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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 
                 Jan. 24, 2000 
               
               
                   
                   
                 for Wireless 
               
               
                   
                   
                 Communication and 
               
               
                   
                   
                 Control 
               
               
                 TH 1599x 
                 60/186,376 
                 Toroidal Choke Inductor 
                 Mar. 2, 2000 
               
               
                   
                   
                 for 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 
                 Mar. 2, 2000 
               
               
                   
                   
                 Control from 
               
               
                   
                   
                 Intelligent Well Data 
               
               
                 TH 1602 
                 60/178,001 
                 Controllable Gas-Lift 
                 Jan. 24, 2000 
               
               
                   
                   
                 Well 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 
                 Jan. 24, 2000 
               
               
                   
                   
                 Fluid Flow Optimization 
               
               
                 TS6185 
                 60/181,322 
                 Optimal Predistortion 
                 Feb. 9, 2000 
               
               
                   
                   
                 in Downhole 
               
               
                   
                   
                 Communications System 
               
               
                 TH 1671 
                 60/186,504 
                 Tracer Injection in a 
                 Mar. 2, 2000 
               
               
                   
                   
                 Production Well 
               
               
                 TH 1672 
                 60/186,379 
                 Oilwell Casing 
                 Mar. 2, 2000 
               
               
                   
                   
                 Electrical Power Pick- 
               
               
                   
                   
                 Off Points 
               
               
                 TH 1673 
                 60/186,375 
                 Controllable Production 
                 Mar. 2, 2000 
               
               
                   
                   
                 Well 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 
                 Mar. 2, 2000 
               
               
                   
                   
                 Casing 
               
               
                 TH 1677 
                 60/186,527 
                 Method for Downhole 
                 Mar. 2, 2000 
               
               
                   
                   
                 Power 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 
                 Mar. 2, 2000 
               
               
                   
                   
                 Hydraulic 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 1600US 
                 09/769,048 
                 Induction Choke 
                 Jan. 24, 2001 
               
               
                   
                   
                 for Power 
               
               
                   
                   
                 Disribution in 
               
               
                   
                   
                 Piping 
               
               
                   
                   
                 Structure 
               
               
                 TH 1602US 
                 09/768,705 
                 Controllable 
                 Jan. 24, 2001 
               
               
                   
                   
                 Gas-Lift Well 
               
               
                   
                   
                 and Valve 
               
               
                 TH 1603US 
                 09/768,655 
                 Permanent, 
                 Jan. 24, 2001 
               
               
                   
                   
                 Downhole, 
               
               
                   
                   
                 Wireless, Two- 
               
               
                   
                   
                 Way Telemetry 
               
               
                   
                   
                 Backbone Using 
               
               
                   
                   
                 Redundant 
               
               
                   
                   
                 Repeaters 
               
               
                 TH 1668US 
                 09/769,046 
                 Petroleum Well 
                 Jan. 24, 2001 
               
               
                   
                   
                 Having Downhole 
               
               
                   
                   
                 Sensors, 
               
               
                   
                   
                 Communication, 
               
               
                   
                   
                 and Power 
               
               
                 TH 1669US 
                 09/768,656 
                 System and Method 
                 Jan. 24, 2001 
               
               
                   
                   
                 for Fluid Flow 
               
               
                   
                   
                 Optimization 
               
               
                   
               
             
          
         
       
     
