38  and the lower choke  32 . 
     A second device terminal  52  is also electrically connected to the tubing  34 , but at a location on an opposite side of the lower choke  32  relative to the electrical connection location for the first device terminal  51 . As described further below with equations, a voltage potential exists across the choke  32  when a time-varying current flows through the tubing. Hence, the device  40  is electrically connected across the voltage potential on the tubing developed by the choke  32  when AC flows in the tubing  34 , which provides current flow through the device  40 . 
     Device  146  may consist of an electrically insulating joint hanger, or a choke in accordance with the present invention. While electrically insulating joint hangers provide true electrical isolation, they must sustain significant mechanical loads on insulating materials such as plastics or ceramics, and are therefore subject to damage from those loads. Chokes cannot provide complete isolation, but are able to sustain high mechanical loads since they are constructed such that all the load-bearing elements are composed of metal. 
     At least a portion of the well casing  36  is electrically conductive. The electrically conductive portion of the well casing  36  is electrically connected to the tubing  34  (e.g., via conductive fluid  82  and/or packer  56 ) and the source of current  38 . Hence, the electrically conductive portion of the well casing  36  acts as part of an electrical return to complete the electrical circuit. 
     Where centralizers are used to control the position of the tubing  34  relative to the casing  36 , such centralizers which are disposed between devices  146  and  32  must not be electrically conductive. Suitable centralizers are typically composed of molded or machined plastic. 
     Therefore, the electrical circuit is formed by the system of the first embodiment, wherein the time-varying current (e.g., AC) can flow from the power source  38  to the tubing  34 , along the tubing  34  between the device  146  and the choke  32 , through the device  40  to the tubing  34  below the lower choke  32 , to the casing  36  via the packer  56  and/or the conductive fluid  82 , and along the well casing  36  to the source  38  to complete the electrical circuit. Thus, the downhole device  40  can receive power, as well as send/receive communication signals, using the tubing  34  between the upper and lower devices  146 ,  32  as one of the primary conductors and as a power and/or communications path. 
     In the application of the first embodiment shown in FIG. 1, the gas-lift oil well extends from the surface  64  through a borehole and extends into a production zone  66  downhole. A production platform  68  is schematically illustrated above the surface  64 . A hanger  54  supports the production tubing string  34  from the well casing  36 . The casing  36  is conventional, i.e., it is typically metal tubing held in place by injecting cement  70  between the casing and the earth in the borehole during well completion. Similarly the tubing string  34  is generally conventional comprising a plurality of elongated tubular metal production pipe sections joined by threaded couplings (not shown) at each end of each tubing section. A gas input throttle  74  is employed to permit the input of compressed gas into the tubing  34  via one or more valves contained within pod  40  for lifting oil during production. Schematically illustrated is a computer system and power source  38  at the surface  64  with power and communication feeds  44  passing through electrically isolating pressure seal  76  and using return connection  78 , which is electrically connected to the casing  36 . The degree of opening of a gas lift valve may be controlled by means of setpoint commands sent by communication from the surface modem to the downhole modem and interpreted by a downhole control interface for the motor of the gas lift valve. Sensor readings from the downhole pod may either be processed locally within the pod to provide autonomous control, or the sensor readings may be conveyed to the surface by means of the communications between the downhole and surface modems, for analysis at the surface. 
     The choke  32  is unpowered and made from a material having a high magnetic permeability (e.g., a relative permeability of 1000 to 150,000), such as a ferromagnetic metal alloy or a ferrite. The choke  32  is electrically insulated from the tubing  34  and acts to create a reactive impedance to AC flow in the tubing. In the case where the upper device  146  is a choke (rather than an electrically insulating joint), its action and construction is essentially the same as the lower choke  32 . The choke  32  (and  146  in the case where it is a choke) are mounted concentric and external to the tubing  34  and are typically coated with shrink-wrap plastic to provide electrical insulation, and may additionally be enclosed within with an epoxy shell (not shown) to withstand rough handling and corrosive conditions. As described in the mathematical analysis below, the size and material of chokes can be chosen to achieve a desired series impedance value. 
