Abstract:
Methods and system are described for providing electric power in a wellbore including at least one thermoelectric generator at a downhole location; and one or more thermally insulated sections of wellbore tubing located such as to reduce heat loss of fluid flowing through the wellbore from the entry point of the fluid to the location of the generator.

Description:
[0001]     The present invention generally relates to an apparatus and a method for generating electrical power inside a wellbore with improved efficiency.  
       CROSS-REFERENCE TO RELATED APPLICATION  
       [0002]     This application claims the benefits of priority from Application Number 0526556.6, entitled “DOWNHOLE THERMOELECTRIC POWER GENERATION,” filed in the United Kingdom on Dec. 30, 2005, which is commonly assigned to assignee of the present invention and hereby incorporated by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0003]     In the oil and gas producing industry there is a long-established need for generation electrical power inside a wellbore, particularly a wellbore in the state of producing hydrocarbon fluids.  
         [0004]     Whilst many possible solutions to the problem have been suggested, the industry is presently using battery packs and cable connection to the surface.  
         [0005]     Among the technical solutions unsuccessfully proposed is the use of thermocouples integrated in or attached to the wall of tubes in the wellbore.  
         [0006]     For example, in the U.S. Pat. No. 6,150,601 there is described a method and apparatus for generating electrical power downhole by applying a temperature gradient across a at least two thermocouples. In the U.S. Pat. No. 6,380,476 a thermocouple is placed inside a side-pocket mandrel and brought into contact either to the inner wall of the casing of a wellbore or the formation. In the U.S. Pat. No. 5,929,327 there is suggest to use a stack of high-temperature bismuth telluride thermocouples heated by the wellbore fluid from within and seawater from the outside to maintain a sufficient temperature gradient across the thermocouples.  
         [0007]     In the light of the know state of the art, it is clear that an applicable thermoelectric generator requires for further improvements to be operable at a downhole location. It is therefore seen as an object of the present invention to provide an apparatus and method for generate electrical power using the thermo-electrical effect with increased efficiency.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with an aspect of the invention, there are provided methods and systems for generating electric power in a wellbore, said system including at least one thermoelectric generator at a downhole location; and one or more thermally insulated sections of wellbore tubing located such as to reduce heat loss of fluid flowing through the wellbore from the entry point of the fluid to the location of the generator.  
         [0009]     The wellbore tubing is preferably production tubing or casing tubing.  
         [0010]     The one or more thermo-insulated sections are characterized by having a lower heat conductivity coefficient than other sections of the wellbore tubing. Compared to conventional steel pipes as used in the oilfield the one or more thermally insulated sections are characterized by having a by a factor of at least 100 lower heat conductivity coefficient. This preferred embodiment is based on the assumption that it is more economical to reduce the heat conductivity only locally, but as a matter of course the benefits of the present invention may also be reaped by providing a complete string of wellbore tubing with such superior insulation.  
         [0011]     The one or more thermo-insulated sections are preferable clad with a layer or layers of thermally insulating material. In a more preferred embodiment double-walled tubing is used with a vacuum or gas filled inner space.  
         [0012]     The effect of the present invention can be increased by making the generator a preferred location of heat exchange between the fluid and the surrounding. In a preferred embodiment the generator is located within a thermally insulated section of wellbore tubing.  
         [0013]     The generator is preferably of a type exploiting the Seebeck effect. It may include Bismuth-Telluride or even microstructured Bismuth-Telluride/Antimony Telluride.  
         [0014]     These and other aspects of the invention will be apparent from the following detailed description of non-limitative examples and drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  illustrates basic elements of a downhole electric power system in accordance with an example of the invention;  
         [0016]     FIGS.  2 A-C illustrate the effect of the present invention;  
         [0017]      FIG. 3  illustrates the increased efficiency of the present invention;  
         [0018]      FIG. 4  is a variant of the system of  FIG. 1 ; and  
         [0019]      FIG. 5  is a further embodiment of the invention used with casing tubing. 
     