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a petroleum well having a downhole modem, and in particular, to a downhole electronics module having one or more sensors which communicate with the surface, whereby the electronics module and sensors are powered using the tubing string and casing as a conductor. 
     2. Description of Related Art 
     Several methods have been devised to place electronics, sensors, or controllable valves downhole along an oil production tubing string, but all such known devices typically use an internal or external cable along the tubing string to provide power and communications downhole. It is, of course, highly undesirable and in practice difficult to use a cable along the tubing string either integral to the tubing string or spaced in the annulus between the tubing string and the casing. The use of a cable presents difficulties for well operators while assembling and inserting the tubing string into a borehole. Additionally, the cable is subjected to corrosion and heavy wear due to movement of the tubing string within the borehole. An example of a downhole communication system using a cable is shown in 
     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 communication scheme for coupling electromagnetic energy in a TEM mode using the annulus between the casing and the tubing. This inductive coupling requires a substantially nonconductive fluid such as crude oil in the annulus between the casing and the tubing. Therefore, the invention described in U.S. Pat. No. 4,839,644 has not been widely adopted as a practical scheme for downhole two-way communication. Another system for downhole communication using mud pulse telemetry is described in U.S. Pat. Nos. 4,648,471 and 5,887,657. Although mud pulse telemetry can be successful at low data rates, it is of limited usefulness where high data rates are required or where it is undesirable to have complex, mud pulse telemetry equipment downhole. Other methods of communicating within a borehole are described in U.S. Pat. Nos. 4,468,665; 4,578,675; 4,739,325; 5,130,706; 5,467,083; 5,493,288; 5,576,703; 5,574,374; and 5,883,516. Similarly, several permanent downhole sensors and control systems have been described in U.S. Pat. Nos. 4,972,704; 5,001,675; 5,134,285; 5,278,758; 5,662,165; 5,730,219; 5,934,371; and 5,941,307. 
     Side-pocket mandrels coupled to the production tubing are known for receiving wireline insertable and retrieval gas lift valves. Many gas lift wells have gas lift valves incorporated as an integral part of the tubing string, typically mounted to a tubing section. However, wireline replaceable side pocket mandrel type of gas lift valves have many advantages and are quite common (such as made by Camco or Weatherford.) See U.S. Pat. Nos. 5,782,261 and 5,797,453. Gas lift valves placed in a side pocket mandrel can be inserted and removed using a wireline and kickover tool either in top or bottom entry. Therefore, it is common practice in oilfield production to shut off production of the well every three to five years and use a wireline to replace gas lift valves. Often, an operator has a good estimate of which valves in the well have failed or degraded and need to be replaced. 
     It would, therefore, be a significant advance in the operation of gas lift wells if an alternative to the conventional bellows type valve were provided, in particular, if the tubing and casing could be used as communication and power conductors to control and operate such a gas lift valve. It would also be advantageous to have sensors and electronics downhole that are powered and communicate using the tubing string and the casing. These sensors and electronics could then be used cooperatively with the controllable gas lift valves to more efficiently operate the well. 
     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. 
     BRIEF SUMMARY OF THE INVENTION 
     The problems outlined above with sensing and communicating downhole in an oil or gas well are addressed by the sensors and electronics module according to the system and method of the present invention. Broadly speaking, an oil or gas well includes a cased wellbore. The wellbore is cased with casing that extends substantially throughout the length of the bore and is held in place by cement between an exterior surface of the casing and the bore. A tubing string is positioned within and longitudinally extends within the casing. An electronics module is coupled to the tubing and includes one or more sensors that work cooperatively with the electronics module to monitor and determine various downhole conditions. Examples of the downhole conditions that can be monitored include tubing fluid pressure, tubing fluid temperature, annulus fluid pressure, annulus fluid temperature, fluid flow rates, valve positions, acoustic data, and seismic data. The electronics module and sensors are powered and communicate with the surface using the tubing and casing as conductors. 
     In more detail, a surface computer includes a master modem that can impart a communications signal to the tubing. The signal is preferably applied below a current limiting device such as a ferromagnetic choke positioned concentrically around the tubing. Low voltage alternating current (AC) power is also supplied to the tubing string below the current limiting device. In a preferred form, the casing is used as a ground return conductor, although an open hole ground to earth is also practical. The power and communications signals are received downhole by the electronics module and sensors. The electronics module includes a downhole slave modem for communicating the sensor measurements to the surface computer. Preferably, the electronics module and sensors are inserted as a wireline retrievable module into a side pocket mandrel in the tubing string. Alternatively, the electronic module and sensors may be mounted directly on the tubing. 
     The ferromagnetic chokes positioned around the tubing act as a series impedance to current flow in the tubing. In a preferred form, an upper ferromagnetic choke is placed around the tubing below a casing hanger near the top of the wellbore. A lower ferromagnetic choke is placed around the tubing downhole, and the electronics module is electrically coupled to the tubing just above the lower ferromagnetic choke. When power and communication signals are applied to the tubing below the upper ferromagnetic choke, the signal is effectively blocked from traveling beyond the lower or upper ferromagnetic chokes. This creates a potential between the tubing and a ground that is then used to power and communicate with the electronics module in the wellbore. 
