Patent Publication Number: US-2016245047-A1

Title: Apparatus, System and Method For Generating Power In A Wellbore

Description:
PRIORITY CLAIM 
     The present application is a continuation of U.S. patent application Ser. No. 13/231,652, filed on Sep. 13, 2011, incorporated herein by reference. 
    
    
     ACKNOWLEDGMENT OF GOVERNMENT SUPPORT 
     The invention described herein was made in the performance of work under a NASA contract NNN12AAO1C, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title. 
    
    
     FIELD OF THE INVENTION 
     The invention is directed to a system, apparatus and method for generating power in a wellbore. 
     BACKGROUND OF THE INVENTION 
     In modern oil and gas production it is desirable to position in a well devices that require a source of electrical power. Energy consuming devices may be employed downhole to perform a variety of tasks. For example, sensors may be positioned within a casing for monitoring fluid flow pressure, temperature, flow rates, and to characterize the fluid content of the flow. Other sensing devices can be useful to measure fluid viscosity and density. 
     With the advent of reservoir management techniques, sophisticated monitoring and valve systems have been integrated into wellbore tubing. This allows portions of a subterranean formation to be open to producing oil/gas up the wellbore, while at the same time other portions of the formation are sealed to avoid production into the wellbore. This may be accomplished by controlling downhole valves. To achieve maximum reservoir performance, it may be necessary to flow from only selected portions of the reservoir at a given time. Subterranean flow monitors and flow control systems may require an external power source to activate such valves during the life of a wellbore. 
     Modern subsea wells may be drilled to depths of as much as 20,000 feet or more, extending from the mudline on the ocean floor downward to the hydrocarbon producing formation. Such wells may utilize downhole valves that must be activated to open or close various portions of the reservoir to production. The determination of when valves should be open or closed may depend upon the character of the fluid being produced by the formation over the life of the well. Such wells may have a life of 20-25 years or more. Downhole sensors may be employed to determine when the wellbore conditions require the opening or closing of such valves, based in part on temperature, pressure, or content of the fluids produced at a given time during the productive life of a well. 
     In the past, it has been common to deploy electrical conductive cables for miles into a reservoir down to a completion string near the bottom of the well. Such electrical conductive cables fail for a variety of reasons, which may require expensive and time consuming workover operations to repair the cables. Such cables are very expensive and heavy, and require substantial expertise and care to deploy downhole. Workover operations to repair damaged cables may require several weeks of effort, and may cost as much as 22 million dollars. 
     There is a need in the industry for an apparatus and system for producing downhole power that can be installed for long term use in a well. The apparatus and system needs to be capable of supplying downhole power in the relatively harsh wellbore environment without the need to run electrical conductive cables the entire length of the well. A device that can generate or supply a steady source of power deep within a reservoir for long periods of time without removal from a well and without requiring electrical conductive cable connections from the power source to the surface would be highly desirable. A downhole power source that can supply a steady and long term source of power to sensors, valves or other wellbore devices is needed. 
     SUMMARY OF THE INVENTION 
     A system for providing electrical power in a subterranean well is disclosed. The system comprises a radioisotope thermoelectric generator positioned and installed in a downhole location. The radioisotope thermoelectric generator comprises a core having a radioisotope for producing heat and a thermocouple positioned adjacent to the core. The radioisotope thermoelectric generator flows heat from the core to the thermocouple to produce electricity. The thermocouple is connected to a conductive cable for transmitting electricity along the conductive cable to an energy storage device or to a component in the well. The radioisotope employed may be selected from one or more of the following radioisotopes: plutonium, uranium, curium, strontium, polonium, promethium, caesium, cerium, ruthenium, cobalt, thulium, and americium. 
     The component may be a valve, sensor, or wireless transmitter, in various embodiments of the invention. If a wireless transmitter is employed, it may be configured for producing acoustic signals or configured for producing electromagnetic signals for propogation upwards within the well. 
     Such a system may provide electrical power in a wellbore, the wellbore having installed therein a completion string. Production tubing within the completion string may be configured for transportation of hydrocarbons from a subterranean formation upwards through a completion string to the surface. 
     The system further may comprise in another embodiment a first downhole receiver for receiving data signals from a wireless transmitter. The first downhole receiver may comprise a processing and transmission system, the processing and transmission system being adapted for processing data signals from the wireless transmitter and further processing and propagating the signals upwards to a second receiver. Signals received by the second receiver may be processed and sent upwards to the ground surface by telemetry communication, in one embodiment of the invention. 
     Telemetry communication may be employed in the practice of the invention. Such telemetry communication may be characterized by generating an electromagnetic signal having a chosen frequency at the proximal end of the electrically conducting pipe such that the electrically conducting pipe radiates the signal at least in the vicinity of the distal end thereof. The frequency may be chosen for the particular application. In some instances, radio frequency is suitable, but in other cases, a frequency band of shorter or longer wavelength may be employed. 
     Then, the radiated signal may be reflected in the vicinity of the distal end of the electrically conducting pipe such that the reflected signal is received by the conducting pipe and transmitted to the proximal end thereof. Then, it is possible to modulate the reflected signal in response to data received from the vicinity of the distal end of the electrically conducting pipe. At that point, it is possible to detect the modulated, reflected signal at the proximal end of the electrically conducting pipe. 
