GB2244330A - Analysis using neutrons - Google Patents

Abstract

A tool suitable for borehole logging, including production oilwell borehole logging, contains a radioisotope neutron source 14, a fast neutron detector 15 and a gamma-ray detector 16. The fast neutron detector is positioned to detect neutrons as they leave the source and before they enter the material to be analysed 12. Subsequent interactions with the material of neutrons scattered by the fast neutron detector give rise to gamma-rays which are detected. Information is registered as to the time delay between detection of a neutron and detection of a gamma-ray. This may be presented simply as an energy spectrum of gamma-ray detection events selected by coincidence or anticoincidence with detection of a neutron, or a time spectrum at selected energy levels. <IMAGE>

Description

Analysis using neutrons In the use of neutron irradiation coupled with gamma-ray detection for analysis of materials, such as in open boreholes, but in particular in the logging of production oilwell boreholes, many factors can combine to make analysis of the resultant gamma-ray spectrum difficult. There is a variety of different reactions possible between the neutrons and the nuclei in the target material, the probability for any particular reaction depending, inter alia, upon the identity and concentration of the target nucleus and the energy distribution of the neutrons. This energy distribution changes as the cloud of neutrons moves through the target material and neutrons lose energy through elastic and inelastic scattering.

In known techniques using neutron interrogation for borehole logging many elements are detectable from the characteristic gamma-rays they emit following neutron capture. Some elements have a low capture cross-section and are difficult or impracticable to detect in this way.

Carbon and oxygen in particular and also magnesium, for example, fall into this category, which is unfortunate because a determination of the concentrations of these elements is an important requirement in borehole logging.

A possibility for determination of these elements is offered by inelastic neutron scattering reactions, in which gamma-rays are emitted together with a scattered neutron and have an energy which is characteristic of the scattering nucleus. However, the gamma-ray spectrum from inelastic neutron scattering reactions and the gamma-ray spectrum from other reactions, notably capture reactions, are superimposed. Interference between a peak in the gamma-ray spectrum due to neutron capture reactions with nuclei of one element and a peak due to inelastic scattering by nuclei of another element makes it difficult to use those peaks in the total gamma-ray spectrum to give a quantitative measure of the concentration of those elements.

Where the logging is to be carried out in a production oilwell borehole, the problems multiply because (a) borehole fluids such as water and the production oil, which are in close proximity to the neutron source, exert a strong influence on the response detected; (b) the borehole lining, similarly in close proximity to the neutron source and containing iron with its high capture cross-section for thermal neutrons, also exerts a strong influence on the response detected; and (c) the logging tool has to have a small diameter, thus restricting the size of detector which can be used.

Timing offers a possible solution. Gamma-rays from inelastic scattering reactions appear a very short time after exposure of the sample to an incident burst of fast neutrons. Thus, a neutron leaving a neutron generator at about 14 MeV will travel at approximately 0.05 m per nanosecond. If the neutron undergoes inelastic scattering, the scattered neutron will be slowed, depending upon the reaction conditions, to a velocity in the range 0.01 m - 0.04 m per nanosecond. The associated gamma-ray, emitted almost instantaneously (within less than 10-16 seconds), travels at the speed of light (approx 0.3 m per nanosecond).On the other hand, gamma-rays from capture reactions emerge after a longer period, there being a delay due to the time taken for the neutrons to reach thermal velocities (by the process of energy loss through elastic and inelastic scattering) at which the cross-sections for neutron capture become significant. Thus it may take typically 100 microseconds to 1 millisecond for neutrons at 14 MeV to be slowed to thermal velocities. Upon capture, the characteristic gemma-ray emission of the particular capture reaction occurs almost instantaneously (again within less than 10-16 seconds).

It is known to make use of these timing effects to separate the inelastic scattering gamma-ray spectrum from the capture gamma-ray spectrum by irradiating the sample with a succession of neutron pulses and time-gating appropriately the response of the gamma detector in synchronism with the generation of the neutron pulses. For this an expensive pulsed neutron generator is required, with the added difficulty, for borehole logging, of providing the generator and detectors in an assembly suitable for feeding down the borehole together with a high voltage supply for the generator.