     In a preferred form, the surface computer can be coupled via its surface master modem, not only to the downhole slave modem, but also to a variety of other data sources outside of the wellbore. These data sources could provide information, for example, on measurements of oil output and measurements of compressed gas input. The measurements could then by used by the surface computer to determine an optimum operating state of the oil well. In a preferred embodiment, the computer could then control the operation of the oil well by varying the amounts of compressed gas input, introducing needed chemicals into the oil well, or controlling downhole valves such as gas lift valves. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic front view of a controllable gas lift well according to one embodiment of the present invention, the gas lift well having a tubing string and a casing positioned within a bore hole. 
     FIG. 2A is an enlarged schematic front view of a side pocket mandrel and a controllable gas lift valve, the valve having an internal electronics module and being wireline retrievable from the side pocket mandrel. 
     FIG. 2B is a cross sectional side view of the controllable gas lift valve of FIG. 2A taken at III—III. 
     FIG. 3 is an enlarged schematic front view of the tubing string and casing of FIG. 1, the tubing string having an electronics module, sensors, and a controllable gas lift valve operatively connected to an exterior of the tubing string. 
     FIG. 4A is an enlarged schematic front view of the tubing string and casing of FIG. 1, the tubing string having a controllable gas lift valve permanently connected to the tubing string. 
     FIG. 4B is a cross sectional side view of the controllable gas lift valve of FIG. 6 taken at VI—VI. 
     FIG. 5 is a schematic of an electrical equivalent circuit diagram for the controllable gas lift well of FIG.  1 . 
     FIG. 6 is a schematic diagram depicting a surface computer electrically coupled to an electronics module of the gas lift well of FIG.  1 . 
     FIG. 7 is a system block diagram of the electronics module of FIG.  6 . 
     FIG. 8 is a schematic front view of a petroleum well having a data monitoring pod and sensors inserted in a side pocket mandrel according to the present invention. 
     FIG. 9 is an enlarged schematic front view of the side pocket mandrel, data monitoring pod, and sensors of FIG.  8 . 
     FIG. 10 is a cross sectional front view of the data monitoring pod and sensors of FIG.  9 . 
     FIG. 11 is a detail view of the data monitoring pod of FIG. 9 showing a geophone for monitoring acoustic data. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 “modem” is used generically herein to refer to any communications device for transmitting and/or receiving electrical communication signals via an electrical conductor (e.g., metal). Hence, the term is not limited to the acronym for a modulator (device that converts a voice or data signal into a form that can be transmitted)/demodulator (a device that recovers an original signal after it has modulated a high frequency carrier). 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 outputs measurements in an analog format, then such measurements may only need to be modulated (e.g., spread spectrum modulation) and transmitted-hence no analog-to-digital conversion is needed. As another example, a relay/slave modem or communication device may only need to identify, filter, amplify, and/or retransmit a signal received. 
     The term “sensor” as used in the present application 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. 
     The term “electronics module” in the present application refers to a control device. Electronics modules can exist in many configurations and can be mounted downhole in many different ways. In one mounting configuration, the electronics module is actually located within a valve and provides control for the operation of a motor within the valve. Electronics modules can also be mounted external to any particular valve. Some electronics modules will be mounted within side pocket mandrels or enlarged tubing pockets, while others may be permanently attached to the tubing string. Electronics modules often are electrically connected to sensors and assist in relaying sensor information to the surface of the well. It is conceivable that the sensors associated with a particular electronics module may even be packaged within the electronics module. Finally, the electronics module is often closely associated with, and may actually contain, a modem for receiving, sending, and relaying communications from and to the surface of the well. Signals that are received from the surface by the electronics module are often used to effect changes within downhole controllable devices, such as valves. Signals sent or relayed to the surface by the electronics module generally contain information about downhole physical conditions supplied by the sensors. 
     Similarly, in accordance with conventional terminology of oilfield practice, the descriptors “upper”, “lower”, “uphole” and “downhole” refer to relative distance along hole depth from the surface, which in deviated wells may or may not accord with absolute vertical placement measured with reference to the earth&#39;s center. 
     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.” 
     Referring to FIG. 1 in the drawings, a petroleum well according to the present invention is illustrated. The petroleum well is a gas lift well  320  having a borehole extending from surface  312  into a production zone  314  that is located downhole. A production platform is located at surface  312  and includes a hanger  22  for supporting a casing  24  and a tubing string  26 . Casing  24  is of the type conventionally employed in the oil and gas industry. The casing  24  is typically installed in sections and is cemented in borehole during well completion. Tubing string  26 , also referred to as production tubing, is generally conventional comprising a plurality of elongated tubular pipe sections joined by threaded couplings at each end of the pipe sections. Production platform also includes a gas input throttle to permit the input of compressed gas into an annular space  31  between casing  24  and tubing string  26 . Conversely, output valve  32  permits the expulsion of oil and gas bubbles from an interior of tubing string  26  during oil production. 