     The invention also may comprise a method for generating power in a completion string of a wellbore by providing a radioisotope thermoelectric generator positioned and installed within a completion string. The radioisotope thermoelectric generator includes a core having a radioisotope for producing heat and a thermocouple positioned adjacent to the core. Heat is generated in the core by radioactive decay of the radioisotope. Then, heat is flowed from the core to the thermocouple to produce electricity. 
     The method may comprise the additional step of employing the produced electricity to activate a valve, a sensor, or a wireless transmitter. Such a wireless transmitter may produce acoustic signals or electromagnetic signals, as non-limiting examples. 
     In one embodiment, it is possible to employ electricity to activate a sensor and then determine the characteristics of fluid production into a well. The electricity may be employed to activate a valve to regulate fluid production into the well. Such a sensor may measure and report temperature and/or pressure characteristics. In yet another embodiment of the invention, the sensor may measure and report water content of production fluids. Such a sensor may be employed to measure and report oil content or gas content of production fluids. In other embodiments of the invention, the sensor may be employed to measure and report the ratio of water produced to the total volume of liquids produced (i.e. the “water cut”) in a subterranean zone within the well. Further, energy storage devices, such as capacitors or batteries could be connected to the power source for receipt and storage of power. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention is illustrated in several embodiments, as set forth in the Figures: 
         FIG. 1  shows a wellbore having suspended therein a production tubing string with a radioisotope thermoelectric generator positioned in the lower portion of the tubing string; 
         FIG. 2  illustrates the internal component parts of a radioisotope thermoelectric generator; 
         FIG. 3  illustrates schematically the manner of electricity generation in a radioisotope thermoelectric generator; and 
         FIG. 4  depicts graphically the electrical power that may be provided by use of such generators as the radioactive component decays over time. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Downhole thermoelectric generators as employed in the practice of the invention employ thermoelectric materials to convert a subsurface heat differential into electrical power. Thermoelectric materials generate a voltage in response to an applied temperature differential. When used in series with an electrical load, current and power are generated. When no current is drawn, thermoelectric materials normally create a voltage correlated to the temperature differential, the coefficient of proportionality being known as the Seebeck coefficient. 
     In application of the invention, it may be possible to avoid the penetration of conductive cable through production packers and/or hanger assemblies, which is a significant advantage in the industry. Such generators may provide a long term and reliable source of power in a downhole environment. Further, the heat may be produced at a predictable and steadily decreasing rate that depends upon the amount of fuel isotope employed, and its half-life. Adequate cooling is provided as well. The generator of the system may be employed in conjunction with sensors, gauges, interval control valves, energy storage systems such as capacitors or batteries, and wireless telemetry systems (acoustic or electromagnetic). 
     In  FIG. 1 , a first embodiment of the invention is illustrated. Wellbore  20  comprises a system for power generation positioned within a subterranean formation. The casing  54  of a well suspends a production tubing  56  that extends to the bottom of the well. Cement  32  seals the bottom of the production tubing  56 , while a float collar  30  is positioned above the cement  32 . A radioisotope thermoelectric generator  22  is positioned in the bottom of the well, and is capable of generating electrical current for long periods of time without intervention or recharge. Power generated by the radioisotope thermoelectric generator  22  may be provided along conductive cable  36  to a number of different devices, including for example valves, sensors (not shown), or other downhole devices that use power. In the case of the embodiment of  FIG. 1 , power is provided to electric interval control valves  26 ,  28  and wireless transmitter  34 . Power from conductive cable  36  may be employed to activate the control valves  26 ,  28  or may be employed to activate the wireless transmitter  34 . An inverted polished bore receptable (“PBR”)  38  is shown below the production packer  40 . The PBR  38  is a seal assembly designed for specific application as an expansion joint, providing stroke length for extreme production tubing  56  movement during well treatment and production. As a separation tool, the PBR  38  facilitates the removal of the production tubing  56  string, while leaving a polished bore and anchor seal assembly set in a packer. 
     The production packer  40  is a component of the completion hardware used to provide a seal between the outside of the production tubing and the inside of the casing, liner, or wellbore wall. Production packers remain in the well during well production. In  FIG. 1 , the production packer  40  is positioned below the second polished bore receptacle  42 , and has been run in close to the bottom end of the production tubing  56 . The production packer  40  is set at a point above the top perforations or sand screens (not shown in  FIG. 1 ). 
     A wireless downhole transmitter/receiver  44  may employ microprocessor controlled frequency synthesis. It may receive signals transmitted from below, and is adapted for two-way communication to the surface (or to another communication device) in an appropriate frequency range, which may be in the range of from about 1 Hz to about 300 GHz. In some other applications, the frequency range that may be employed is from about 1 Hz to about 50 MHz. Data gathered by the downhole transmitter/receiver  44  may be encoded in digital format and transmitted electromagnetically and wirelessly to the surface or to relay station/transmitter  50  by a signal processor unit (not shown), or alternately may be encoded for acoustic transmission to relay station/transmitter  50 . Relay station/transmitter  50  receives power from conductive cable  52  run from the top of the wellbore  20 . 