     FIG. 2 illustrates in greater detail the preferred embodiment of the invention outlined in FIG. 1 as it is applied to a gas-lift oil well. FIG. 2 illustrates such a well consisting of casing  36  extending from the surface and containing production tubing  34 . At the well head the upper portion of the production tubing is electrically isolated from the lower portion by means of an electrically insulating joint hanger  146 . At depth within the well the annular space between casing  36  and tubing  34  contains completion fluid  82 , and an electrically conductive packer  56  which hydraulically isolates the completion fluid from the production zone  66 . Fluids from the production zone  66  are conveyed to the surface by passage through the production tubing  34 . In FIG. 2 the disposition of two chokes  32  are shown at depth within the well, each of which is used to power electrical pods  40 . These pods implement any combination of communication, measurement and control functions to assist well production operations. 
     Referring still to FIG. 2, the general disposition of surface equipment is illustrated, consisting of an AC power source  48 , a 1:10 power transformer  86 , and a modem  39 . One output side of the surface power transformer and modem circuits are connected by means of conductor  44  through a pressure sealed electrical isolation feedthrough  76  to the production tubing section below the electrically isolating hanger. The other output sides of the power transformer and the surface master modem circuits are electrically connected to the well casing. 
     FIG. 2 shows each pod being used to power and control a motorized gas lift valve  24 . For this purpose a suitable implementation of the pod consists of a power transformer  111  with a winding ratio such that 2 Volts on the tubing side creates 15 Volts on the electronics module side (and vice versa), and a main printed circuit board (PCB)  112  having a modem  122  and other electrical components to power and control the motorized gas lift valve  24 . The downhole modems within the pods communicate with the modem at the surface, and possibly with each other, allowing data to be transferred from each pod to the surface or between pods, and instructions to be passed from the surface to control each gas lift valve. Each modem is individually addressable, and each control or sensor device within each pod is individually addressable. 
     While FIG. 2 illustrates the case where two downhole modules are operated in the well, it will be readily apparent that the same principle may be used to provide an arbitrary number of downhole modules. This is useful in an application where a conductive completion fluid  82  is present in the annulus before unloading a gas-lift well. Each choke will not work sufficiently to develop a voltage potential at its respective device when the choke is submerged in conductive fluid, but as the conductive fluid is progressively removed during the unloading process, each device can receive power and/or communications (thus being controllable) when the respective choke is no longer submerged in conductive fluid. Hence, as the conductive fluid level drops during unloading, the devices sequentially become controllable, which aids in achieving a more controlled unloading procedure. 
     Referring to FIG. 3, the electrical equivalent circuit of the power and communications path of FIG. 2 may be analyzed. The casing and tubing form the major transmission paths for both the power and communication signals. The casing is represented by the conductor  101 . The tubing is represented by conductor  102 . Resistor  218  represents the combined distributed resistance offered by casing and tubing, and is typically of the order of 1 Ohm. The choke impedances are represented by inductors  32 . At the frequency of the AC power the reactive impedance offered by each choke is of the order of 2 Ohms. 
     Referring still to FIG. 3, the surface master modem ensemble  39  is represented by resistor  212  for its receiver, and an AC source  214  for its transmitter. AC power input at the surface is represented by AC source  216 . The downhole electronic pods associated with each choke are represented by power converter and modem ensembles  122 , composed of resistors  106  for the power converters and modem receivers, and AC sources  108  for the modem transmitters. The circuit is completed by the metal packer  56  which has a negligibly small electrical impedance. 
     It is seen from FIGS. 2 and 3 that the downhole pods are powered by the AC voltage developed on the tubing by each choke, caused by the back-EMF created by the passage of current along the tubing which passes through the choke. The chokes are designed to develop about 2 Volts from the AC which passes through them, and this AC is converted to DC in the power conditioning circuit which is coupled through the power supply input transformer, following standard practice for such AC-to-DC power conversion and conditioning circuits. This DC power is typically supplied to the pod sensors, modem, and control circuits at about 15 Volts, and of the order of 10 Watts is typically available to power these downhole sub-systems. 
     Referring to FIG. 4 a,  the construction of a suitable choke may be described. A choke for a given application may be divided into multiple pieces along its length (L). In other words, stacking multiple sub-sections of chokes  134  together along the choke axis  60 , as shown in FIGS. 4 a  and  4   b,  provides the same effect as have one large choke of length (L). Multiple sub-sections  134  stacked on top of one another act as a series of impedances, which added together provide the same total impedance as a single choke having the same total length of ferromagnetic material as the aggregated sub-sections. 