     An upper ferromagnetic choke  40  and lower ferromagnetic chokes  41 ,  42  are installed on tubing string  26  to act series impedances to alternating current flow. The size and material of ferromagnetic chokes  40 ,  41 ,  42  can be altered to vary the series impedance value. The section of tubing string  26  between upper choke  40  and lower choke  42  may be viewed as a power and communications path (see also FIG.  8 ). All chokes  40 ,  41 ,  42  are manufactured of high permeability magnetic material and are mounted concentric and external to tubing string  26 . Chokes  40 ,  41 ,  42  are typically protected with shrink-wrap plastic and fiber-reinforced epoxy to provide electrical insulation and to withstand rough handling. 
     A computer and power source  44  having power and communication feeds  46  is disposed at the surface  312 . Where feed  46  passes through the hanger  22  it is electrically isolated from the hanger by pressure feedthrough  47  located in hanger  22  and is electrically coupled to tubing string  26  below upper choke  40 . The neutral connection  46  is connected to well casing  24 . Thus power and communications signals are supplied to tubing string  26  from computer and power source  44 , and casing  24  is regarded as neutral return for those signals. 
     A packer  48  is placed within casing  24  downhole below lower choke  42 . Packer  48  is located above production zone  314  and serves to isolate production zone  314  and to electrically connect metal tubing string  26  to metal casing  24 . Similarly, above surface  12 , the metal hanger  22  (along with the surface valves, platform, and other production equipment) electrically connects metal tubing string  26  to metal casing  24 . Typically, the electrical connections between tubing string  26  and casing  24  would not allow electrical signals to be transmitted or received up and down the well using tubing string  26  as one conductor and casing  24  as another conductor. However, upper ferromagnetic choke  40  acts as an impedance to AC flow, and thus directs AC signals down tubing  26 . Intermediate chokes  41  and bottom choke  42  also impede current flow on the sections of tubing  26  where it passes through each choke, and thus AC carried on tubing  26  generates a potential difference on the tubing above and below each choke. This voltage is used to pass power and communication signals to downhole electronic modules  50  and  52 . Chokes  41  and  42  also provide a means to couple communication signals generated by modules  50  and  52  onto the tubing, to provide a means for transmitting signals from downhole modules to surface equipment  44 . In summary the chokes  40 ,  41 ,  42  around tubing string  26  alter the electrical characteristics of tubing  26 , providing a system and method to conduct power and communication signals up and down the tubing and casing of gas lift well  320 . 
     A plurality of motorized gas lift valves  132  are operatively connected to tubing string  26 . The number of valves  132  disposed along tubing string  26  depends upon the depth of the well and the well lift characteristics. Each valve and its associated control module  50  is energized from the surface, and thus each valve may be individually addressed and its degree of opening controlled from the surface using commands sent from the surface over a communication link between a surface modem, and downhole modems in electronics modules  50 . 
     Referring now to FIG. 2 a  in the drawings, the downhole configuration of controllable valve  52 , as well as the electrical connections with casing  24  and tubing string  26 , is depicted. The pipe sections of tubing string  26  are conventional and where it is desired to incorporate a gas-lift valve in a particular pipe section, a side pocket mandrel  54 , such as those made by Weatherford or Camco, is employed. Each side pocket mandrel  54  is a non-concentric enlargement of tubing string  26  that permits wireline retrieval and insertion of controllable valves  52  downhole. 
     Any centralizers located between upper and lower chokes  40 ,  42 , must be constructed such as to electrically isolate casing  24  from tubing string  26 . 
     A power and signal connector wire  64  electrically connects controllable valve  52  to tubing string  26  at a point above its associated choke  41 . Connector  64  must pass outside the choke  41 , as shown in FIG. 2A, for the choke to remain effective. A connector wire  66  provides an electrical return path from controllable valve  52  to tubing  26 . Each valve  52  and its associated electronics module is powered and controlled using voltages generated on the tubing  26  by the action of chokes  41 ,  42 . 
     It should be noted that the power supplied downhole tubing  26  and casing  24  is effective only for choke and control modules that are above the surface of any electrically conductive liquid that may be in annulus  31 . Chokes and modules that are immersed in conductive liquid cease to receive signals since such liquid creates an electrical short-circuit between tubing and casing before the signals reach the immersed chokes and modules. 
     Use of controllable valves  52  is preferable for several reasons. Conventional bellows valves often leak when they should be closed during production, resulting in wasteful consumption of lift gas. Additionally, conventional bellows valves  50  are usually designed with an operating margin of about 200 psi per valve, resulting in less than full pressure being available for lift. 