     Electrical submersible pump  46  also may reside within the production string, and is positioned between the downhole transmitter/receiver  44  and the surface controlled subsurface safety valve  48 . 
     The radioisotope thermoelectric generator  22  of the invention is shown in  FIG. 2 , which shows a partial cut-away view of the device. A radioisotope  62  is held within the core  61 . The radioisotope employed may be selected from one or more of: plutonium, uranium, curium, strontium, polonium, promethium, caesium, cerium, ruthenium, cobalt, thulium, and americium. In some applications, plutonium may be employed, but for other applications, americium may be beneficial. The identity of the isotope employed may depend upon radioisotope total cost, half-life, environmental concerns, and other considerations. 
     One structure of a radioisotope generator  22  is shown, although it is recognized that other specific structures could be employed. Core  61 , containing the radioisotope, is held upon support frame  66 . A thermocouple  64  of two different metals is provided in close proximity to the core  62 , and may be wound upon the core  62 . Heat moves from the radioisotope  62  to the thermocouple  64  as a consequence of radioactive decay of the radioisotope  62 . 
     One type of thermocouple  64  that may be employed is a silicon-germanium thermocouple, but other metals may be employed depending upon the configuration and process specifications of the radioisotope generator  22 . In general, a thermocouple is a device consisting of two different conductors (usually metal alloys or semiconductors) that produce a voltage proportional to a temperature difference between either end of the pair of conductors. In this application, thermocouple  64  is used to convert a heat gradient into electricity. In contrast to most other methods of temperature measurement, thermocouples are self-powered and require no external form of excitation. The two metals may be connected to each other in a closed electrical loop. A junction of dissimilar metals is capable of producing an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys may be used for different temperature ranges. Properties such as resistance to corrosion may also be important when choosing the type of thermocouple to be employed. Electronic instruments may be employed to compensate for the varying characteristics of the thermocouple, and so improve the precision and accuracy of measurements. 
     The radioisotope generator  22  is provided with a mounting flange  68 , and an outer shell assembly  70 , which may be construed of a metal such as aluminum. Cooling tubes  72  act in concert with an active cooling system manifold  76  to maintain temperature control. A pressure relief valve  74  is provided for relief of pressure, and will be set to open when needed to relieve pressure. 
     A schematic showing the mechanism of operation of the radioisotope generator  22  is shown in  FIG. 3 . A heat source, in this case a radioisotope  62 , is applied to dissimilar metals in thermocouple  64  to produce a voltage proportional to a temperature difference between either end of the pair of conductors. This voltage provides electrical current  80  in circuit  78 . This electrical current  80  may be applied to charge an energy storage device, or may be used immediately, depending upon the application. A processor (not shown) may be provided to regulate the flow of energy in the system, which may in some instances include transfer to an energy storage device (not shown). 
       FIG. 4  provides an illustration of the long term power generation characteristics of the radioisotope thermoelectric generator  22 . Power reduction is experienced over time due to radioactive decay. Thermocouples are generally reliable and long-lasting, but are relatively inefficient in converting the heat to power (about 3-7% efficiency on average). The power and activity of various radioisotopes may be seen in  FIG. 4 . Cost, power production, and environmental issues may determine the most appropriate radioisotope to be employed for a given downhole power application. 
     It is recognized that multiple radiosotope generators  22  could be deployed and installed adjacent to a component to which the respective generator(s)  22  provide power. This may be particularly useful for instances in which there are long distances between components that are to receive downhole power. In such applications, generators  22  could be positioned adjacent to their respective components. 
     Persons of skill in the art may recognize that U.S. Pat. No. 6,766,141 describes a telemetry-based method of communication in the wellbore. Such a system may be employed in the practice of this invention. The method includes an apparatus and method for telemetry communication with oil-well monitoring and recording instruments located in the vicinity of the bottom of gas or oil recovery pipes. Modulated reflectance provides a wireless communication method that does not require signal transmission power from the telemetry package, and will provide a long-lived and reliable way to monitor down-hole conditions. Such a system may be employed in the practice of the present invention, as further described herein. Although modulated reflectance reduces or eliminates the loss of energy at the sensor package because energy is not consumed during the transmission process, additional stored extra energy downhole may be applied. The electrical energy generated downhole by the radioisotope thermoelectric generator of the invention may be employed to power various devices, including for example a well communications system. Telemetry communication may be employed in the practice of the invention. Such telemetry communication may be characterized by generating a radio frequency signal having a chosen frequency at the proximal end of the electrically conducting pipe such that the electrically conducting pipe radiates the signal at least in the vicinity of the distal end thereof. Then, it is possible to modulate the reflected radio frequency signal in response to data received from the vicinity of the distal end of the electrically conducting pipe. This facilitates detection of the modulated, reflected signal at the proximal end of an electrically conducting pipe. 
     The invention is shown by example in the illustrated embodiments. However, it is recognized that other embodiments of the invention having a different configuration but achieving the same or similar result are within the scope and spirit of the claimed invention.