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TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to a downhole apparatus and method for generating electricity and, in particular to, a downhole electrical generator that uses lift fluid pressure to produce electricity which is used to operate other downhole devices. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the invention, its background is described in connection with the operation of downhole electrical devices, as an example. The control and operation of oil and gas production wells constitute an important and ongoing concern of the petroleum industry. As an example, well control has become particularly important and more complex in view of the industry wide development of multilateral wells. Generally speaking, multilateral wells have multiple branches each having discrete production zones which produce fluid into common or independent production tubing. In either case, there is a need for controlling zone production, isolating specific zones and otherwise monitoring each zone in a particular well. As a result, the methods and devices used for controlling wells are growing more complex. In fact, downhole control systems which include downhole computerized modules employing downhole computers for commanding downhole tools such as packers, sliding sleeves and valves are becoming more common. 
     For example, using downhole sensors, a downhole computer controlled system may monitor actual downhole parameters such as pressure, temperature and flow to automatically execute control instructions based upon the monitored downhole parameters. As should apparent, operating such a well control systems will require electrical power. It has been found, however, that presently known methods of supplying or generating electricity downhole suffer from a variety of problems and deficiencies. 
     In one method, electricity may be supplied downhole by lowering a tool on a wireline and conducting electricity through one or more conductors in the wireline from the surface to the tool. Similarly, hardwires may be attached on the exterior of the tubing running from the surface to the desired downhole location. These techniques, however, are not desirable due to their cost and complexity. In addition, in deep wells, there can be significant energy loss caused by the resistance or impedance in the wires. 
     Downhole electrical circuits utilizing batteries housed within a downhole assembly have also been attempted. These batteries, however, can only provide moderate amounts of electrical energy at the elevated temperatures encountered downhole. In addition, batteries have relatively short lives requiring frequent replacement and/or recharging. 
     Other attempts have been made to provide a downhole mechanism which continuously generates and supplies electricity. For example, systems using radioisotopes, fuel cells and piezoelectric techniques have been attempted. These systems, however, have raised safety and environmental concerns, are expensive and complex and/or do not generate suitable amounts of electricity. 
     A more promising approach to supplying electricity downhole appears to be the use of downhole electrical generators. Previous attempts to operate downhole generators, however, have met with limited success. Specifically, many downhole generators are installed within the tubing string which prevents the passage of other tools or equipment therethrough. Other downhole generators have been proposed that are installed in side pockets thus allowing passage of equipment through the tubing. 
     All of these downhole generators, however, suffer from a serious drive problem. Specifically, the turbines of these downhole generators are rotated by the upward flow of production fluids. Not only does this create an undesirable pressure drop in the production fluids, but use of production fluids to drive turbines significantly limits the life expectancy of these downhole generators. Specifically, the mechanical and chemical qualities of production fluids tend to erode and corrode the turbine as well as other components of these downhole generators. In addition, tars and suspended solids in the production fluid tend to clog flow passageways within these downhole generators and prevent proper rotation of the rotors. Also, the amount of the electrical output of these production fluid driven downhole generators is controlled by the flow rate of production fluid through the tubing which is dependent, in part, upon the pressure in the formation which decreases over time. 
     Therefore, a need has arisen for a downhole generator that is not driven by the flow of production fluids through the tubing. A need has also arisen for such a downhole generator that does not cause a pressure drop within the production fluids. Further, a need has arisen for such a downhole generator wherein the electrical output is not dependent upon the pressure in the formation from which the production fluids are produced. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises a lift fluid driven downhole electrical generator that does not use the flow of formation fluids to drive a turbine. As such, the lift fluid driven downhole electrical generator of the present invention does not choke the flow of formation fluids up through the tubing. In addition, the electrical output of the lift fluid driven downhole electrical generator of the present invention is not dependent upon the flow rate of formation fluids or the pressure in the formation from which the formation fluids are produced. 