     Referring to FIG. 4 b,  the details of a suitable choke assembly are illustrated, though it will be clear to one familiar with the art that alternative designs are feasible. The tubing section  34  is composed of type  316  stainless steel and typically has an outer diameter of 3.5 inches and a length of 10 feet. Each end of the production tubing section  34  is furnished with New VAM male threads by which mating sections of conventional production tubing are attached. (New VAM is a registered Trademark of Vallourec Mannesman Oil &amp; Gas France, and defines a thread form suitable for this purpose). At the upper and lower extremities of the choke section are welding collars  50  with internal diameter 3.55 inches, length 2 inches, and wall thickness one quarter of an inch. The section of tubing  34  between the welding collars is covered with PTFE heat-shrink tubing  20  of 0.020 inches wall thickness, and thus tubing  20  lies between the production tubing section  34  and the internal walls of all the choke sub-sections  134 . Each end of the choke assembly is furnished with a machined plastic centralizer  114 . A suitable machinable plastic is polyetheretherketone (PEEK) which is a commodity material available from many commercial sources 
     Choke sub-sections  134  are formed by winding 60 sheet laminations of a high-permeability ferromagnetic alloy such as Permalloy (Permalloy is a registered Trademark, of Western Electric Company). Permalloy is a nickel/iron alloy with a nickel content in the range 35% to 90% and is available as a commodity material from many commercial sources. A suitable alloy is composed of 86% nickel/14% iron, and the laminations are 0.014 inches thick and 2.35 inches wide such that the final dimensions of each choke section are 3.6 inches internal diameter, 5.45 inches external diameter, and 2.35 inches in the direction of the choke axis  60 . Fifteen such choke sections are stacked to form a total choke assembly suitable for usual power frequencies, 50 or 60 Hertz. At power frequencies up to a few hundred Hertz, laminated ferromagnetic alloy can be used for construction of the choke sections, as in standard transformer design practice, and as described above. Lamination is required to reduce eddy current losses which would otherwise degrade the effectiveness of the choke. For material with absolute magnetic permeability of 50,000 operating at 60 Hertz the required lamination thickness for 2 skin depths is 0.8-millimeters (0.031 inches), which is realistic and practical. 
     Between each choke section is a polytetrafluoro-ethylene (PTFE) washer  136  with internal diameter 3.6 inches, external diameter 5.45 inches, and thickness 0.030 inches. After all the chokes are threaded onto the tubing, the entire section of chokes is covered with PTFE heatshrink tubing  138  having 0.020 inches wall thickness. The stainless steel rod  51  is 0.125 inches diameter covered with polyethylene (PE) heat-shrink tubing and extends along the length of the completed choke assembly. It is attached to the upper welding collar  10  and passes through holes in the centralizers  114 . Its lower end is electrically connected to the input of the electrical pod which is below the choke assembly. 
     The impedance offered by the choke is a critical implementation issue, since this determines what proportion of total power supplied to the pipe will be lost to leakage through the choke, and what proportion will be available to power and communicate with the devices installed in the isolated section of the pipe. Since the impedance presented by an inductor increases with frequency, the AC power frequency is used in both the theoretical analysis and the testing of alternative choke configurations, as this is normally equal to or lower than the communication frequencies. 
     FIGS. 5 a-d  indicate the parameters used in the choke design analysis. FIGS. 5 a  and  5   b  illustrate the case where the choke is placed within the annulus  58  between the tubing  34  and the casing  36 . FIGS. 5 c  and  5   d  illustrate an alternative case where the choke is placed outside the casing  36 . The basis for the analysis is the same in both cases, but it is important to realize that the electrical current value (I) used in the design analysis is the net current linked by the choke. In the case where the choke is disposed in the annulus  58  (FIGS. 5 a  and  b ), the current is that on the tubing alone. When the choke is disposed external to the casing (FIGS. 5 c  and  5   d ), the current is the vector sum of the separate currents on the casing and tubing. Thus if these currents were to be equal but opposite in phase there would be no net choking effect with the configuration shown in FIGS. 5 c  and  5   d.    
     The defining variables and a self-consistent set of physical units are: 
     L=length of choke, meters; 
     a=choke inner radius, meters; 
     b=choke outer radius, meters; 
     r=distance from choke axis, meters; 
     I=r.m.s. net current through choked section, Amperes; 
     Ω=angular frequency of leakage current, radians per second; 
     μ=absolute magnetic permeability of choke material at radius r, equal to the absolute permeability of free space (4π×10 −7  Henrys per meter) multiplied by the relative permeability of the magnetic material of the choke. 