Furthermore, problems in the spectral analysis are not completely avoided because signals derived from gamma-rays emitted in high energy neutron reactions principally with oxygen nuclei (from, for example, oxygen in the formation water, and oxygen in the host rock) interfere with signals derived from the 4.43 MeV gamma-rays from carbon.

Our patent specification No. 2 212 264 discloses an invention based upon the appreciation that, for a useful proportion of inelastic neutron scattering events, it is possible, using a gamma detector and a neutron detector, to detect in coincidence the scattered neutron and the associated gamma-ray.

A problem with the invention of patent specification No. 2 212 264 and with all techniques involving use of coincidence counting is to meet the conflicting needs for accumulating an adequate count in a short time period whilst keeping the intensity required from the neutron source as low as possible. This problem is particularly significant when operating in a cased borehole, because of the attenuation of scattered neutrons returning front the formation towards the neutron detector.

The present invention is based upon the appreciation that a significant improvement in sensitivity can be achieved by positioning the neutron detector so as to detect fast neutrons leaving the source before they reach the material to be analysed. Coincidence detection can then identify a gamma-ray resulting when the neutron is subject to a subsequent scattering event within a short time interval (eg of the order of 20 nanoseconds), without the need to detect the scattered neutron.

The invention provides, in one of its aspects, apparatus for use in the analysis of materials comprising a source of neutrons, a neutron detector capable of detecting without absorbing at least a proportion of neutrons which pass therethrough, which detector is positioned to detect neutrons after they have left the source and before they enter the material to be analysed, means for detecting an event produced by interaction of a neutron with the said material, and registration means for registering the event together with information related to the time delay between detection of a neutron leaving the source and detection of a said event.

In one arrangement according to the invention, the said means for detecting an event comprise a gamma-ray detector, and the registration means register the number of gamma-ray detection events at at least one selected energy which occur in coincidence or anti-coincidence with the detection of a neutron.

By coincidence we mean that the time interval between the occurrences is less than a predetermined time interval chosen to be such as is likely to encompass expected differences in the detection times for the neutron and the respective gamma-ray resulting from scattering of the neutron, whilst being short enough to exclude fortuitous coincidences from other reactions. By anticoincidence we mean gamma detection events which occur outside the predetermined time interval set for defining coincidence.

In another arrangement according to the invention, the said means for detecting an event comprise a gamma-ray detector, and the registration means register the number of gamma-ray detection events which occur at at least one selected energy and include in the registration information as to which, if any, of two or more predetermined time intervals after detection of a neutron encompassed detection of a gamma-ray.

Alternatively, for each event detected, the registration means registers information as to energy of the gamma-ray and the time delay between detection of a neutron leaving the source and detection of the gamma-ray event.

For the detection of inelastic scattering reactions, it will be evident that the source of neutrons is such as to provide neutrons at an energy in excess of the threshold energy for the inelastic scattering reaction which, amongst those reactions of interest for the analysis, has the highest threshold energy. For example if inelastic scattering reactions with carbon and oxygen are of interest, neutrons in excess of 6.1 MeV are required to excite the inelastic scattering reaction with oxygen. If only carbon is of interest then it is only necessary for the incident neutron energy to exceed 4.4 MeV and, indeed there is advantage, where practicable, in avoiding excitation of higher energy reactions which may introduce additional complication of the resulting gamma-ray spectrum.

Where practicable in the particular conditions of use, it may be advantageous that the source is a radioisotope neutron source.

In one arrangement according to the invention, the neutron detector is constructed and positioned so as to be adjacent to and surrounding the source.

In another arrangement according to the invention the neutron detector is constructed and positioned so as to be adjacent to the source and to define a predetermined limited range of directions in which neutrons emanating from the source pass through the neutron detector, thereby to define a directional limitation in the analysis.