     Broadly characterized, the lift fluid driven downhole electrical generator, once positioned downhole in a tubing string, converts the lift fluid pressure into electricity. For example, the lift fluid may be used to create rotary motion by impinging the lift fluid against a rotor. The rotary motion may then be converted to electricity by rotating a first portion of an electromagnetic assembly relative to a second portion of the electromagnetic assembly. 
     The lift fluid driven downhole electrical generator comprises a housing having one or more lift fluid ports in a sidewall portion thereof for receiving the lift fluid from the annulus surrounding the tubing string. A flow control device that is slidably disposed within the housing is used to selectively allow and prevent the flow of lift fluid through the lift fluid port. The openness of the lift fluid port may be controlled by the operation of an actuator that is operably coupled to the flow control device. The actuator may infinitely vary the openness of the lift fluid port between the fully open and fully closed positions in response to a signal from the surface received by a downhole telemetry system, a signal from a downhole sensor or a timer. Alternatively, a controller may be used to monitor the electrical output of the downhole generator and then send a signal to adjust the position of the flow control device relative to the lift fluid port to vary the electrical output of the downhole generator if desired. 
     When the lift fluid ports are open, a rotor, rotatably disposed within the housing, converts the lift fluid pressure to rotary motion as the lift fluid impinges the rotor. The rotation of the rotor is imparted on the first portion of the electromagnetic assembly which is rotatable relative to the second portion of the electromagnetic assembly, which is stationary with the housing. This relative rotation within the electromagnetic assembly converts the rotary motion to electricity. The first portion of the electromagnetic assembly includes a plurality of electrical windings wrapped around a core. One end of the electrical windings is electrically coupling to a first portion of a commutator and the other end of the electrical windings is electrically coupling to a second portion of the commutator. The second portion of the electromagnetic assembly includes magnets and at least two contact members that are stationary with the housing of the downhole electrical generator. In operation, when the first portion of the electromagnetic assembly is rotated relative to the second portion of the electromagnetic assembly, a first contact member sequentially engages the first portion of the commutator then the second portion of the commutator while a second contact member simultaneously sequentially engages the second portion of the commutator then the first portion of the commutator. As such, electricity is generated by the lift fluid driven downhole electrical generator of the present invention. 
     In addition, the present invention may be used to control the electrical output of a lift fluid driven downhole electrical generator. This is achieved by positioning the downhole electrical generator within a tubing string, injecting a lift fluid down an annulus surrounding the tubing string, providing a fluid communication path through the downhole electrical generator by varying the position of a flow control device relative to a lift fluid port, communicating lift fluid through the lift fluid port, rotating a rotor and an electromagnetic assembly such that electricity is generated in response to the flow of lift fluid through the fluid communication path, sensing the generated electricity to determine the electrical output of the downhole electrical generator and adjusting the flowrate of lift fluid through the fluid communication path by selectively varying the position of the flow control device relative to the lift fluid port, thereby controlling the electrical output of the downhole generator. 
     More specifically, the step of sensing the generated electricity to determine the electrical output of the downhole electrical generator may include receiving a signal indicative of the magnitude of the electricity being generated with a controller, processing the signal in the controller and generating a control signal with the controller to vary the position of the flow control device relative to the lift fluid port. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a schematic illustration of an offshore oil and gas production platform operating a lift fluid driven downhole electrical generator of the present invention; 
     FIG. 2 is a partial cross sectional view of a lift fluid driven downhole electrical generator of the present invention in its closed position; 
     FIG. 3 is a partial cross sectional view of a lift fluid driven downhole electrical generator of the present invention in its fully open position; and 
     FIG. 4 is a partial cross sectional view of a lift fluid driven downhole electrical generator of the present invention in a partially open position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
     Referring to FIG. 1, an offshore oil and gas production platform operating a lift fluid driven downhole electric generator is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . Wellhead  18  is located on deck  20  of platform  12 . Well  22  extends through the sea  24  and penetrates the various earth strata including formation  14  to form wellbore  26 . Disposed within wellbore  26  is casing  28 . Disposed within casing  28  and extending from wellhead  18  is production tubing  30 . A pair of seal assemblies  32 ,  34  provide a seal between tubing  30  and casing  28  to prevent the flow of production fluids therebetween. During production, formation fluids enter wellbore  26  through perforations  36  in casing  28  and travel into tubing  30  to wellhead  18 . 