    
     DETAILED DESCRIPTION  
       [0020]     Downhole thermoelectric generators use thermoelectric materials to convert a subsurface heat flux into electrical power. Thermoelectric materials generate a voltage in response to an applied temperature differential, and when in used in series with an electrical load, current and hence power, are generated. When no current is drawn, the thermoelectric materials normally used for generation create a voltage that is proportional to the temperature differential, the coefficient of proportionality being the Seebeck coefficient
 
 V   0   =SΔT   [1]
 
         [0021]     When a current is drawn, the voltage drops according to
 
 V=V   0   −IR   0   [2]
 
 where R 0  is the internal resistance of the thermoelectric generator. 
 
         [0022]     If, for instance, the generator is connected to a resistive load R L , then V=IR L  and hence  
             V   =         S   ⁢           ⁢   Δ   ⁢           ⁢     TR   L           R   0     +     R   L         .             [   3   ]             
 
         [0023]     The power generated is VI, and hence is quadratic in the temperature difference.  
         [0024]     From the above analysis it may be seen that maximizing the temperature difference across the thermoelectric elements is crucially important in thermoelectric generator design.  
         [0025]     In accordance with an example of the present invention as illustrated schematically in  FIG. 1 , it is therefore proposed to provide a thermally insulated zone  121  between the fluid  11  flowing through the wellbore  10  upstream of the location of a thermoelectric generator  13 .  
         [0026]     In the example of  FIG. 1 , the thermoelectric generator  13  is mounted on production tube  12 . At the bottom of the production tubing  12  there is placed a packer  14  preventing the entry of the produced fluid into the annulus  102  between production tubing  12  and a casing tube  101 . Between the entry point  122  of the fluid flow into the production tube  12  and the mounting point of the thermoelectric generator  13  there is zone  121  of well tube that is specifically adapted to provide increased thermal insulation when compared to conventional wellbore tubing. Produced hydrocarbons enter the production tubing at a temperature either equal to that of the surrounding rock  100 - 1 , or higher. As the fluid move up the production tubing, it will be hotter than the surrounding annulus, which is cooled by the surrounding rock  100 - 2  at the local geothermal temperature. This temperature difference generally increases as the fluid passed through tubing higher in the wellbore.  
         [0027]     An example of this effect is shown in  FIG. 2A , based on a theoretical calculation. The solid line shows the temperature of the fluid in the production tubing, entering the tubing at 100C., at a depth of 3000 m. The geothermal gradient is the dash-dotted line, and the dashed line shows the calculated temperature in the annulus. In the example, the fluid entering the production tubing is 3C. above the geothermal temperature at that depth.  
         [0028]     In order to maximize the temperature difference, the flowing fluid should be kept as close to its initial temperature as possible, and heat flow to the annulus minimized. Both effects can be achieved by using tubing that is more insulating than steel upstream of the thermoelectric generator.  
         [0029]     Suitable methods for insulating the tubing are either cladding the tubing in a thermal insulator that is not affected by the well temperature or mechanical environment—for instance fibrefrax products available from Unifrax, or employing dual-tubing with an insulating gap, for instance vacuum insulated tubing (VIT) available from Grant-Prideco division Tube-Alloy.  
         [0030]     In  FIG. 2B , the calculation has been repeated with the conductivity of the tubing reduced by a factor of 100, corresponding to surrounding the tubing with some form of fibrous or ceramic insulation. The temperature of the flowing oil is not greatly affected, but there is a noticeably greater temperature difference with the annulus.  
         [0031]     In  FIG. 2C , the conductivity of the tubing has been reduced by a further factor of  80 , corresponding to vacuum insulated tubing with insulated joints. In this example the flowing oil looses far less heat, and the temperature difference to the annulus is significantly larger.  
         [0032]     The absolute temperature difference between the flowing fluid and the annulus for steel tubing (solid line), insulated tubing (dashed line) and vacuum insulated tubing (dash-dot line) versus distance from the bottom of the production tubing is shown in  FIG. 3 . It can be seen that for instance at 100 m above the entry point of production tubing, there is less than 0.5C. temperature difference between the flowing fluid and the annulus with steel tubing, whereas with vacuum insulated tubing a difference or temperature gradient of around 4C. is available.  
         [0033]     These calculations are only intended to be representative, and for any particular well geometry, flow rates and fluid compositions, the particular results will vary.  