It will generally be necessary to include shielding positioned to prevent direct passage of ionising radiation from the source to the means for detecting an event.

Preferably a tubular housing is provided for containing the apparatus, the tubular housing being suitable for feeding into an oil production borehole.

The invention includes a method for analysis of material comprising irradiating the material with neutrons, detecting neutrons after they have left the source and before they enter the material to be analysed, detecting events produced by interaction of a neutron with the said material, and registering events together with information related to the time delay between detection of a neutron leaving the source and detection of an event.

Preferably gamma-ray events are detected and in one aspect of the invention registration is effected of the number of gamma-ray events detected at at least one selected energy which occur in coincidence or anti-coincidence with the detection of a neutron.

In a development, registration is effected of the number of gamma-ray events detected which occur at at least one selected energy, and the registration includes information as to which, if any, of two or more predetermined time intervals after detection of a neutron encompassed detection of a gamma-ray event. In a more advanced arrangement, for each event detected registration is effected of information as to the energy of the gamma-ray and the time delay between detection of a neutron leaving the source and detection of the event.

A specific construction of apparatus and a method embodying the invention will now be described by way of example and with reference to the drawings filed herewith, wherein: Figure 1 is a diagrammatic representation of an oilwell borehole logging tool, Figure la is a section on the line I-I of Figure 1, Figure lb is similar to Figure la, but showing a modification, Figure 2 is a block diagram of the signal processing circuit used with the apparatus of Figure 1, Figures 3 and 4 show gamma-ray spectra using the apparatus of Figures 1 and 2, illustrating respectively laboratory simulation of open and cased-hole borehole operation, Figure 5 is a gamma-ray spectrum obtained using anti-coincidence counting with apparatus as shown in Figure 1, Figure 6 is a block diagram of a development of the circuit shown in Figure 2, and Figure 7 is a block diagram of another form of signal processing circuit for use with the apparatus of Figure 1.

Figure 1 shows a logging tool 10 in a borehole 11 within formation rock 12. Mounted within a casing 13 of the logging tool 10 are a neutron source 14, neutron detector 15, and a gamma-ray detector 16, this being represented diagrammatically as a scintillation detector, with photomultiplier at 19. The neutron detector 15 surrounds the neutron source 14 so that all neutrons emanating from the source pass through the detector 15 before passing into the borehole and on into the formation 12. The gamma-ray detector 16, on the other hand is protected from direct exposure to the neutrons from the source 14 by shielding 17. In this example, the neutron source is a radioisotope source 241Am/Be. The neutron detector 15 is of the liquid scintillator type and the gamma-ray detector 16 is a bismuth germanate scintillation detector.

As illustrated diagrammatically in Figure 1, a proportion of the reactions in which incident neutrons from the source are inelastically scattered by nuclei in the formation rock 12 will result in detection of the associated gamma-ray by gamma-ray detector 16.

Other reactions of neutrons with nuclei in the formation rock 12 will generate gamma-rays. Of particular interest are neutron capture reactions which principally involve those neutrons which have an energy reduced to a level in thermal equilibrium with the formation material.

Using a coincidence technique it is possible to select for registration only those gamma-rays detected in detector 16 which are in delayed (ie by about 20 nano-seconds) coincidence with detection of a neutron by the neutron detector 15. It is, of course, necessary for the spectrum analysis to measure the energy of the detected gamma-rays and this is done in a conventional manner by integration of the total light output in the light pulse produced by the gamma-ray in the detector 16.

It is also important for the logging tool 10 to produce information from neutron capture reactions. These can be derived from gamma-ray events detected in detector 16 in anticoincidence with neutrons detected in detector 15.

Referring to Figure 2 the fast pulse indicating detection of a neutron from source 14 passing through the neutron detector 15 is fed on line 27 to an interval timer selector 22, a device, the construction of which will be evident from the description that follows of its function.