     By definition, ω=2πf where f=frequency in Hertz. 
     At a distance r from the current I, the r.m.s. free space magnetic field H, in Henries per meter, is given by: 
     
       
           H=I/ 2π r   
       
     
     The field H is circularly symmetric about the choke axis, and can be visualized as magnetic lines of force forming circles around that axis. 
     For a point within the choke material, the r.m.s. magnetic field B, in Teslas, is given by: 
     
       
           B =μH=μI/ 2π r   
       
     
     The r.m.s. magnetic flux F contained within the choke body, in Webers, is given by: 
     
       
         
           F=∫BdS 
         
       
     
     where S is the cross-sectional area of the choke in square meters as shown in FIGS. 5 a  and  5   c  and the integration is over the area S. Performing the integration from the inner radius of the choke (a), to the outer radius of the choke (b), over the length of the choke (L), we obtain: 
     
       
           F=μLI  1 n ( b/a )/2 n   
       
     
     where 1n is the natural logarithm function. 
     The voltage generated by the flux F, in Volts, is given by: 
     
       
           V=ΩF= 2 πf F=μLIf  1 n ( b/a ) 
       
     
     Note that the back-e.m.f. (V) is directly proportional to the length (L) of the choke for constant values of (a) and (b), the choke element internal and external radii. Thus by altering the length of the choke, any desired back-e.m.f. can be generated for a given current. 
     Inserting representative values: 
     μ=50,000×(4π×10 −7 ), L=1 meter, I=10 Amperes, f=60 Hertz, 
     a=0.045 meters (3.6 inch inner diameter), b=0.068 meters (5.45 inch external diameter): 
     then the back-e.m.f. developed V=2.6 Volts 
     showing that such a choke is effective in developing the required downhole voltage, and does so when realistic and safe currents and voltages are impressed upon the tubing and transmitted from the well head to downhole equipment. 
     FIG. 6 shows a possible application of the system for defining an electrical circuit in accordance with the first embodiment of the present invention. A gas-lift oil well extends from the surface  64  through a borehole and extends into a production zone  66  downhole. A hanger  54  supports a production tubing string  34  from a well casing  36 . The casing  36  is conventional, i.e., it is typically metal tubing held in place by injecting cement  70  between the casing and the earth in the borehole during well completion. Similarly the tubing string  34  is generally conventional comprising a plurality of elongated tubular metal production pipe sections joined by threaded couplings (not shown) at each location of the tubing sections. A gas input throttle  74  is employed to permit the input of compressed gas into the tubing  34  via valves  40  for lifting oil during production. Schematically illustrated is a computer system and power source  38  at the surface  64  with power and communication feeds  44  passing through pressure seal  76  in the hanger  54  and using return connections  78 , which are electrically connected to the casing  36 . Ferromagnetic induction chokes  30 ,  31 ,  32  are installed on the production tubing  34  to act as series impedances to AC flow and to define an electrical path along the tubing string  34  between the upper choke  30  and lowest choke  32 . As previously explained with reference to FIG. 1, the electrical effect of the upper choke  30  is similar to that of the insulating tubing joint  146  illustrated in FIG.  1 . 
     Referring to FIG. 6, in a typical manner, a packer  56  is placed downhole in the casing  36  above the production zone  66 . The packer  56  hydraulically isolates the production zone, but electrically connects the production tubing  34  with the outer metal casing  36 . Similarly, above the surface  64  the metal hanger  54  (along with the surface valves  80 , platform  68 , and other production equipment) electrically connects the production tubing  34  to the outer metal casing  36 . Typically, such a configuration would not allow AC power or electrical signals to be passed up or down the well using the tubing  34  as one conductor and the casing  36  as the other conductor. However, the use of induction chokes  30 ,  31 ,  32  alters the electrical characteristics of the well&#39;s piping structure providing a system and method to pass AC power and communication signals up and down the borehole of the oil well via the tubing  34  and the casing  36 , and to make this power available to downhole modules  40 . An electrical potential is formed between the tubing  34  and the casing  36  between the upper choke  30  and the lowest choke  32 , and on the tubing  34  at each choke  31 . Hence, devices  40  can be powered by connecting a first device terminal  51  to the tubing  34  above the chokes  31 ,  32  and connecting a second device terminal  52  to the tubing below each choke  31 , 32 . 