     Coupled within tubing  30  is a lift fluid driven downhole electrical generator  38 . Downhole electrical generator  38  is driven by lift fluid communicated thereto from surface installation  40 , through fluid conduit  42  and the annulus between casing  28  and tubing  30  as will be explained in greater detail below. 
     In addition, the lift fluid may be used to enhance the recovery of hydrocarbons from formation  14  by decreasing the hydrostatic head of the column of formation fluid in wellbore  26 . Decreasing the hydrostatic head enhances recovery by reducing the amount of pressure required to lift the formation fluids to the surface. Decreasing the density of the column of fluid extending from formation  14  to the surface reduces the hydrostatic head of this fluid column. As such, mixing a lower density fluid into the formation fluids reduces the overall density of the fluid column and consequently decreases the hydrostatic head. Accordingly, low density fluids, including liquids such as a hydraulic fluid or gases may be used. 
     Even though FIG. 1 depicts a vertical well, it should be noted by one skilled in the art that the present invention is equally well-suited for slanted wells, deviated wells or horizontal wells. Also, even though FIG. 1 depicts an offshore operation, it should be noted by one skilled in the art that the present invention is equally well-suited for use in onshore operations. 
     Referring now to FIG. 2, therein is depicted a lift fluid driven downhole electrical generator of the present invention that is generally designated  50 . Generator  50  has an outer housing  52  that is a substantially cylindrical tubular member that is threadedly and sealingly coupled to tubing string  30 , as seen in FIG. 1, at its upper and lower ends. It should be apparent to those skilled in the art that the use of directional terms such as top, bottom, above, below, upper, lower, upward, downward, etc. are used in relation to the illustrative embodiments as they are depicted in the figures, the upward direction being toward the top of the corresponding figure and the downward direction being toward the bottom of the corresponding figure. As such, it is to be understood that the downhole components described herein may be operated in vertical, horizontal, inverted or inclined orientations without deviating from the principles of the present invention. 
     Housing  52  has a primary flow passageway  54  extending longitudinally therethrough. Housing  52  also has one or more lift fluid ports  56  radially extending through the side wall thereof. In the illustrated embodiment, multiple ports  56  are disposed around the same circumference of housing  52 , however, other ports could be disposed either above or below ports  56  along the length of housing  52  if desired. 
     Housing  52  can be made of any suitable material, such as metal, plastic or ceramic capable of withstanding the pressures, temperatures and substances downhole. The material for housing  52  may be machined or formed to have a desired shape and size including a radially expanded inner diameter region  58  and interior cavities  60 ,  62  and  64  for purposes to be described below. 
     Disposed within radially expended inner diameter region  58  of housing  52  is an inner subassembly  70  that is rotatably and axially moveable relative to housing  52 . Inner subassembly  70  has a primary flow passageway extending longitudinally therethrough that preferable has the same inner diameter as primary flow passageway  54  of housing  52 . Inner subassembly  70  includes a flow control device  72  for selectively allowing fluid flow or preventing fluid flow through ports  56 . Flow control device  72  is disposed in housing  52  such that flow control device  72  is moveable between a closed position, fully obstructing ports  56 , as best seen in FIG. 2, a fully open position, completely unobstructing ports  56 , as best seen in FIG. 3, and a partially open position partially obstructing ports  56 , as best seen in FIG.  4 . As will be explained below, the position of flow control device  72  is infinitely variable relative to ports  56  such that the electrical output of generator  50  may be controlled. 
     In the illustrated embodiment, flow control device  72  is an annular body made of a suitable material providing for a bearing seal between the exterior surface of flow control device  72  and the interior surface of housing  52 , such as a metal-to-metal seal. As illustrated, the height of flow control device  72  is sufficient to overlie ports  56  when ports  56  are to be closed. 
