Near-field sensitivity of formation and cement porosity measurements with radial resolution in a borehole

Abstract

A neutron porosity tool having an electronic neutron generator arrangement and a control mechanism used to provide voltage and pulses to an electronic neutron tube is provided, the neutron generator arrangement including: at least one vacuum tube; at least one ion target; at least one radio-frequency cavity; at least one high-voltage generator; at least two neutron detectors; at least one pulser circuit; and at least one control circuit. A method of controlling a neutron porosity tool having an electronic neutron generator arrangement and a control mechanism that provides voltage, and pulses to an electronic neutron tube, the method including at least: controlling a bipolar neutron tube to produce two distinct neutron reactions; using a control circuit to modify the output of a pulser circuit; and using a plurality of neutron detectors to determine formation response offsets.

Claims

1. A neutron porosity tool comprising: a first vacuum tube enclosing a first ion target, a target electrode coupled to the first ion target, a first radio-frequency (RF) cavity, and a first cathode; a plurality of neutron detectors; a current source coupled to the first cathode; a first pulser circuit coupled to the first RF cavity and to the plurality of neutron detectors, the first pulser circuit to drive the first RF cavity and to gate a response of the plurality of detectors; and a first high-voltage generator coupled to the target electrode, the first high-voltage generator to provide a voltage to the target electrode.

2. The tool of claim 1, further comprising a second pulser circuit, wherein the first and second pulser circuits are to provide two differing ion accelerating voltages, such that deuterium-deuterium reactions and deuterium-tritium reactions are generated within a single reactance plane while the tool is positioned within a borehole.

3. The tool of claim 1, further comprising a second pulser circuit, wherein the first and second pulser circuits are to provide substantially equivalent ion accelerating voltages, such that deuterium-deuterium reactions are generated within a single reactance plane while the tool is positioned within a borehole.

4. The tool of claim 1, further comprising a second pulser circuit, wherein the first and second pulser circuits are to provide substantially equivalent ion accelerating voltages, such that deuterium-tritium reactions are generated within a single reactance plane while the tool is positioned within a borehole.

5. The tool of claim 1, further comprising a second vacuum tube enclosing a second cathode and a second ion target co-located with the first ion target, wherein the target electrode is a common target electrode that is coupled to the first and second ion targets, and the first high-voltage generator is to provide the voltage to the common target electrode, such that deuterium-deuterium reactions and deuterium-tritium reactions are generated within a single reactance plane while the tool is positioned within a borehole.

6. The tool of claim 1, wherein the plurality of detectors comprise helium-3 gas.

7. The tool of claim 1, wherein the plurality of detectors comprise Lithium-6 glass.

8. The tool of claim 1, further comprising: a second vacuum tube that forms a conjoined tube with the first vacuum tube, wherein the second vacuum tube encloses a second ion target, a second RF cavity, and a second cathode, and the target electrode is a common target electrode that is coupled to the first and second ion targets.

9. The tool of claim 8, further comprising: a second pulser circuit coupled to the second RF cavity, wherein the first and second pulser circuits are coupled to a common ground potential, and the second pulser circuit is to drive the second RF cavity.

10. The tool of claim 9, wherein the plurality of neutron detectors includes a first neutron detector and a second neutron detector, the first pulser circuit is to gate a response of the first neutron detector, and the second pulser circuit is to gate a response of the second neutron detector.

11. The tool of claim 9, wherein the first pulser circuit operates in phase with the second pulser circuit.

12. The tool of claim 1, further comprising: a second high-voltage generator, wherein the first cathode is coupled between the second high-voltage generator and the current source.

13. A method of operating a neutron porosity tool, the method comprising: causing a vacuum tube of the tool to produce two distinct neutron reactions by: driving a first radio-frequency (RF) cavity with a first pulser circuit of the tool, and providing a voltage to a target electrode with a first high-voltage generator of the tool, the vacuum tube enclosing the first RF cavity and the target electrode; and gating a response of a plurality of neutron detectors with the first pulser circuit to determine formation response offsets.

