Abstract:
Disclosed is a radiation logging tool, comprising a tool housing; a compact generator that produces radiation; a power supply coupled to the compact generator; and control circuitry. Embodiments of the compact generator comprise a generator vacuum tube comprising a source generating charged particles, and a target onto which the charged particles are directed; and a high voltage supply comprising a high voltage multiplier ladder located laterally adjacent to the generator vacuum tube. The high voltage supply applies a high voltage between the source and the target to accelerate the charged particles to a predetermined energy level. The compact generator also includes an electrical coupling between an output of the high voltage supply and the target of the generator vacuum tube to accommodate the collocated positions of the generator vacuum tube and the high voltage power supply.

Description:
CROSS-REFERENCE TO OTHER APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Application No. 61/261,398, filed provisionally on Nov. 16, 2009 (Attorney Docket 49.0415). This application also claims priority to and the benefit of U.S. Provisional Application No. 61/261,394 (Attorney Docket 49.0434), filed provisionally on Nov. 16, 2009. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates generally to borehole/oilfield electronic radiation generators. 
       BACKGROUND 
       [0003]    Ionizing radiation has been used in the oilfield for well over half a century to enable measurements not possible with other formation probing techniques (such as electrical resistivity, acoustic, electro-magnetic, NMR, etc.). The broad types or kinds of ionizing radiation include charged  p articles, X-rays, gamma-rays, and neutrons. 
         [0004]    Historically, the first method employed to produce ionizing radiation for oilfield applications involved radioactive sources. These radioactive sources (e.g.  137 Cs,  241 Am,  252 Cf,  238 Pu, etc.), were chosen, based on the application, for their emitted radiation type and in quantities sufficient for the desired intensity. Few naturally occurring radioactive sources emit neutrons during their decay, generally as a consequence of spontaneous fission. One example of a radioactive material with a significant probability for spontaneous fission is  252 Cf. While  252 Cf is used in some applications, its short half-life of only 2.6 y is generally considered a drawback. Many neutron sources rely on the generation of neutrons through a nuclear reaction, typically the  9 Be(alpha, n) 12 C reaction. Such sources consist of an alpha emitter, such as  241 Am or  238 Pu, mixed with Be-powder. The small nuclear reaction probability necessitates the use of a large amount of radioactive alpha emitter to generate a sufficient neutron output. 
         [0005]    The small size and simplicity of the source housing, the reliability of continuous radioactive decay, and the typically low purchase cost make such radiation sources very attractive. The shortcomings, however, include limited radiation output, (particularly for neutron sources), issues of radiation safety because the source remains ON at all times, as well as the increase in the cost of the material and its disposal, the accountability (tracking/inventory requirements) and, for some like  252 Cf, a short useful life. The absence of the ability to fast pulse ON/OFF a chemical source also limits the measurements that can be made. 
         [0006]    To overcome these disadvantages of radioactive sources, electronic radiation generators were developed over the last several decades. Electronic radiation generators function by accelerating and colliding charged particles inside of sealed tubes to induce radiation-producing reactions. The two approaches to accelerating charged particles are to use linear accelerators (e.g., Linac) or circular accelerators (e.g., cyclotron). The linear accelerators employed in the oilfield, and of relevance to this particular invention disclosure, are electrostatic-based. Linear accelerators operate by creating a sufficiently large voltage difference between a source of charged particles and a target. Once the source is turned ON, charge particles entering the region of high voltage (HV) are accelerated to the predetermined desired energy and made to collide with a suitable target material to produce radiation. If either the ion source or the voltage are turned OFF, the radiation stops. The accelerated charged particles can be electrons, protons and heavier ions, which will generate ionizing radiation when they impinge on the target. 
         [0007]    The intensity and energy of the radiation produced depends on the high voltage used to accelerate the charged particles. Typically, the higher the “high voltage” is, the higher the energy of the charged particles. High voltage power supplies capable of producing from tens to hundreds of kilovolts are desirable. A common power supply design employed is based on the Cockcroft-Walton voltage multiplier, also known as the Greinacher multiplier. Such a voltage multiplier includes an array of stages in series, shaped like a ladder, with each stage  100  comprising a pair of capacitors  102  and a pair of diodes  104  (see  FIG. 1 ). 