     Alternatively, instead of using an integral flow control device such as flow control device  72 , the flowrate of lift fluid into lift fluid ports  56  may be controlled by lift fluid valves installed within lift fluid ports  72  or in a side pocket mandrel adjacent thereto. The openness of the lift fluid valves may be controlled using known techniques, but are preferably electrically controlled. 
     Inner subassembly  70  includes a rotor  74  that provides an interface with the lift fluid whereby rotor  74  is driven by the lift fluid entering generator  50  through ports  56 . Rotor  74  is used to convert fluid flow to mechanical power. Specifically, rotor  74  is connected to flow control device  72  such that as flow control device  72  opens ports  56 , flow of a lift fluid into ports  56  impinges rotor  74  to rotate rotor  74 . In one embodiment, the connection between rotor  74  and flow control device  72  is such that both move linearly and rotate together. In another embodiment, joint linear movement occurs but rotor  74  can rotate relative to flow control device  72  using, for example, a sealed bearing coupling. 
     In the illustrated embodiment, rotor  74  has two degrees of motion. Rotor  74  can rotate about its longitudinal axis and rotor  74  can move linearly or axially within housing  52 . In the illustrated embodiment, this linear movement occurs simultaneously with and in conjunction with the longitudinal movement of flow control device  72 . As illustrated, flow control device  72  and rotor  74  are linearly disposed and adjoin each other within radially expended inner diameter region  58  of housing  52 . 
     Rotor  74  of the illustrated embodiment has a cylindrical squirrel cage configuration comprising a plurality of angled vanes  76  that are circumferentially separated such that the spaces between vanes  76  permit radial fluid flow between the outside and the inside of rotor  74  and such that an axial channel is defined through rotor  74  to permit axial flow between adjoined vanes  76  as well as through generator  50 . As such, rotor  74  is driven by lift fluid flowing into generator  50  through open ports  56 . The resulting mechanical power of rotor  74  is used to generate electricity as explained below. 
     As mentioned above, rotor  74  and flow control device  72  are connected such that they can be moved linearly within housing  52 . In the illustrated embodiment, this movement is caused by an actuator  78 . Actuator  78  moves flow control device  72  and rotor  74  linearly to variably adjust the openness of ports  56  and to provide infinite flow control throughout the continuum between fully closed and fully opened. 
     Actuator  78  is mounted within interior cavity  64  of housing  52  and is coupled to inner subassembly  70  linking actuator  78  with rotor  74 . Operation of actuator  78  moves inner subassembly  70 , including rotor  74  and flow control device  72  axially within housing  52  to displace flow control device  72  relative to ports  56 . 
     In the illustrated embodiment, actuator  78  includes a motor  80 . Motor  80  includes a rotating element  82  having a threaded inner surface which engages a threaded outer surface of a ring  84 . Ring  84  is axially fixed with respect to linear movement relative to mandrel  86  of inner subassembly  70  by retaining rings  88 ,  89 . Ring  84  is rotatably coupled on mandrel  86  such that mandrel  86  can rotate inside ring  84 . To obtain axial movement, ring  84  is maintained rotationally stationary relative to rotating element  82  of motor  80  so that operation of rotating element  82  drives ring  84  and mandrel  86  up or down as desired. 
     Alternatively, linear movement of inner subassembly  70  inside housing  52  could be achieved manually using a shifting tool. For example, such a shifting tool can be connected to either end of inner subassembly  70  and operated to mechanically pull or push inner subassembly  70  up or down. 
     In the illustrated embedment, when actuator  78  has moved flow control device  72  to a partially or fully open position, lift fluid induced rotation of rotor  74  may now occur. Such rotation, in turn, causes operation of an electromagnetic assembly  90 . Electromagnetic assembly  90  provides an electrical interface which converts mechanical power to electricity. 
     Electromagnetic assembly  90  includes a mandrel  92  that provides support for a plurality of electrical windings  94 , a plurality of pole pieces  96  and a commutator  98 , which are also considered to be part of electromagnetic assembly  90 . Mandrel  92  is connected to rotor  74 . As illustrated, mandrel  92  and rotor  74  are integral and unitary, being constructed with the same tubing piece. Mandrel  92  is also coupled to mandrel  86 . 