14. The method of claim 13, further comprising causing the first pulser circuit and a second pulser circuit of the tool to provide two differing ion accelerating voltages, such that deuterium-deuterium reactions and deuterium-tritium reactions are generated within a single reactance plane while the tool is positioned within a borehole.

15. The method of claim 13, wherein the tool comprises a second vacuum tube enclosing a second cathode and a second ion target co-located with the first ion target, the target electrode is a common target electrode that is coupled to the first and second ion targets, and the method further comprising providing the voltage to the common target electrode with the first high-voltage generator, such that deuterium-deuterium and deuterium-tritium reactions are generated within a single reactance plane while the tool is positioned within a borehole.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a downhole tool housing located within a cased borehole, where the casings are cemented to each other and to the formation; in this example, two spheres illustrate the difference in depth of investigation of high and low neutron energies.

(2) FIG. 2 illustrates a schematic layout of a typical pulsed neutron tube.

(3) FIG. 3 illustrates one embodiment of a bipolar pulsed neutron tube, illustrating the ability to combine two high-voltage generators to produce a higher tube voltage without changing the outer diameter of the downhole tool housing.

(4) FIG. 4 illustrates one embodiment of a unipolar pulsed neutron tube, illustrating the ability to combine two tubes into a single package with a common target electrode. Further illustrating the ability to effectively double the output of a single target-plane while using a single generator.

(5) FIG. 5 illustrates two embodiments of pulsing schemes that may be used to control either a single-polar tube or a pair of tubes with a common target. Further illustrating the ability to select neutron-burst energies through the selection of pulse-schemes to a common target.

(6) FIG. 6 illustrates one embodiment of a bipolar pulsed neutron tube, illustrating the ability to combine two tubes into a single package with a common target electrode, with each tube capable of developing differing tube voltages. Further illustrating the ability to select which tube and, therefore, which energy neutron would be emitted by the common linked targets, through use of two pulser and radio-frequency cavities.

BRIEF DESCRIPTION OF SEVERAL EXAMPLE EMBODIMENTS

(7) The methods and means described herein enable pulsed neutron generators to substantially increase their output and rapidly switch between output neutron energies while maintaining a single reactance plane, within the environment of a borehole. Control mechanisms for various neutron tube geometries and high-voltage generators that power said neutron tubes are provided, the tool including at least a pulsed neutron tube, a radio-frequency cavity, a high-voltage generator, and an electronic pulsing scheme to selectively switch said radio-frequency cavity.

(8) With reference now to the attached Figures, FIG. 1 illustrates an electronic neutron source located within a downhole tool pressure housing [101], which is located within a well casing [103] filled with well or drilling fluid [102]. The first well casing [103] is cemented [104] to a further casing [105] that is again cemented [106] to the well formation [107]. The energy of produced neutrons [108, 109] determines the depth of investigation of the measurement and, therefore, the offset of the detectors of optimum sensitivity. In this example, the lower energy neutrons produced through a deuterium-deuterium (DD) reaction would produce a measurement that is more sensitive to porosity changes within a near-field region [108], such as the cement immediately surrounding a borehole [104]. Whereas higher energy neutrons produced by a deuterium-tritium (DT) reaction would be sensitive to a near-and-outer-field zone [109], such as the formation [107] and outer cemented casing annuli [106] of a well. The bias between the DD measurement and the DT measurement can be used to indicate whether increased porosity (as could be expected by a fluid channel in the cement) is in the near-field zone [108] or outer-field zone [109].

(9) FIG. 2 illustrates a typical example of a contemporary neutron porosity logging tool, wherein a near space [201] and far space [202] neutron detector are located upon the axis within a tool housing, along with a neutron tube [203]. A replenisher current [206] causes the production of deuterium gas within a vacuum chamber within the tube [203]. A radio-frequency (RF) cavity within the chamber is driven by a pulser circuit [205] (operating at 1 kHz and 10% duty cycle, for example) which acts to ionize the deuterium gas into positively charged deuterons that are accelerated toward a negative grid, powered by a high voltage generator [204], and onto a target. The target is typically a metal halide that is doped with tritium, such that the bombardment of the tritium atoms with deuterons produces helium ions and 14.1 MeV neutrons. The pulsing technique means that pulses of neutrons are produced and the near [201] and far [202] space detectors operate during the time when the generator is not pulsing to collect signals coming back from the surrounding formation without being swamped by primary neutrons arriving directly from the source. To achieve this effect, the pulser signal is typically used to gate the response of the detectors.