         [0008]    During each ON cycle, the A/C current supplied by the step up transformer  106  charges each respective capacitor  102 , according to the rectification provided by the diodes  104 . The maximum admissible voltage for each stage  100  depends on the capacitor and diode component ratings as well as on the AC voltage provided by the secondary of the step up transformer  106 . To achieve a given high voltage, a ladder is constructed such that, to first order (i.e., neglecting losses, etc.), enough stages are used to sum up to the predetermined desired voltage. The high voltage output  108  of the series of stages, or ladder, can then be fed to a voltage divider (bleed resistors, discussed in further detail in related application Ser. No. 61/261,398, non-provisional application filed concurrently herewith, (Atty. Docket 49.0415)) to obtain a measurement of the high voltage fed out to neutron generator tube. 
         [0009]    One electrical constraint for the design of high voltage power supplies is that electrical hold off between high voltage and ground must be sufficient over the whole length of the power supply. Given the limited diameter available in downhole tools, the stage component size must be small enough to accommodate the necessary high voltage insulation over a very limited radial distance. Typically, the size of high voltage components increases with their voltage rating. A tradeoff must be made between the stage (and component) size and the necessary high voltage insulation based upon the voltage difference to neighboring ground potential. For borehole-size radiation generators, high voltage capacitor ratings are typically on the order of around a few kilovolts (e.g., five kilovolts). Typically, multi-layer capacitors are used to achieve the required capacitance, and are generally rectangular or box-shaped with radial leads. Thus, typically to produce from tens to hundreds of kilovolts requires a power supply with several dozens of stages. When the stages are placed end to end, this adds up to a significant overall length for the power supply. 
         [0010]    Currently, the most frequently employed electronic radiation generator in the oilfield is the pulsed neutron generator (PNG), as shown in  FIG. 2 . Aside from the supporting low voltage electronics (current and pulsing supplies), a PNG comprises a high voltage power supply  202  (comprising comprises the step up transformer and the voltage multiplier), a sealed neutron generator tube  204  and a high voltage insulation system  206 . The sealed neutron generator tube  204  includes an ion source  208  which is at or near ground potential, and separated from a target  210  by an HV acceleration column. The configuration of a PNG is generally cylindrical, as dictated by the borehole geometry, with the target  210  of the neutron generator tube  204  in contact with the high voltage end of the high voltage power supply  202 . The necessary high voltage insulation (solid, liquid and/or gaseous or any combination thereof) surrounds the neutron generator tube  204  and high voltage power supply  202 , with the entirety being contained in a hermetically-sealed pressure housing  212  (not to be confused with an overall tool pressure housing of a tool in which the present generator is utilized). Preferably, the pressure housing is cylindrical in shape. The conventional arrangement of these components is in a rectilinear configuration, where the high voltage power supply  202  is connected in line (on one axis or close to on one axis) to the high voltage end of the neutron generator tube  204  (see  FIG. 2 ), frequently with a protective surge resistor  214  therebetween. The typical length of such a generator is often in the range from about 36 to 48 inches, or more. 
         [0011]    The considerable length of a typical PNG configured as in  FIG. 2  can impose significant limitations on the tool design and consequently the range of measurements possible to obtain. Because of the length of the high voltage power supply in the typical PNG, only the neutron generator tube end affords close proximity to the radiation source (i.e., the target). For most practical purposes, this limits detector placement to a single axial direction relative to the target. If a tool is intended to perform multiple different measurements with the radiation provided by the electronic source, it is likely that not all the required detectors can be placed at the optimal axial and/or radial position. A short radiation source would allow detector placements axially on both sides of the radiation source, and thus a better independent optimization of multiple measurements through optimal placement of multiple detectors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a design of a prior art Cockcroft Walton voltage multiplier generator. 
           [0013]      FIG. 2  shows a conventional prior art layout of a Pulsed Neutron Generator for downhole use. 
           [0014]      FIG. 3  shows a layout of a compact generator with collocated linear high voltage power supply and a neutron generator tube in one embodiment in accordance with the present disclosure. 
           [0015]      FIG. 4  shows a compact generator with an external step-up transformer in another embodiment in accordance with the present disclosure. 
           [0016]      FIG. 5  shows a high voltage turn-around from end of ladder back to the neutron generator tube in another embodiment in accordance with the present disclosure. 
           [0017]      FIG. 6  shows a “split-and-flip” voltage multiplier ladder providing a reduced electrical potential at the ends allowing the shortest axial distance to ground in another embodiment in accordance with the present disclosure. 
           [0018]      FIG. 7  shows a schematic layout of a neutron generator with a collocated “split-and-flip” voltage multiplier ladder in another embodiment in accordance with the present disclosure. 
           [0019]      FIG. 8  shows an alternate implementation of “split and flip” ladder in accordance with the present disclosure. 
           [0020]      FIG. 9  shows a fold-back high voltage supply. 