     Referring more specifically to FIGS. 2A and 2B, a more detailed illustration of controllable gas-lift valve  52  and side pocket mandrel  54  is provided. Side pocket mandrel  54  includes a housing  68  having a gas inlet port  72  and a gas outlet port  74 . When controllable valve  52  is in an open position, gas inlet port  72  and gas outlet port  74  provide fluid communication between annular space  31  and an interior of tubing string  26 . In a closed position, controllable valve  52  prevents fluid communication between annular space  31  and the interior of tubing string  26 . In a plurality of intermediate positions located between the open and closed positions, controllable valve  52  meters the amount of gas flowing from annular space  31  into tubing string  26  through gas inlet port  72  and gas outlet port  74 . 
     Controllable gas-lift valve  52  includes a generally cylindrical, hollow housing  80  configured for reception in side pocket mandrel  54 . An electronics module  82  is disposed within housing  80  and is electrically connected to a stepper motor  84  for controlling the operation thereof. Operation of stepper motor  84  adjusts a needle valve head  86 , thereby controlling the position of needle valve head  86  in relation to a valve seat  88 . Movement of needle valve head  86  by stepper motor  84  directly affects the amount of fluid communication that occurs between annular space  31  and the interior of tubing string  26 . When needle valve head  86  fully engages valve seat  88  as shown in FIG. 2B, the controllable valve  52  is in the closed position. 
     O-rings  90  are made of an elastomeric material and allow controllable valve  52  to sealingly engage side pocket mandrel  54 . Slip rings  92  surround a lower portion of housing  80  and are electrically connected to electronics module  82 . Slip rings  92  provide an electrical connection for power and communication between tubing string  26  and electronics module  82 . 
     Controllable valve  52  includes a check valve head  94  disposed within housing  80  below needle valve head  86 . An inlet  96  and an outlet  98  cooperate with inlet port  72  and outlet port  74  when valve  52  is in the open position to provide fluid communication between annulus  31  and the interior of tubing string  26 . Check valve  94  insures that fluid flow only occurs when the pressure of fluid in annulus  31  is greater than the pressure of fluid in the interior of tubing string  26 . 
     Referring to FIG. 3 in the drawings, an installation configuration for a controllable gas lift valve and its associated electronics support module is shown. In FIG. 3, tubing  26  includes an annularly enlarged pocket, or pod  100  formed on the exterior of tubing string  26 . Enlarged pocket  100  includes a housing that surrounds and protects controllable gas lift valve  132  and an electronics module  106 . In this mounting configuration, gas lift valve  132  is rigidly mounted to tubing string  26  and is not insertable and retrievable by wireline. Jumper  64  is fed through enlarged pocket  100  to connect electronics module  106  to the tubing above the choke  41  and thus receive power and communications signals, in conjunction with jumper  66  returned to the tubing below the choke. Electronics module  106  is rigidly connected to tubing string  26  and is not insertable or retrievable by wireline. 
     Controllable valve  132  includes a motorized needle valve head  108  and a check valve head  110  that are schematically illustrated in FIG.  2 . The valve heads  108 ,  110  cooperate to control fluid communication between annular space  31  and the interior of tubing string  26 . 
     A plurality of sensors are used in conjunction with electronics module  106  to control the operation of controllable valve  132  and gas lift well  320 . Pressure sensors, such as those produced by Three Measurement Specialties, Inc., can be used to measure internal tubing pressure, internal pod housing pressures, and differential pressures across gas lift valves. In commercial operation, the internal pod pressure is considered unnecessary. A pressure sensor  112  is rigidly mounted to tubing string  26  to sense the internal tubing pressure of fluid within tubing string  26 . A pressure sensor  118  is mounted within pocket  100  to determine the differential pressure across needle valve head  108 . Both pressure sensor  112  and pressure sensor  118  are independently electrically coupled to electronics module  106  for receiving power and for relaying communications. Pressure sensors  112 ,  118  are potted to withstand the severe vibration associated with gas lift tubing strings. 
     Temperature sensors, such as those manufactured by Four Analog Devices, Inc. (e.g. LM-34), are used to measure the temperature of fluid within the tubing, Chousing pod, power transformer, or power supply. A temperature sensor  114  is mounted to tubing string  26  to sense the internal temperature of fluid within tubing string  26 . Temperature sensor  114  is electrically coupled to electronics module  106  for receiving power and for relaying communications. The temperature transducers used downhole are rated for −50 to 300° F. and are conditioned by input circuitry to +5 to +255° F. The raw voltage developed at a power supply in electronics module  106  is divided in a resistive divider element so that 25.5 volts will produce an input to the analog/digital converter of 5 volts. 
     A salinity sensor  116  is also electrically connected to electronics module  106 . Salinity sensor  116  is rigidly and sealingly connected to the housing of enlarged pocket  100  to sense the salinity of the fluid in annulus  31 . 
     It should be understood that the embodiment illustrated in FIG. 3 could include or exclude any number of the sensors  112 ,  114 ,  116  or  118 . Sensors other than those displayed could also be employed in either of the embodiments. These could include gauge pressure sensors, absolute pressure sensors, differential pressure sensors, flow rate sensors, tubing acoustic wave sensors, valve position sensors, or a variety of other analog signal sensors. Similarly, it should be noted that an electronics module similar to electronics module  106  could be packaged with various sensors and deployed independently of controllable valve  132 . 