     FIG. 7 illustrates a method for applying the system of FIG. 6 to an unloading process for a gas lift well. Typically the unloading process starts with the annulus  58  filled with completion fluid  82 , to level  1  of the well as illustrated in FIG.  7 . The completion fluid  82  is normally a brine which is electrically conductive, and thus creates an electrical connection between tubing  34  and casing  36 . Each downhole module  40  controls a motorized gas lift valve which may be opened to permit fluid, either liquid or gas, to pass from the annulus  58  to the interior of tubing  34 . At the start of the unloading process all of these lift gas valves are open, but none of the modules  40  can be powered at this point in time since the completion fluid creates an electrical short circuit between the tubing  34  and the casing  36  at a point above all of the chokes  31 . 
     To initiate the unloading process, lift gas under pressure from a surface supply is admitted to the annulus  58 , and starts to displace the completion fluid through the open lift gas valves of each of the downhole modules  40 , thus driving down the level of the completion fluid. When the level of the completion fluid has reached level  2  shown on FIG. 7, the first module  40  immediately above level  2  has become powered and thus controllable, since the tubing and casing above level  2  are no longer electrically short-circuited above level  2 . The lift gas valve associated with the module immediately above level  2  may now be regulated to control the flow of lift gas into the tubing  34 . The rising column of lift gas bubbles lightens the liquid column between this first valve and the surface, inducing upwards flow in the production tubing. At this point in the unloading process therefore, the uppermost lift gas valve is passing gas under control from commands sent from surface equipment  38 , and the other lift gas valves are open to pass completion fluid but cannot yet be controlled. 
     Completion fluid continues to be expelled through the lower open valves until the completion fluid level reaches level  3 . At this point the module  40  immediately above level  3  becomes powered and controllable as described with reference to the valve at level  2 , so that lift gas flow through the valve at level  3  may now be regulated by commands sent from the surface. Once this flow is established, the lift gas valve at level  2  may be closed, and lift of fluids in the tubing  34  is thus transferred from level  2  to level  3 . 
     In like manner, as the completion fluid continues to be expelled and its surface passes levels  4  and  5 , the gas lift valves at these levels become powered and controllable at progressively greater depths. As gas lift progresses down the tubing, the valves above are closed to conserve lift gas, which is directed to only the lowermost open valve. At the end of the unloading process, only the gas lift valve at choke  32  is open, and all valves above it are closed. 
     This method for controlling the unloading process ensures that each valve is closed at the correct moment. In existing practice and without benefit of means to control directly the lift gas valves, the cycling of the intermediate valves between open and closed is implemented by using pre-set opening and closing pressures. These preset values are chosen using design calculations which are based on incomplete or uncertain data. The consequence is that in existing practice the valves frequently open and close at inappropriate times, causing lift instability, excessive wear or total destruction of the valves, and also inefficiencies in lift gas usage from the need to specify the valve presets with pressure margins which reduce the range of gas pressures which can be made available for lift during the unloading and production processes. 
     The method described for the unloading process provides a similar benefit for well kickoff. In this case the assumed starting condition is with the annulus pressurized by lift gas and therefore cleared of conductive fluid, but with lift gas flow stopped either because the well has been shut in, or is killed. The supply pressure of the lift gas source is normally insufficient to initiate lift gas flow immediately through the bottom valve, associated with choke  32  in FIG. 7, since with no lift gas flowing the tubing  34  is filled with a static column of produced liquids which exert a head pressure greater than the available lift gas pressure. To initiate lift, the intermediate valves at levels  2  though  5  of FIG. 7 must be cycled open progressively to lighten the fluid column in tubing  34 , and then closed when gas injection has been achieved from a lower valve. The benefits of the powered and controllable valves for kickoff are the ability to cycle the valves in the correct sequence, to be sure that each is positively open or closed at the correct point in the kickoff process, to be able to use the available lift gas pressure to best advantage, and to use lift gas quantity in the most economical manner consistent with obtaining lift. 
     Referring to FIG. 3, it will be seen that the downhole modules are electrically in series with each other. When there are several downhole modules as in FIGS. 6 and 7, the voltages on the tubing generated by the chokes  30 ,  31 ,  32  combine additively to determine the voltage which must be applied at the well head by the surface equipment. There are alternative embodiments in which the downhole equipment is arranged in electrical parallel, which in certain applications may be desirable to reduce the voltage which must be applied at the wellhead. 