     The plurality of electrical windings  94  are wound on mandrel  92 . The plurality of pole pieces  96  are disposed radially outwardly of windings  94  so that pole pieces  96  overlie windings  94 . Commutator  98  serves as a brush ring and is connected to electrical windings  94  in a known manner so that one end of windings  94  is connected to one or more electrically parallel segments of commutator  98  and the other end of windings  94  is connected to one or more different electrically parallel segments of commutator  98 . Commutator  98  is made of suitable electrically conductive material. 
     Electromagnetic assembly  90  also includes a plurality of magnets  100  mounted within interior cavity  60  of housing  52  such that magnets  100  interact with electromagnetic fields generated by electrical windings  94 . The position of cavity  60 , and thus of magnets  100  within cavity  60 , is such that magnets  100  and pole pieces  96  are substantially aligned throughout the linear travel of inner subassembly  70  within housing  52 . 
     Electromagnetic assembly  90  also includes a plurality of contacts  102  mounted within interior cavity  62  of housing  52 . In the illustrated embodiment, contacts  102  are electrically conductive members such as brushes, that overlie and engage respective segments of commutator  98 . At least one contact  102  engages one section of commutator  98  connected to one end of windings  94  and at least one other contact  102  engages a different section of commutator  98  connected to the other end of windings  94 . Contacts  102  and commutator  98  are sized sufficiently so that electrical contact is made throughout the linear movement of inner subassembly  70  relative to housing  52 . Contacts  102  provide an interface to electrical wires such as wires  104 ,  106 . Electricity generated by the present invention travels within wires  104 ,  106 . This electricity can be used for powering devices for sensing parameters of the production fluid such as temperature, pressure, flow, density and the like using downhole sensors  108 ,  110 . Likewise, the electricity may be used to power a downhole telemetry system  112  that may communicate with the surface via pressure pulses, acoustics, electromagnetic waves or other suitable wireless techniques. In addition, the electricity may be used to recharge batteries  114 . 
     To keep the lift fluid within the rotor section of inner subassembly  70  and to isolate the electrical components of electromagnetic assembly  90  from the lift fluid, the illustrated embodiment includes three seals. An O-ring seal  116  is mounted in a groove defined around the upper end of flow control device  72 . This places seal  116  above ports  56 . Seal  116  provides a fluid seal between flow control device  72  and the inner surface of housing  52 . 
     An O-ring seal  118  is mounted in a groove in mandrel  92  near the juncture of rotor  74  and mandrel  92 . Seal  118  provides a fluid seal between mandrel  92  and the inner surface of housing  52  between cavity  60  and ports  56 . This places seal  118  below ports  56 , and thus on the opposite side of ports  56  from seal  116 , thereby limiting the axial travel of the lift fluid therebetween. 
     O-ring seal  120  is mounted in a groove on mandrel  86  between commutator  98  and upper retaining ring  88  of actuator  78 . Seal  120  provides a fluid seal between mandrel  86  and the inner surface of housing  52  between cavities  62 ,  64 . 
     An additional O-ring seal  122  is mounted in a groove on the lower end of inner subassembly  70  to prevent the entry of dirty formation fluids between inner subassembly  70  and housing  52 . 
     Generator  50  can be operated remotely using an onboard controller  124  housed within housing  52 . Controller  124  is of any suitable type to provide the necessary control and signal processing associated with the operation of generator  50  such as a microprocessor, however, other types of digital or analog controllers can be used. 
     In the illustrated embodiment, controller  124  receives electricity from wires  104 ,  106 . Controller  124  can be used to distribute the electricity to the various electrical components associated with generator  50 . For example, controller  124  may be used to provide electricity as well as operation information to sensors  108 ,  110  to obtain reading for pressure, temperature, density, flow rate or similar parameters associated with the production fluids. This information may then be returned to controller  124  and stored in a memory device associated with controller  124 . Thereafter, controller  124  may provide electricity and operating parameters to telemetry device  112  such that information received from sensors  108 ,  110  may be wirelessly sent to the surface via pressure pulses, acoustics, electromagnetic waves or other suitable techniques known in the art. In addition, controller  124  may direct electricity to batteries  114  for storage and later use when, for example, generator  50  is not generating electricity. 