     Referring now to FIGS. 4A and 4B in the drawings, a controllable gas lift valve  132  having a valve housing  133  is mounted on a tubing conveyed mandrel  134 . Controllable valve  132  is mounted similar to most of the bellows-type gas lift valves that are in use today. These valves are not wireline replaceable, and must be replaced by pulling tubing string  26 . An electronics module  138  is mounted within housing  133  above a stepper motor  142  that drives a needle valve head  144 . A check valve  146  is disposed within housing  133  below needle valve head  144 . Stepper motor  142 , needle valve head  144 , and check valve  146  are similar in operation and configuration to those used in controllable valve  52  depicted in FIGS. 2A and 2B In similar fashion to FIG. 2B, an inlet port  148  and an outlet port  150  are furnished to provide a fluid communication path between annulus  31  and the interior of tubing string  26 . 
     Power and communications are supplied to electronics module  138  by a power and signal connector  136  which connects by jumper  64  to the tubing above choke  41 . Jumper  140  is connected between electronics module  138  and housing  133 , thus being effectively connected to tubing string  26  below choke  41 . 
     Although not specifically shown in the drawings, electronics module  138  could have any number of sensors electrically coupled to the module  138  for sensing downhole conditions. These could include pressure sensors, temperature sensors, salinity sensors, flow rate sensors, tubing acoustic wave sensors, valve position sensors, or a variety of other analog signal sensors. These sensors would be connected in a manner similar to that used for sensors  112 ,  114 ,  116 , and  118  of FIG.  3 . 
     Referring now to FIG. 5 in the drawings, an equivalent circuit diagram for gas lift well is illustrated and should be compared to FIG.  1 . Computer and power source  44  includes an AC power source  120  and a master modem  122  electrically connected between casing  24  and tubing string  26 . Electronics modules  50  are independently and permanently mounted in an enlarged pocket on tubing string  26 . Although not shown, the equivalent circuit diagram could also include depictions of electronics module  106  of FIG.  2 . 
     For purposes of the equivalent circuit diagram of FIG. 5, it is important to note that although electronics modules  50  appear identical, they may contain or omit different components and combinations such as sensors  112 ,  114 ,  116 ,  118  of FIG.  3 . Additionally, the electronics modules may or may not be an integral part of the controllable valve. Each electronics module includes a power transformer coupled to a diode bridge to generate DC which energizes the modem and other electronic components such as the valve motor controller. Each electronics module also contains a data transformer which is capacitively coupled to the input/output of the slave modem slave modem. 
     Referring to FIG. 6 in the drawings, a block diagram of a communications system  152  according to the present invention is illustrated. FIG. 6 should be compared and contrasted with FIGS. 1 and 5. Communications system  152  includes master modem  122 , AC power source  120 , and a computer  154 . Computer  154  is coupled to master modem  122 , preferably via an RS 232  bus, and runs a multitasking operating system such as Windows NT and a variety of user applications. AC power source  120  includes a 120 volt AC input  156 , a ground  158 , and a neutral  160  as illustrated. Power source  120  also includes a fuse  162 , preferably 7.5 amp, and has a transformer output  164  at approximately 6 volts AC and 60 Hz. Power source  120  and master modem  122  are both connected to casing  24  and tubing  26 . 
     Communications system  152  includes an electronics module  165  that is analogous to module  50  in FIG. 1, and module  106  in FIG.  3 . Electronics module  165  includes a power supply  166  and an analog-to-digital conversion module  168 . A programmable interface controller (PIC)  170  is electrically coupled to a slave modem  171  (analogous to slave modem of FIG.  5 ). Couplings  172  are provided for coupling electronics module  165  to casing  24  and tubing  26 . 
     Referring to FIG. 7 in the drawings, electronics module  165  is illustrated in more detail. Amplifiers and signal conditioners  180  are provided for receiving inputs from a variety of sensors such as tubing temperature, annulus temperature, tubing pressure, annulus pressure, lift gas flow rate, valve position, salinity, differential pressure, acoustic readings, and others. Some of these sensors are analogous to sensors  112 ,  114 ,  116 , and  118  shown in FIG.  3 . Preferably, any low noise operational amplifiers are configured with non-inverting single ended inputs (e.g. Linear Technology LT1369). All amplifiers  180  are programmed with gain elements designed to convert the operating range of an individual sensor input to a meaningful 8 bit output. For example, one psi of pressure input would produce one bit of digital output, 100 degrees of temperature will produce 100 bits of digital output, and 12.3 volts of raw DC voltage input will produce an output of 123 bits. Amplifiers  180  are capable of rail-to-rail operation. 
     Electronics module  165  is electrically connected to master modem  122  at the surface via casing  24  and tubing string  26 . Address switches  182  are provided to address a particular device from master modem  122 . As shown in FIG. 7, 4 bits of addresses are switch selectable to form the upper 4 bits of a full 8 bit address. The lower 4 bits are implied and are used to address the individual elements within each electronics module  165 . Thus, using the configuration illustrated, sixteen modules are assigned to a single master modem  122  on a single communications line. As configured, up to four master modems  122  can be accommodated on a single communications line. 
     Electronics module  165  also includes PIC  170 , which preferably has a basic clock speed of 20 MHz and is configured with  8  analog-to-digital inputs  184  and 4 address inputs  186 . PIC  170  includes a TTL level serial communications UART  188 , as well as a stepper motor controller interface  190 . 