     FIG. 8 illustrates schematically a well similar to that of FIG. 6, furnished with a plurality of downhole electrical control, measurement and communication modules  40 . In this embodiment the power for each pod is derived from the voltage developed between the tubing  34  and the casing  36 , by the chokes  30  and  32 . In contrast to the serial connections of the embodiment of FIG. 7, in the embodiment of FIG. 8 the electrical connections to the downhole modules  40  are in parallel. In this embodiment therefore the voltage which must be applied at the wellhead by the surface equipment  38  through the conductor  44  remains the same regardless of the number of downhole modules, but the current which must be supplied is in proportion to the number of downhole modules. This embodiment would be inoperable so long as conductive fluid were present in the annulus above the lower choke  32 , but it has the advantage that the wellhead electrical potential remains low and therefore safe regardless of the number of downhole modules. 
     FIGS. 9 a  and  9   b  illustrate schematically the difference that the induction chokes  30 ,  32  make in the electrical circuit. FIG. 9A is a simplified schematic representing the electrical circuit of the embodiment shown in FIG.  8 . Whereas, FIG. 9 b  is a simplified schematic of what the electrical circuit would be without the use of the induction chokes  30 ,  32  in accordance with the present invention. The preferred or natural path of AC in the circuit shown in FIG. 9B is to take the upper short path of least resistance rather than travel through the tubing  34 , through the devices  40 , and through the casing  36 . 
     Looking back to FIG. 9A in comparison, instead of traveling across the upper short path that now has a large inductor (the upper induction choke  30  along a first location  41  of the tubing  34 ), most of the current will be forced to flow along the tubing  34 . The impedance provided by the induction effect of the upper choke  30  creates a substantial barrier to passage (back-e.m.f.) of most of the AC current. Again looking at FIG. 9B, without the lower choke  32 , there is a lower short path from the tubing  34  to casing  36 , such as a packer  56  (see FIGS. 1 and 6) or a conductive fluid  82 . With the lower choke  32  in place, instead of traveling across the lower short path that now has a large inductor (lower induction choke  32 ), most of the current will be forced to flow across the device  40  to reach the electrical return (e.g., casing  36 ). Thus, the goal of providing electrical power and/or communications to a device  40  downhole using the tubing  34  and casing  36  as electrical paths in the circuit is accomplished by using this embodiment of the present invention. 
     It will be clear to those skilled in the art that combinations of serial and parallel power and communication connections are possible by using combinations of the disposition of chokes described in reference to FIGS. 6 and 8. By using electrical switches within the downhole modules  40 , controlled from the surface, the configuration of downhole power and communication signals may be dynamically configured to be either series or parallel for each individual downhole module. 
     FIGS. 6 and 8 may be used to illustrate another application of the present invention. Devices  40  contain communications modems used to relay a signal up or down the well. In another possible application, devices  40  may also contain various sensors to measure the characteristics of the well downhole conditions, including but not limited to: temperature, pressure, chemical composition, flow rate, pool depth, or any combination thereof. For example, if a downhole device  40  comprises a sensor, a valve, and a logic circuit, a closed loop system can operate downhole to optimize the oil flow by varying the gas input according to the sensor readings using logic rules. As another example, if a downhole device  40  comprises a sensor, a communications modem, and a controllable valve, another closed loop system can be formed. The device  40  can measure one or more characteristics of the well conditions using the sensor, and send the data obtained by the sensor uphole to a computer system using the modem. With the well condition data from the downhole sensor, the computer system at the surface can analyze the data (and possibly in combination with other data from other similar downhole sensors), and provide instructions for controlling the valve downhole. The control signals for the valve can be transmitted downhole to the modem in the device, which can be used to control the valve (e.g., change valve settings). Such controllable devices  40  downhole have the potential to greatly increase the production efficiency of a gas-lift oil well, as well as provide more controllable unloading and/or kick-off, which may translate into cost savings and increased earnings for the oil production company. 
     A system in accordance with the present invention is relatively robust and reliable due to its low number of additional parts needed (in addition to typical, existing equipment being used in the oil fields). Because the induction chokes  30 ,  32  are unpowered and have no moving parts, there are few failure modes. Also, a system in accordance with the present invention has the advantage that it can be adapted to use much of the existing petroleum well equipment designs (e.g., tubing  34 , packers  56 , casing  36 ). 