           [0021]      FIG. 10  shows a fold-back high voltage supply with intermediate electrode in insulating support in another embodiment in accordance with the present disclosure. 
           [0022]      FIG. 11  shows a compact generator with insulating support with coolant channel in another embodiment in accordance with the present disclosure. 
           [0023]      FIG. 12  shows a neutron generator with collocated voltage multiplier ladder connecting an intermediate potential from the ladder to the intermediate electrode of the neutron generator tube in another embodiment in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In the following description, numerous details are set forth to provide an understanding of the present disclosure. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible. 
         [0025]    In light of the above discussion, it would be advantageous to reduce radiation generators, including a PNG, to as short and compact a size as possible. 
         [0026]    Most pulsed neutron generators generate neutron radiation through the reaction of energetic deuterium with tritium (d-T) reaction. However, other known reactions, like d-D and t-T can be employed as well. X-rays are generated by directing energetic electrons onto a target. The scattering of the energetic electrons on the target leads to the emission of Bremsstrahlung x-rays and x-rays characteristic of the target material. 
         [0027]    The present disclosure discusses solutions for a compact generator collocated with a linear multi-stage power supply, such as a Cockcroft-Walton (CW) power supply. In a most preferred embodiment of the present disclosure, a multi-stage high voltage power supply, such as a Cockcroft-Walton styled voltage multiplier, would be employed, however parallel work on single-staged voltage multipliers indicates that single-staged voltage multipliers are also contemplated in the present disclosure, as any other axially collocated supplies could be used. An embodiment of a pulsed neutron generator collocated with its high voltage power supply is shown in  FIG. 3 . The neutron generator  300  is enclosed in a hermetically sealed housing  302  which contains the step-up transformer  304 , which is powered by an external voltage source (not shown in  FIG. 3 ). A stepped up high voltage on the order of several kilovolts powers the voltage multiplier  306 , which appears above the neutron generator tube  300  in  FIG. 3 , though for practical purposes, the voltage multiplier  306  is situated substantially side-by-side, collocated with the neutron generator tube  300 , thereby shortening the overall length required for the pulsed neutron generator. The neutron generator tube  300  contains an ion source  308  on one side and a target  310  on the other side on which accelerated beam particles impinge. The ion source  308  and the target  310  are separated by an accelerating region, which is surrounded by the insulating wall of the neutron generator tube  300 . The wall of the neutron generator tube  300  may comprise, for example, a ceramic. The high voltage generated by the voltage multiplier  306  is fed to the target  310  through a protective resistor  314  (i.e., a surge resistor). The end of the neutron generator tube  300  having the target  310  is supported by an axial insulator  316  that may also serve as the mechanical support for the protective resistor  314 . A high voltage turn-around  318  couples between the output of the high voltage multiplier  314  to the input of the neutron generator tube  300  (often through a protective surge resistor  314 ). 
         [0028]    Referring now to  FIG. 4 , in an alternative embodiment, the step up transformer  404  may be located outside of the generator housing  402  at a second location to allow a further reduction of the overall length of the pulsed neutron generator, wherein the step-up transformer  404  is coupled to the input of the high voltage multiplier  306  via high voltage wires  420  passing into the housing  402  via high voltage feed-through passages  422 . One effect of such a configuration is that high voltages of 10 kV or more have to be connected over a distance of several feet to the generator. In the limited space of a downhole tool, such a may pose problems due to the required diameter of the wires for such a connection (including high-voltage insulation) as well as noise induced by high voltage wiring. Despite these factors, locating the step up transformer  404  outside of the generator housing is a possible embodiment. 
         [0029]    Referring now to the embodiment of  FIG. 5 , the electrical connection from the end of the voltage multiplier  506  to the target  510  necessitates a turn-around  516  of the high voltage line. Given the proximity of the turn-around  516  to the ground plane at the end (shown right side of  FIG. 5 ) of the generator housing  502 , resulting electrical fields have the potential to lead to uncontrolled arcing. The turn-around  503  may therefore be protected by a corona shield  505  positioned so as to reduce electrical field stresses. A possible approach is shown in detail in  FIG. 5 . 
         [0030]    As shown in  FIG. 5 , the target  510  end of the neutron generator tube  500  are shown contained in the wall  501 . Adjacent to the neutron generator tube  500  is the voltage multiplier  506  with a high voltage line  503  exiting the voltage multiplier  506 . The corona shield  505  is provided about the high voltage line  503  (and including about the surge resister  514 ). The corona shield  505  surrounding the high voltage wire  503  (or other components at high electric potential) serves to reduce the electric field around such components or wires. Typically, it is a metallic conductor (other conducting materials can be used as well) with a highly polished surface and a large radius of curvature. The increased radius of curvature and the absence of surface irregularities (sharp edges and rough surface) minimize the electric field at the surface and reduce the probability of electron emission from the surface, which may lead to an electrical breakdown of the insulation. Additionally, the corona shield  505  may be coated by a thin insulating layer to further reduce the probability of electron emission from the surface. 