(10) FIG. 3 illustrates one embodiment, wherein a cathode (filament) within a source tube [304] is held at a high direct current potential (such as 85 kV, for example) by a positive high-voltage generator [305], in addition to the target [extractor] electrode being held at 85 kV, by a negative high-voltage generator [306]. The result would be a potential difference across the tube [304] cavity of 190 kV, enough to enable a DD reaction (2 MeV), if the target is doped with deuterium. A mid-space detector [302] can be added between the near [301] and far [303] space detectors, such that sensitivity to the 2 MeV neutron physics is optimized. In another embodiment, the pulser's DC level is elevated to a high positive potential, such that the potential difference between the RF cavity and the target electrode is at a sufficient potential to accelerate the deuterons to fusion energies. In another embodiment, the target is doped with both tritium and deuterium, such that the either DT or DD-DT output can be selected by simply enabling or disabling the non-target multiplier [305], by a control circuit.

(11) FIG. 4 illustrates one embodiment where the neutron generation tube [403, 404] is mirrored around the target, such that a single high voltage generator [405] is required, to operate two halves of a tube with a common target electrode (even if the two physical targets are distinct and separate). By using two pulsers [406, 407] that are out of phase with each other, the effective pulse rate would double, thereby doubling output neutron flux from a pair of co-located targets. This effect ensures that the target region for the conjoined tubes is collocated. Power-wise, the beam current delivery of the high voltage generator would effectively double, but with a half-contribution interlaced from each tube-half [403,404].

(12) FIG. 5 illustrates an example embodiment of the conjoined common target region scheme illustrated in FIG. 4. A single pulsing regime [501] is illustrated as a function of voltage between ground [504] and high-voltage output [505] versus time [503]. Two pulser circuits with a common ground [504] would operate at a set frequency and duty-cycle but with the each side of the pulser operating [/2] out of phase with the other [502], such that the positive high-voltage [506] output is out of phase with the negative high-voltage [507] output. In another embodiment, the pulsers operate in phase with each other,

(13) FIG. 6 illustrates an example embodiment wherein an additional high voltage generator [607] is included on one side of the conjoined tube [604, 605], and the associated target doped with both tritium and deuterium, and an interleaved pulser scheme used as illustrated in FIG. 5, such that every pulse of neutrons out of the target alternates between 14 MeV neutrons and 2 MeV neutrons. In this manner, the response of the mid-space [602] and far-space [603] detectors is individually gated to the separate timing signals of the pulsers [609, 610] controlled by a control circuit, such that a separate profile for near-field and far-field porosity response is determined during the same logging run.

(14) In one example embodiment, a near space, mid space and far space neutron detector are located upon the axis within a tool housing, along with a neutron generator. A radio-frequency cavity within the chamber is driven by a pulser circuit (operating at 1 kHz and 10% duty cycle, for example) which acts to ionize the deuterium gas into positively charged deuterons that are accelerated toward a negative grid (powered by a voltage multiplier) and onto a target. The target is typically a metal halide that is doped with tritium, such that the bombardment of the tritium atoms with deuterons produces helium ions and 14.1 MeV neutrons. The pulsing technique means that pulses of neutrons are produced, as it permits the near and far space detectors to operate during the time when the generator is not pulsing, to collect signals coming back from the surrounding formation without being swamped by primary neutrons directly from the source. To achieve this, the pulser signal is typically used to gate the response of the detectors.

(15) In one embodiment the cathode circuit's DC level is held at a high potential (such as 85 kV), in addition to the target [extractor] electrode being held at 85 kV. The result is a potential difference across the RF cavity of 190 kV, enough to enable a DD reaction (2.5 Mev), if the target is doped with deuterium. A mid-space detector is located between the near and far space, such that sensitivity to the 2.5 MeV neutron physics is optimized. In another embodiment, the pulser's DC level is elevated to a high positive potential, such that the potential difference between the RF cavity and the target electrode is at a sufficient potential to accelerate the deuterons to fusion energies.