     FIG. 10 illustrates an alternative embodiment in accordance with the methods of the present invention. This embodiment uses only one choke  30  along a first location  41  of a piping structure. In the example shown in FIG. 10, the piping structure is a production tubing string  34 , and the electrical return comprises a well casing  36 . This embodiment is similar to the embodiment described in reference to FIG. 6 in that it works the same as that embodiment at the first location  41  of the piping structure. However, instead of having a choke  32  at the second location  42  (downhole) of the piping structure, a current transformer  90  is used to couple power and communications signals between device  40  and the tubing  34 . Current flowing in tubing  34  links the current transformer  90 , and passes to the lower part of the well where either packer  56  or conductive fluid  82  or both connect the tubing  34  to the casing  36 , thus providing a return path for the power and communication signals to the surface equipment  38 . 
     The embodiment of FIG. 10 is an alternative method for providing electrical power to devices  40  at depth within a well containing a conductive fluid  82  (e.g., saline solution) in the annulus  58  between the casing  36  and the production tubing  34 . When the current transformer  90  is above the conductive fluid  82 , AC current flow within the tubing  34  acts as the primary winding of the transformer and induces secondary current flow in the toroidal secondary winding of the current transformer  90 . This secondary current can be used to provide electrical power and/or communications to the device  40  electrically connected to the transformer  90 . Electrical isolation at the well head can take the form of choke  30  as illustrated in FIG. 10, or, as illustrated in FIG. 1, an electrically insulating tubing joint  146 . 
     FIG. 11 shows details of the current transformer  90  of FIG.  10 . The transformer  90  comprises a cylindrical ferromagnetic core  94  wound such that the main lengths of the windings  92  are generally parallel to the axis of the core  96 , following conventional practice for such a transformer. Effectively the tubing  34  acts as the primary winding of such a transformer  90 , creating a circular magnetic field axially symmetric about the tubing axis, which is aligned with the transformer axis  96 . This magnetic field induces an electrical current in the secondary winding  92 , and this current is available to power and communicate with electrical or electronic equipment within the device  40  electrically connected to the current transformer secondary winding  92 . The geometry, number of turns, length, and materials can vary for the transformer  90 , depending on the application needs. Since the action of such a transformer is reversible, communication signals generated by a modem within module  40  may be coupled to tubing  34  for transmission to surface equipment. Thus, the goal of providing electrical power and/or communications to a device  40  downhole using the tubing  34  and casing  36  as electrical paths in the circuit is also accomplished by using this embodiment of the present invention. 
     The embodiment described in reference to FIGS. 10 and 11 is not limited to a single current transformer, but may be extended to multiple downhole transformers in a manner analogous to the multiple downhole chokes illustrated in reference to FIG.  6 . Multiple such downhole power transformers provide a method to energize lift gas valves sequentially as annulus fluid level drops during the unloading or kickoff processes, as previously described in reference to FIG. 7, and with the same benefits. 
     FIG. 12 shows a petroleum well application in accordance with another embodiment of the present invention, where the chokes are external to the casing. In this embodiment, the piping structure used to carry the electrical current for the downhole device  40  comprises the casing  36 , which is a conductive metal tubing in this case, and the electrical return comprises the earthen ground  72 . Thus, in this embodiment, the chokes  30 ,  32  are located about the casing  36  rather than being located only about the tubing  34  as in the embodiments described previously. In this embodiment, the current flows from the power source  38  to the casing  36  below the upper choke  30 , along the casing  36  to a first device terminal  51  (due to the upper choke  30 ), through the device  40  (due to a voltage potential developed across the lower choke  32 ) to the earth ground  72  via the casing  36  below the lower choke  32 , and back to the source  38  via the earthen ground  72  (and vice versa because AC). 
     In the choke design analysis previously described with reference to FIGS. 5 c-d,  current in both the casing  36  and the tubing  34  is impeded by a choke such as  30  or  32  of FIG. 9 a  since currents on both the casing and the tubing link the choke. The tubing cannot be used as the current return path for power applied to the casing since the magnetic fields from the supply and return currents would balance within the chokes, which would become ineffective. It is for this reason that that the ground return path  72  is necessary. 