         [0031]    An alternate approach for reducing electrical stress at the turn-around is to “split-and flip” the voltage multiplier back on itself. The schematic in  FIG. 6  illustrates this approach, which affords a reduction in the turn-around voltage stress, but at the expense of moving the electrical field stress midway through the voltage multiplier and adding to the complexity by feeding back high voltage wires.  FIG. 6  shows the split  600  of the voltage multiplier at a certain stage, with feedback from the end back towards the middle, with the HV output at an intermediate of the voltage multiplier. 
         [0032]      FIG. 7  shows the schematic layout of a neutron generator employing a “split-and-flip” voltage multiplier as outlined in  FIG. 6 . The region of high electric stress is indicated. An alternate embodiment of the “split-and-flip” approach is shown in  FIG. 8  where the split divides the high voltage multiplier into a first section  806 A located outside, but proximate, the sealed neutron generator tube  804 , and a second section  806 B located in-line with the sealed neutron generator tube  804  in a support  850 . 
         [0033]    Yet another approach consists of folding the voltage multiplier itself and bringing it back as shown in  FIG. 9 . This means that some of the multiplier stages are located underneath the main support of the high voltage supply that contains the majority of the multiplier stages. This results in a similar reduction of the high voltage stresses at the position closest to the end of the generator as in  FIG. 7 . In both cases, the high voltage at the far end of the HV supply is lower than the maximum high voltage. This may allow a reduction in the length of the insulating mechanical support and as a consequence a shorter overall length of the PNG. In this case, the portion of the voltage multiplier that is folded back requires additional room underneath the supply. 
         [0034]    For both embodiments, an additional improvement can be accomplished by making sure that an axial plane coinciding with the fold of the voltage multiplier is at the same electrical potential as the folding point in the voltage multiplier. This can be accomplished by inserting a conductive plane into the insulating support that connects the end of the generator tube to the end of the generator housing. The additional electrically conducting plane is shown in  FIG. 10 . 
         [0035]      FIG. 8  can also be used for the folded voltage multiplier. The supporting insulator (shown in  FIGS. 7-12 ) extending from the target end of the generator tube to the end of the generator housing is included not only for mechanical strength but also as a heat conducting medium to remove heat from the target. Typically, electrical insulators are poor thermal conductors and the Alumina ceramic is no exception to this rule. However, Aluminum nitride (AlN) is a much better conductor of heat while at the same time being a good electrical insulator. The insulating mechanical support  730 ,  830 , etc. could therefore be manufactured of Aluminum nitride. Matters of heat conduction are critical for the reliable operation of the PNG at elevated temperature. A traditional d-T generator with a titanium-based target will stop working when the target temperature exceeds 250° C. At an ambient temperature of 150 or 175° C., the target temperature can reach or exceed 250° C. unless there is a proper heat sink. The insulating mechanical support, in particular if made of AlN, offers an excellent solution to this problem. Alternatively or additionally, one could envisage circulating an insulating fluid (gas or liquid) through the insulating support in order to evacuate heat ( FIG. 11 ) 
         [0036]    The ceramic support can be made in such a way that the heat evacuation from the CW voltage multiplier is facilitated to reduce the temperature seen in particular by the diodes but also by the capacitors. 
         [0037]    Collocation of the voltage multiplier with the generator tube has additional advantages. It is feasible to tap off from intermediate stages of the CW generator to provide a well defined potential to one or more intermediate electrodes of the generator tube. Radiation generator tubes with multiple intermediate electrodes are described in more detail in the concurrently pending U.S. patent application Ser. No. 12/618,821 filed Nov. 16, 2009, entitled “Electrode Configuration for Downhole Nuclear Radiation Generator” (Atty. Docket 49.0388). An example of such an arrangement using an intermediate-electrode-neutron-tube is shown in  FIG. 12 . In order to protect the voltage multiplier from transients due to arcs to the intermediate electrode, it may be necessary to add a protective (surge) resistor between the intermediate voltage multiplier point and the intermediate electrode (not shown in  FIG. 12 ). It is evident that the concept can be expanded to multiple electrodes with multiple intermediate potentials. 
         [0038]    While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.