     Electronics module  165  also contains a power supply  166 . A nominal 6 volts AC line power is supplied to power supply  166  along tubing string  26 . Power supply  166  converts this power to plus 5 volts DC at terminal  192 , minus 5 volts DC at terminal  194 , and plus 6 volts DC at terminal  196 . A ground terminal  198  is also shown. The converted power is used by various elements within electronics module  165 . 
     Although connections between power supply  166  and the components of electronics module  165  are not shown, the power supply  166  is electrically coupled to the following components to provide the specified power. PIC  170  uses plus 5 volts DC, while slave modem  171  uses plus and minus 5 volts DC. A stepper motor  199  (analogous to stepper motor  84  of FIG.  2 B and stepper motor  142  of FIG. 4B) is supplied with plus 6 volts DC from terminal  196 . Power supply  166  comprises a step-up transformer for converting the nominal 6 volts AC to 7.5 volts AC. The 7.5 volts AC is then rectified in a full wave bridge to produce 9.7 volts of unregulated DC current. Three-terminal regulators provide the regulated outputs at terminals  192 ,  194 , and  196  which are heavily filtered and protected by reverse EMF circuitry. Modem  171  is the major power consumer in electronics module  165 , typically using 350+milliamps at plus/minus 5 volts DC when transmitting. 
     Modem  171  is a digital spread spectrum modem having an IC/SS power line carrier chip set such as models EG ICS1001, ICS1002 and ICS1003 manufactured by National Semiconductor. Modem  171  is capable of 300-3200 baud data rates at carrier frequencies ranging from 14 kHz to 76 kHz. U.S. Pat. No. 5,488,593 describes the chip set in more detail and is incorporated herein by reference. While they are effective and frequently employed in applications such as this, spread-spectrum communications are not a necessity and other communication methods providing adequate bandwidth would serve equally well. 
     PIC  170  controls the operation of stepper motor  199  through a stepper motor controller  200  such as model SA1042 manufactured by Motorola. Controller  200  needs only directional information and simple clock pulses from PIC  170  to drive stepper motor  199 . An initial setting of controller  200  conditions all elements for initial operation in known states. Stepper motor  199 , preferably a MicroMo gear head, positions a rotating stem control needle valve  201  (analogous to needle valve heads  86  of FIG. 2B and 144 of FIG.  4 B), which is the principal operative component of the controllable gas lift valve. Stepper motor  199  provides 0.4 inch-ounce of torque and rotates at a maximum of 500 steps per second. A complete revolution of stepper motor  199  consists of  24  individual steps. The output of stepper motor  199  is directly coupled to a 989:1 gear head which produces the necessary torque to open and close needle valve  201 . The continuous rotational torque required to open and close needle valve  201  is 3 inch-pounds with 15 inch-pounds required to seat and unseat the valve  201 . 
     PIC  170  communicates through digital spread spectrum modem  171  to master modem  122  via casing  24  and tubing string  26 . PIC  170  uses a MODBUS 584/985 PLC communications protocol. The protocol is ASCII encoded for transmission. 
     The present invention makes use of an electronics module and a combination of sensors to provide valuable information about the downhole characteristics of a well. Sensors such as sensors  112 ,  114 ,  116 , and  118  of FIG. 3 are used with electronics modules similar to those illustrated in FIGS. 3,  6  and  7 . 
     It will be apparent to those skilled in the art that the systems and methods defined by reference to FIGS. 1-7 may be applied in alternative embodiments. 
     Referring now to FIGS. 8 and 9 in the drawings, well  200  is different than petroleum well  320  of FIG.  1 . Well  200  includes a conventional wellhead  202  at the surface  12  above hanger  22 . Gas inlet valve  204  is fluidly connected to annulus  31  for injecting compressed gas between tubing string  26  and casing  24 . Those skilled in the art will appreciate that surfactants and other chemicals can be injected through gas inlet valve  204  into the well. An upper ferromagnetic choke  40  is disposed between casing  24  and tubing string  26  below the casing hanger  22  to impede current flows as previously explained. An electrically isolated tubing section  210  is provided as an alternative to a lower ferromagnetic choke. Since tubing section  210  acts an impedance to current flow, a power and communications path is established in a section of tubing string  26  between upper choke  40  and tubing section  210 . 
     Petroleum well  200  has one or more data monitoring pods, or electronics modules  214  on tubing string  26 , the number and type of each pod depending on the requirements of the individual well  200 . Each of the data monitoring pods  214  are individually addressable via wireless spread spectrum communication through tubing string  26  and casing  24 . A master surface modem  206  and an AC low voltage power supply  208  are electrically connected to tubing string  26  below upper choke  40  to power and communicate with sensors and the data monitoring pods  214  downhole. 
     Well  200  includes a side pocket mandrel  212  integrally coupled to tubing string  26  downhole. Data monitoring pod  214  has a housing  215  and is electrically coupled to tubing section  26  between upper choke  40  and isolated tubing section  210 . This connection is schematically illustrated as an insulated wire  216 . A grounding lead  218  is connected to bow spring centralizer  60  for grounding data monitoring pod  214  to casing  24  as shown. Preferably, data monitoring pod  214  is wireline insertable and retrievable in side pocket mandrel  212  using conventional slick-line methods. Data monitoring pod  214  is connected to the side pocket mandrel  212  by slip rings to provide power and communication connections between the housing of the mandrel  212  and the data monitoring pod  214 . 