     The potential developed on the casing across the choke  32  is connected by electrical conductors  51  and  52  to power and communicate with an instrument pod  40  located external to the casing. Chokes  30 ,  32 , and the instrument pod  40 , are set in the well with the casing and before the cement  70  is injected. As in the previous embodiments the instrument pod  40  may provide bidirectional communication through a modem to return data to the surface from sensors to measure conditions such as formation pressure, temperature, acoustic signals etc connected to pod  40 , and to accept control commands from the surface. 
     Even though many of the examples discussed herein are applications of the present invention in petroleum wells, the present invention also can be applied to other types of well, including but not limited to: water wells and natural gas wells. 
     Also in a possible embodiment (not shown) of the present invention, the piping structures of two adjacent wells can be used to form a current loop for the electrical circuit. For example, a second location of a piping structure of a first well may be electrically connected (e.g., via a wire, conductive fluid, and/or the earth) to a second location of a piping structure of a second well adjacent to the first well, and a first location of the piping structure of the first well is electrically connected to a first terminal of a power source and a first location of the piping structure of the second well is electrically connected to a second terminal of the source, such that the electrical circuit is formed by using the piping structures of both wells. Hence, one of the piping structures will act as an electrical return. In another possible embodiment (not shown), two piping structures of the same well (e.g., two adjacent lateral branches) can be used to form a current loop for an electrical circuit. For example, the piping structure can be a first lateral branch and the electrical return can be a second lateral branch. 
     One skilled in the art will see that the present invention can be applied in many areas where there is a need to provide power and/or communication within a borehole, well, or any other area that is difficult to access. As discussed above, a production tubing string, as used in oil fields for withdrawing oil from a reservoir, is an example of a well with limited access downhole. Another example is the use of the present invention to provide power and/or communications to a device within a borehole of a machine part, where access within the borehole is limited. For example, when looking for cracks in a steam turbine using nondestructive testing techniques (e.g., ultrasonics, eddy current), there is often a need to provide power and communications to a sensor deep within a borehole of the steam turbine rotor that may be three to six inches in diameter and thirty feet long. The piping structure can comprise a rod or tube that physically supports the sensor, and the electrical return can comprise the machine part being inspected. Hence, the use of the present invention can provide a system and method of providing power and communications to a sensor deep within the borehole where access is limited. 
     Also, one skilled in the art will see that the present invention can be applied in many areas where there is an already existing conductive piping structure and a need to route power and/or communications in a same or similar path as the piping structure. A water sprinkler system or network in a building for extinguishing fires is an example of a piping structure that may be already existing and having a same or similar path as that desired for routing power and/or communications. In such case another piping structure or another portion of the same piping structure may be used as the electrical return. The steel structure of a building may be used as a piping structure and/or electrical return for transmitting power and/or communications in accordance with the present invention. The steel rebar in a concrete dam or a street may be used as a piping structure and/or electrical return for transmitting power and/or communications in accordance with the present invention. The transmission lines and network of piping between wells or across large stretches of land may be used as a piping structure and/or electrical return for transmitting power and/or communications in accordance with the present invention. Surface refinery production pipe networks may be used as a piping structure and/or electrical return for transmitting power and/or communications in accordance with the present invention. Thus, there are numerous applications of the present invention in many different areas or fields of use. 
     It will be appreciated by those skilled in the art having the benefit of this disclosure that this invention provides a system that uses at least one unpowered induction choke to form an electrical circuit in a piping structure. It will also be appreciated by those skilled in the art having the benefit of this disclosure that this invention provides a system for providing power and/or communications to a device downhole in a borehole of a well using an electrical circuit formed in a piping structure by using at least one unpowered induction choke. It will be further appreciated by those skilled in the art having the benefit of this disclosure that this invention provides a system and method of providing a downhole electrical circuit in a well or borehole formed by using at least one unpowered induction choke and at least one power transformer about an existing piping structure. It should be understood that the drawings and detailed description herein are to be regarded in an illustrative rather-than a restrictive manner, and are not intended to limit the invention to the particular forms and examples disclosed. On the contrary, the invention includes any further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments apparent to those of ordinary skill in the art, without departing from the spirit and scope of this invention, as defined by the following claims. Thus, it is intended that the following claims be interpreted to embrace all such further modifications, changes, rearrangements, substitutions, alternatives, design choices, and embodiments.