     Controller  124  may also be used to control the electrical output of generator  50 . Specifically, controller  124  may monitor a characteristic of the generated electricity, for example magnitude. This sensed electricity can be correlated to the flow rate of lift fluid through ports  56 . As such, the degree of openness of ports  56  may be adjusted to create the desired electrical output. For example, if it is desired to produce more electricity based upon the electricity characteristic monitored by controller  124 , then controller  124  can send a signal to actuator  78  to upwardly shift inner subassembly  70  and increase the degree of openness of ports  56 . Alternatively, if it is determined by controller  124  that less electricity should be produced, then controller  124  can send a signal to actuator  78  to downwardly shift inner subassembly  70  and decrease the degree of openness of ports  56 . 
     In operation, generator  50  generates electricity by at least partially unobstructing ports  56  by upwardly shifting flow control device  72  such that lift fluid in the annulus outside generator  50  flows through ports  56  into the flow channel inside rotor  74 , as best seen in FIG.  4 . This is performed in the illustrated embodiment of generator  50  by wirelessly sending a signal from the surface to telemetry system  112  to open ports  56 . This signal is sent to controller  124  where it is processed and sent to motor  80 . Motor  80  receives electricity from batteries  114  then operates rotating element  82  to axially upwardly shift ring  84 . This upwardly moves rotor  74  and flow control device  72  to open ports  56 . Alternatively, controller  124  can have an internal timer by which it is programmed to respond at preset time intervals to turn motor  80  on and off. Likewise, controller  124  may prompt motor  80  to operate based upon changes in the production fluid parameters sensed by sensors  108 ,  110 . 
     The present invention uses feedback regarding the amount of electricity being generated by generator  50  in response to the lift fluid flow through rotor  74  with controller  124 . When the electrical signal indicates the desired electrical parameter is being achieved, motor  80  can be de-energized to stop the linear movement of inner subassembly  70 . Alternatively, motor  80  can be used to move inner subassembly  70  up and down to, respectively, increase or decrease the electrical output of generator  50  as desired. 
     When flow control device  72  has at least partially opened ports  56 , lift fluid drives rotor  74  which, in turn, rotates windings  94  and pole pieces  96  relative to magnets  100  and rotates commutator  98  relative to contacts  102  such that electricity is generated. 
     Another aspect of the operation of the present invention is moving flow control device  72 , together with rotor  74 , to selectively block ports  56 . As explained above, these components are moved together axially within housing  52 . The axial movement occurs in response to any suitable force which can be internally generated or externally applied. In the illustrated embodiment, motor  80  can be energized to drive inner subassembly  70  downwardly within housing  52  such that flow control device  72  closes ports  56  and prevents lift fluid from entering ports  56 . 
     It should be noted by those skilled in the art that even though the illustrated embodiments have depicted a rotatable electromagnetic assembly as the means for generating electricity, lift fluid could alternatively be used to provide the energy to generate electricity using other types of electricity generating devices including, but not limited to, expandable bladders, vibrating reeds, piezoelectric wafer stacks and the like, all of which are contemplated and considered within the scope of the present invention. 
     While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.

Summary:
A lift fluid driven downhole electrical generator and method for generating and controlling the electrical output from the electrical generator is disclosed. The electrical generator comprises a housing having a lift fluid port in a sidewall portion thereof for allowing the flow of lift fluids therethrough. A rotor is rotatably disposed within the housing. The rotor converts lift fluid pressure to rotary motion when the lift fluid travels through the lift fluid port and impinges the rotor. The electrical generator also includes an electromagnetic assembly having a first portion that is rotatable with the rotor and a second portion that is stationary with the housing. The electromagnetic assembly converts the rotary motion to electricity as the first portion of an electromagnetic assembly rotates relative to the second portion of the electromagnetic assembly.