     Referring still to FIGS. 8 and 9 in the drawings, but also to FIG. 10, the disposition of data monitoring pod  214  in side pocket mandrel  212  is illustrated in more detail. In a preferred form, one or more sensors are coupled to data monitoring pod  214 , similar to the electronic sensors illustrated in FIG.  3 . Data monitoring pod  214  includes electronics for a sensor package  220  that measures physical conditions in the well bore such as seismic conditions, acoustic conditions, pressure conditions, temperature conditions, and others. An annulus pressure port  222  and transducer  223  are disposed in housing  215  of data monitoring pod  214  for sensing the pressure of the annulus fluid, i.e., the fluid between casing  24  and tubing string  26 . Other measurements of specific characteristics of the annulus fluid can easily be made through annulus pressure port  222  such as temperature via the transducer  223  as shown. In similar fashion, a tubing pressure port  224  and transducer  225  are disposed in housing  215  for sensing the fluid pressure inside tubing  26 . Other physical characteristics of the tubing fluid can also be made through tubing pressure port  224 . 
     Data monitoring pod  214  is slidably received within the cylindrical bore of side pocket mandrel  212  such that a stepped portion of housing  215  is in abutting engagement with a landing shoulder  226 . As mentioned previously, data monitoring pod is insertable and retrievable by wireline. After installation in side pocket mandrel  212 , a ground pin feed-through at  228  forms an electrical connection between mandrel  212  and data monitoring pod  214 . 
     Referring more specifically to FIG. 10, data monitoring pod  214  is illustrated in more detail. A power and communication circuits board  230  includes sensor inputs  232  for connecting to sensor packages  220  (connection not shown) so that physical data such as pressure, temperature, acoustic, and seismic data can be measured. The input signals are conditioned at a signal conditioner  234  before transfer to a programmable interface controller (PIC)  236 . A spread-spectrum modem  238  connects to tubing string  26  though a modem coupling network  240 . A power supply transformer  244  is adapted for receiving low voltage AC power along tubing string  26  and is electrically connected to a DC power supply  242 . 
     Referring again to FIG. 8 in the drawings, and also to FIG. 10, well  200  includes a geophone  246 , which is operatively attached to casing  24  with its power and communication leads connected to data monitoring pod  214  through a pressure seal in side pocket mandrel  212 . A rubber hanger  248  operates as an acoustic end coupler to connect geophone  246  to electronics module  214 . A spring steel bracket  250  provides the mechanical force to impinge geophone  246  on casing  24 . The ground and geophone electromagnetic wiring (not shown) is passed through a feed-through  252  and is electrically connected to data monitoring pod  214 . 
     Master modem  206  at surface  312  and an associated controller communicate to one or more slave modems  238  located in or adjacent to each data monitoring pod  214 . Data monitoring pod  214  and slave modem  238  report measurements from the various sensors downhole to master modem  206  via tubing string  26  and casing  24 . A surface computer (not shown) continuously combines and analyzes downhole data as well as surface data to compute a real-time tubing pressure profile. Many uses of this data from the sensors are possible. For example, an optimal gas lift flow rate for each controllable gas lift valve may be computed from this data. Preferably pressure measurements are taken on locations uninfluenced by gas lift injection turbulence. Acoustic sensors (sounds less than 10 kilohertz) listen for tubing bubble patterns. Data is sent via slave modem  238  directly to surface modem  206 . Alternatively, data can be sent to a mid-hole data monitoring pod  214  and relayed to the surface computer. 
     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 wells, including but not limited to water wells and natural gas wells. 
     One skilled in the art will see that the present invention can be applied in many areas where there is a need to provide sensors, power, and communication within a borehole, well, or any other area that is difficult to access. 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 communications to sensors 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 may have a same or similar path as that desired for routing power and communications to a plurality of sensors. 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 also be used as a piping structure and/or electrical return for transmitting power and communications to sensors 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 communications to sensors 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 communications to sensors 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 communications to sensors in accordance with the present invention. Thus, there are numerous applications of the present invention in many different areas or fields of use. 
     It should be apparent from the foregoing that an invention having significant advantages has been provided. While the invention is shown in only a few of its forms, it is not just limited but is susceptible to various changes and modifications without departing from the spirit thereof.

Summary:
A petroleum well has an electronic module and a number of sensors which communicate with the surface using the tubing string and casing as conductors. Induction chokes at the surface and downhole electrically impede AC flow through the (tubing or casing if so configured) with the resulting voltage potential useful for power and communication. A high bandwidth, adaptable spread spectrum communications system is used to communicate between the downhole electronics module and a surface master spread spectrum modem. Downhole sensors, such as pressure, temperature, acoustic and seismic sensors accurately assess downhole physical conditions. In a preferred form, the electronics module and sensors are wireline insertable and retrievable into a side pocket mandrel in the tubing string. Permanent downhole sensors that can communicate with the surface allow such diverse applications as optimizing well and field performances, monitoring and assessing the geophysics of the fomrations around the well, assessing well and reservoir reserves, assessing reservoir conditions.