For an experimental set up, this function has been achieved by combining a standard time to amplitude converter with a standard single channel analyser. Two signals are derived from the gamma detector 16. One is a fast pulse signal generated at the start of a gamma detection event. This is supplied on line 21 to the interval timer selector 22. The other signal derived from the gamma detector 16 is a full signal pulse which is fed on line 23 to an amplifier/shaper circuit 24 of conventional form. Output from the amplifier/shaper 24 drives a pulse height analyser 25 under control of a linear gate 26.

If the signal on line 21 and that on line 27 are within the short time interval set in the selector 22, then a trigger pulse is generated on a coincidence line 28 at the output of the selector 22.

Thus, if the control terminal of linear gate 26 is connected via switch 31 to line 28, the pulse height analyser 25 will receive only those signals from the gamma detector 16 which are in coincidence with detection of a neutron by the neutron detector 15. If the control terminal of the linear gate 26 is connected via switch 31 to line 29, then the pulse height analyser 25 will receive only those signals from the gamma detector 16 which are in anti coincidence with detection of a neutron by neutron detector 15.

Figure 3 shows gamma-ray spectra generated by the pulse height analyser 25 for experimental samples respectively of sand, dolomite and a high carbon (eg Oil bearing) material, representing formation rock 12. The spectra were obtained with the apparatus set in the coincidence counting mode.

Figure 4 shows gamma-ray spectra generated by the pulse height analyser 25 for experimental samples respectively of sand and high carbon material, but with an intervening layer of steel for simulating cased hole operation. Again the spectra were obtained with the apparatus set in the coincidence counting mode.

The spectral quality (ie the ratio of the signal to background) for carbon at 4.43 MeV is greatly improved for a given source intensity and counting time as compared with the arrangement of patent specification No. 2 212 264.

Figure 5 shows an anticoincidence spectrum from a sample of sandstone containing oil. Comparison with Figures 3 and 4 demonstrates the excellent separation by the coincidence method of inelastic scattering events from a spectrum (Figure 5) dominated by thermal neutron capture in which it is difficult to determine for any peak what proportion of the count derived from the inelastic scattering reaction responsible for that peak and what proportion derived from interfering reactions.

Whilst a penalty is paid in loss of sensitivity due to the reduction in countrate because of the coincidence requirement, the loss is mitigated by the improved discrimination between inelastic scattering events and those due to neutron capture. Thus, within the typical commercial parameters of a neutron flux of 108 per second and a 15 minute counting period, the technique of the invention is capable of sensitivity to a minimum volume fraction of oil in sandstone of about 6% in a cased hole.

The timing resolution of the apparatus of the example (Figure 2) was approximately 2 to 3 nanoseconds. A faster gamma ray detector, such as can be secured using a high efficiency barium fluoride scintillator, coupled with improved design and/or improved electronics is capable of improving the timing resolution to about 1 nanosecond. In either example, with such timing resolution (ie to 3 nanoseconds or better), it is possible to include in the analysis a measure of the delay within the coincidence time interval between detection of a neutron and detection of the associated gamma-ray. This delay is related to the spatial region in which the inelastic scattering reaction took place. This appreciation can be used to eliminate unwanted signals from fluids within the borehole and from the borehole lining.

Figure 6 shows a development of the apparatus of Figure 2 illustrating one arrangement for taking advantage in this way of improved timing resolution, and also provides for logging data indicative of neutron energy.

In Figure 6 components which perform the same function (albeit with greater timing precision) as the corresponding components in Figure 2 are referenced with the same numerals.

In the Figure 6 example, time to amplitude converter 37 is capable of registering time delay between a start signal on line 27 and a stop signal on line 21, the signal amplitude fed to an analogue to digital converter 39 on line 38 being proportional to the delay.

A digitised form of the time delay is derived in turn by analogue to digital converter 39 and used to address and increment the contents of a two-dimensional memory 40. The complete memory address also requires a component of gamma-ray energy information which is derived from a digitised form of the gamma-ray detector signal. This is provided from amplifier shaper 24 on line 42 using analogue to digital converter 41. The stored contents of the two-dimensional array 40 are transferred at regular intervals to microprocessor 35.