(16) In another example embodiment, the target is doped with both tritium and deuterium, such that either the DT or DD-DT output is selected by simply enabling or disabling the non-target multiplier.

(17) In another embodiment, the neutron generation tube is mirrored around the target, such that a single multiplier is required to operate two halves of a tube with a common target electrode (even if the two physical targets are distinct and separate). By using two pulsers disposed out of phase with each other, the effective pulse rate is doubled, thereby doubling the output neutron flux from a pair of co-located targets. This effect ensures that the target region for the conjoined tubes is collocated. Power-wise, the beam current delivery of the multiplier effectively doubles, but with a half-contribution interlaced from each tube-half. In the conjoined common target region scheme, two pulser circuits with a common ground operate at a set frequency and duty-cycle, but with the each side of the pulser operating [/2] out of phase with each other.

(18) In another example embodiment, the pulsers operate in phase with each other, the benefit being to double the output of the tube without increasing the thermal dissipation load on the individual target faces. A doubling of neutron output flux permits a doubling of possible logging speeds (up to 7,200 ft/hr., for example) without reducing the statistical quality (accuracy) of the measured porosity response.

(19) In another example embodiment, an additional multiplier is included on one side of the conjoined tube, and the associated target doped with both tritium and deuterium, and an interleaved pulser scheme used, so that every pulse of neutrons out of the target alternates between 14.1 MeV neutrons and 2.5 MeV neutrons. In this way, the response of the mid-space and far-space detectors is individually gated to the separate timing signals of the pulsers, such that a separate profile for near-field and far-field porosity response could be determined during the same logging run.

(20) Those of ordinary skill in the art will understand that in this context, D+T.fwdarw.n+4He (E.sub.n=14.1 MeV).

(21) Ordinarily skilled artisans will also appreciate that in this context, D+D.fwdarw.n+3He (E.sub.n=2.5 MeV).

(22) As the energy of produced neutrons determines the depth of investigation of the measurement, and therefore the offset of the detectors of optimum sensitivity, the lower energy neutrons produced through a D-D reaction will produce a measurement more sensitive to porosity changes within a near-field region, such as the cement immediately surrounding a borehole, whereas higher energy neutrons produced by a D-T reaction are sensitive to a near-and-outer-field zone such as the formation and outer cemented casing annuli of a well. The bias between the DD measurement and the DT measurement is used to indicate whether increased porosity (as could be expected by a fluid channel in the cement) is in the near-field zone or outer-field zone.

(23) In another embodiment, multiple detector positions are used to further increase the dimensionality of the received data (alternating between 2 MeV and 14 MeV) such that radial resolving ability can be increased.

(24) In another embodiment, the neutron detectors are helium-3-filled detectors.

(25) In another embodiment, the neutron detectors are lithium-6 glass detectors.

(26) In another embodiment, the source tube is shielded in all but one direction and is manipulated such that it rotates around the tool major axis, such that azimuthal porosity information may be determined through directional bias of the source and/or detectors using neutron moderating or shielding materials.

(27) In another embodiment, the source tubes and detectors are shielded in all but one direction and are manipulated such that they rotate around the tool major axis, such that azimuthal porosity information may be determined through directional bias of the source and/or detectors using neutron moderating or shielding materials.

(28) In another embodiment, technique is combined with ultrasound or x-ray density techniques. Radially resolved porosity could be highly advantageous when combined with x-ray, ultrasonic and azimuthal neutron techniques, to map-out (in 3D) the porosity associated with channels or fluid defects in cement surrounding a borehole.

(29) The foregoing specification is provided only for illustrative purposes, and is not intended to describe all possible aspects of the present invention. While the invention has herein been shown and described in detail with respect to several exemplary embodiments, those of ordinary skill in the art will appreciate that minor changes to the description, and various other modifications, omissions and additions may also be made without departing from the spirit or scope thereof.