         [0034]     It should be noted that all the arguments detailed above for a thermoelectric generator mounted on production tubing also apply for a thermoelectric generator mounted on fluid injection tubing designed to carry fluid from the surface, except the sign of the heat difference is reversed, and the preferred location for insulation changing to being above the thermoelectric generator and not below it as shown in  FIG. 1 .  
         [0035]     For a particular location of thermoelectric generator, to maximize the electric output, the heat flux through the generator must be maximized. This is a particular concern in an environment such as downhole, where the material between the heat source (produced hydrocarbons) and the heat sink (the annulus, or the surrounding formation) is thermally conductive. Most thermoelectric materials with a high Seebeck coefficient S are also poor thermal conductors, and thus, for instance, mounting a thermoelectric generator by itself on the outside of production tubing will result in a very small temperature gradient across the generator, even if the outer surface of the generator matches the annular temperature. Because of the thermal conductivity contrast between the metal and the thermocouples, most of the temperature gradient will be within the tubing itself, and only a very small portion within the thermocouples.  
         [0036]     A further advantage of providing an insulating environment for the generator is that if the flow of produced fluid is interrupted, the time taken for the local heat to dissipate will increase, and thus the generator will continue to provide power for longer before the local temperature gradient is too low.  
         [0037]     A suitable technology for borehole thermoelectric generators is Bismuth-Telluride, which is a semiconductor and can be doped to be either P type or N type. Many companies make thermoelectric modules from this material, for instance Hi-Z of San Diego, Calif. More recently microstructured materials have been created with improved thermoelectric characteristics, for instance the superlattice Bismuth-Telluride/Antimony Telluride structures created by RTI International and Nextreme thermal solutions of North Carolina.  
         [0038]     In order to maximize the heat difference actually across the thermoelectric generator, the area surrounding the generator should be sufficiently thermally insulated, so that instead of heat preferentially being conducted around the generator, it is preferentially conducted through the generator. The thermal path that leads through the generator should be as thermally conductive as possible, and in as good thermal contact with the heat sink as possible, so that for instance a high conductivity radiator element, made out of Aluminum for example may be employed on the heat sink side of the generator.  
         [0039]     The example of  FIG. 4  illustrates an improved variant of the example of  FIG. 1 .  
         [0040]     The apparatus shown includes all elements of  FIG. 1  denoted by the same numerals. However in accordance with the observations described above the apparatus shown is adapted to optimize the heat flow through the generator  13  itself. Hence, there is additional insulated tubing  123  shown above the thermoelectric generator  13  before the tubing reverts to standard production tubing  12 . As a consequence of this modification, a greater temperature difference can be maintained at the location of the generator  13 .  
         [0041]     Following the above description, it will be appreciated that the insulated tubing need not be deployed all the way from the thermoelectric generator down to the bottom of the production tubing, however the larger heat-flux into the annulus below the insulated casing could increase the inverted heat gradient in the annulus, which could then cause increased convection currents—reducing the effectiveness of the insulated casing higher up.  
         [0042]     Insulated casing can also be deployed below the bottom of the production tubing, in order to both maintain fluid temperature and reduce the heat flow to the rock, however an economic solution may restrict the deployment of insulated tubing along extended stretches of the wellbore.  
         [0043]     In another apparatus for combining a thermoelectric generator and insulated tubing shown in  FIG. 5 , the thermoelectric generator  13  may be mounted on the wellbore casing  101  below the bottom of the production tubing  12  and cemented into the well  10 . The temperature difference between production layer  100 - 1  and the layer  100 - 2  at the location of the generator  13  will be reduced compared to a generator employed higher up, and thus maximizing the heat flow is even more important. In this deployment, a length  103  of casing upstream of the generator  13  and preferably also a length  104  of casing downstream of the generator  13  will be highly insulating, either clad in insulating material or through the use of double walled casing employing vacuum insulation.