Useful information can also be gleaned from the amplitude of the signal from the neutron detector 15. This is processed by amplifier shaper 43 and neutron energy selector or single channel analyser 44. A gating signal from the neutron energy selector on line 45, applied whenever neutrons deposit energy at the detector 15 within preset upper and lower limits, enables the passage of gamma-ray energy and time delay information from the analogue to digital converters 39 and 41 to the memory 40.

It will be appreciated that the energy transferred to the neutron detector by the neutron scattered is useful in that, where this is large the neutron will move into the formation with relatively lower energy and vice versa, thus providing some degree of control over the energy of neutrons entering the formation.

Whilst the apparatus of Figure 6 as described is capable of time delay analysis into 128 time windows in this example, for some applications it may be adequate to combine the counts so as to group them into fewer time windows. A selection of three windows is capable of providing useful information as the following discussion illustrates and a suitable data collection configuration is shown in Figure 7.

The first time interval starts with detection of a neutron registered by a signal on line 27 and ends after a time correponding with the likely delay in detecting gamma-rays from neutron scattering reactions occuring mainly within the borehole and its lining. The second time interval starts with the end of the first time interval and ends after a further delay chosen, in this example, to be long enough to encompass detection of gamma-ray events corresponding to reactions in the formation up to a reasonable distance from the source but not so long that the count is dominated by random events in which the detected gamma-ray event is not associated with the detected neutron. The third time interval starts with the end of the second time interval and ends after a delay appropriate for returning the circuit within a reasonable time to readiness for detection of the next gamma-ray event. The majority of events in which a gamma-ray is detected in this time interval are random, the detected gamma-ray event being unassociated with the detected neutron.

The three time intervals are chosen according to the particular requirements and circumstances of the analysis.

However, for production oilwell borehole logging, the first time interval might typically be of the order of 15 nanoseconds, the second time interval of the order of 50 nanoseconds and the third time interval in the range 200 to 500 nanoseconds. This is illustrated diagrammatically at 36 in Figure 7 by a time spectrum showing a typical form for the number of counts against time. The first, second and third time intervals are illustrated by the numerals I, II, III. It will be seen that most of the counts (in time interval I) derive from reactions close to the apparatus ie within the borehole and its lining. There is a contribution to the peak from reactions in the formation but these fall off quite rapidly with time (corresponding to distance from the source/detector 16) which during time interval III have settled to the random background level.

It will be appreciated that display or print-out from the microprocessor can be programmed as desired to produce spectra from the data based upon any selection of time interval or combination of time intervals or alternatively a time spectrum at a selected energy level can be generated.

The arrangement of Figure la, in which the neutron detector 15 surrounds the neutron source 14, will give a symmetrical measurement response. Useful directional effects can be generated by appropriate limitation of the detector around the neutron source 14. This is illustrated in Figure lb which shows, by way of example, a neuton detector 15b which extends through only 1800 around the source.

The invention is not restricted to the details of the foregoing examples. For instance, a radioisotope neutron source is preferred because it is simpler, smaller and cheaper than a neutron generator. However, a neutron generator can be used as a neutron source in accordance with the invention, and can be used, if desired, in continuous rather than pulsed mode. Whilst commercially available neutron generators produce neutrons at 14 MeV, a lower energy generator would be advantageous for avoiding unwanted high energy reactions, as discussed above. On the other hand, there are circumstances when it is helpful to have the greater penetration and speed of neutrons at 14 MeV.

Whilst a neutron detector of the liquid scintillator type is described, any neutron detector of the proton recoil type which is capable of detecting without stopping neutrons would be suitable. Proton recoil detectors are usually organic and a high temperature plastic scintillator could be used instead of a liquid scintillator.

Gamma-ray detection using bismuth germanate or barium fluoride scintillator is descibed and these detectors have advantages but also some disadvantages such as limited tolerance to operation at elevated temperatures. The detector has therefore to be chosen according to the intended use and, for particular applications scintillators of doped sodium iodide or caesium iodide, with its capability for tolerating higher temperature operation, may be more suitable.

Production oilwell borehole logging is an application of importance for the apparatus of the foregoing examples, and, in particular, detection of carbon and oxygen.

However, the apparatus is readily adapted for detection of other elements and/or for different types of analysis for example, the apparatus can be used in geological survey boreholes.

Claims (16)

Claims

1. Apparatus for use in the analysis of materials comprising a source of neutrons, a neutron detector capable of detecting without absorbing at least a proportion of neutrons which pass therethrough, which detector is positioned to detect neutrons after they have left the source and before they enter the material to be analysed, means for detecting an event produced by interaction of a neutron with the said material, and registration means for registering the event together with information related to the time delay between detection of a neutron leaving the source and detection of a said event.

2. Apparatus as claimed in claim 1, wherein the said means for detecting an event comprise a gamma-ray detector, and the registration means register the number of gamma-ray detection events at at least one selected energy which.

occur in coincidence or anti-coincidence with the detection of a neutron.

3. Apparatus as claimed in claim 1, wherein the said means for detecting an event comprise a gamma-ray detector, and the registration means register the number of gamma-ray detection events which occur at at least one selected energy and include in the registration information as to which, if any, of two or more predetermined time intervals after detection of a neutron encompassed detection of a garr6a-ray.

4. Apparatus as claimed in claim 1, wherein the said means for detecting an event comprise a gamma-ray detector, and the registration means register for each event detected information as to energy of the gamma-ray and the time delay between detection of a neutron leaving the source and detection of the event.

5. Apparatus as claimed in any of claims 1 to 4, wherein the source of neutrons is such as to provide neutrons at an energy in excess of the threshold energy for the inelastic scattering reaction which, amongst those reactions of interest for the analysis, has the highest threshold energy.

6. Apparatus as claimed in any of claims 1 to 5 wherein the source is a radioisotope neutron source.

7. Apparatus as claimed in any of claims 1 to 6, wherein the neutron detector is constructed and positioned so as to be adjacent to and surrounding the source.

8. Apparatus as claimed in any of claims 1 to 6 wherein the neutron detector is constructed and positioned so as to be adjacent to the source and to define a predetermined limited range of directions in which neutrons emanating from the source pass through the neutron detector, thereby to define a directional limitation in the analysis.

9. Apparatus as claimed in claim 7 or claim 8, wherein shielding is positioned to prevent direct passage of ionising radiation from the source to the means for detecting an event.

10. Apparatus as claimed in any of the preceding claims, wherein a tubular housing is provided for containing the apparatus, the tubular housing being suitable for feeding into an oil production borehole.

11. A method for analysis of material comprising irradiating the material with neutrons, detecting neutrons after they have left the source and before they enter the material to be analysed, detecting events produced by interaction of a neutron with the said material, and registering events together with information related to the time delay between detection of a neutron leaving the source and detection of an event.

12. A method as claimed in claim 11, wherein gamma-ray events are detected and registration is effected of the number of gamma-ray events detected at at least one selected energy which occur in coincidence or anti-coincidence with the detection of a neutron.

13. A method as claimed in claim 11, wherein gamma-ray events are detected, and registration is effected of the number of gamma-ray events detected which occur at at least one selected energy, and the registration includes information as to which, if any, of two or more predeterlained time intervals after detection of a neutron encompassed detection of a gamma-ray event.

14. A method as claimed in claim 11, wherein gamma-ray events are detected, and for each event detected registration is effected of information as to the energy of the gamma-ray and the time delay between detection of a neutron leaving the source and detection of the event.

15. Apparatus substantially as herein described with reference to and illustrated in, Figure 1 and Figure 2, or Figure 6 of the drawings filed herewith.

16. A method substantially as herein described with reference to Figure 1 to 5 or Figure 6 of the drawings filed herewith.