JPS62473B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Broadly speaking, the present invention relates to the well logging method and device;
More specifically, an improved method for obtaining additional, more accurate information about the location and mining potential of underground geological hydrocarbon reserves through the combined rocking of neutron property data and gamma ray spectroscopy data. and regarding equipment.

In general, logging to identify oil or gas reserves not only detects the presence of hydrocarbons, but also determines their relative abundance (saturation) and ease of recovery. is important. In this connection, it is desirable to obtain information on formation parameters such as stratigraphy, shaliness, porosity and salinity. This is because this type of information is useful for quantitatively assessing the saturation levels of hydrocarbons and water, and for assessing the productivity of any given formation.

An example of a nuclear logging device for providing such information is described in US Pat. No. 3,521,064, of which the applicant is the patentee. The device described in this patent matches a detected gamma-ray energy spectrum of a formation of unknown composition, such as a thermal neutron capture gamma-ray spectrum, to a composite spectrum made up of weighted spectra of materials of known composition. It is analyzed by Accurate analysis of formation components can be performed by comparing the magnitude of the detected gamma-ray energy spectrum at a number of separate points or energy levels to the magnitude of the composite spectrum to obtain the best possible match. By appropriately selecting the component spectra, it is also possible to obtain a spectral output representative of properties such as porosity, shale content, salinity content, stratigraphy, etc. of the formation of interest.
However, conventional devices for implementing the technical ideas presented in the above-mentioned U.S. patents are subject to statistical uncertainties and perturbations, which impair the usefulness of such devices in some situations. Ta.

Other forms of casing-penetrated core formations include one or more selected neutron properties of the formation, including, for example, the thermal neutron lifetime or decay time τ, the macroabsorption (capture) cross-section Σ, and the neutron deceleration time. This includes the determination of A device useful for taking measurements of those properties is disclosed in U.S. Pat. No. 3,566,116;
No. 3,662,179 and US Pat. These devices provide much more valuable information that is particularly useful in distinguishing between brine and oil and detecting changes in water saturation. However, the interpretation of neutron property logs, such as τ and Σ logs, is improved by reliable and correlatable data of formation stratigraphy, porosity, and shale content. This is especially true in low salinity formations where the logs of τ and Σ are unreliable and different formations exhibit similar τ and Σ values.

Although it is generally accepted that a useful correlation exists between thermal neutron lifetime logs and certain gamma spectroscopy log data, as disclosed in U.S. Pat. No. 3,413,471, a single instrument There has been no integration of neutron signature logging and gamma-ray spectroscopic logging that would serve to provide sufficient information about the various formation parameters to allow a complete and accurate study of the hydrogen zone.

As can be seen from the above points, it is known how to produce gamma ray spectrometry measurements within a hole by irradiating the tissue from within the hole with pulses of neutrons that create a localized neutron distribution. As neutrons interact with matter, they lose energy and generate heat. When a neutron encounters an atomic nucleus, the thermal energy causes neutron-neutron interaction to form gamma rays. The energy of this gamma ray depends on the characteristics of the atomic elements. These gamma rays with characteristic energies are later analyzed by spectral techniques to identify the elements that give rise to the detected gamma rays.

The neutron capture cross section of the tissue being investigated varies depending on the material of the tissue (e.g. sandstone, shale, marble, brine and oil all have different thermal neutron capture cross sections and therefore different thermal neutron decay times). . Since the neutron source is placed in the hole, the gamma rays resulting from neutron-neutron interactions in the hole always appear first at a time before the gamma rays are generated in the tissue, but only at different relative times depending on the identity of the tissue material. survive. Since we wish to perform gamma ray spectral measurements of tissue components rather than hole components, we believe that detecting hole gamma rays can be avoided by delaying the time the detector responds to gamma rays. However, Applicants also know that because the number of gamma rays decreases over time, it is undesirable to delay them for long periods of time following a neutron pulse. Too long a delay time will seriously reduce the measurement efficiency due to a decrease in the count rate.

It is an object of the present invention to provide improved spectral analysis of tissue components surrounding the hole by neutron-gamma spectroscopy techniques.

Another object of the present invention is to provide a hole neutron-gamma ray spectroscopy technique that provides an improved gamma ray energy spectrum for spectral analysis.

These objectives are achieved by measuring the thermal neutron decay time of the tissue and calculating the pulse cycle and the detection cycle of the decay time, and by controlling the detector on period of the detection cycle according to the measured decay time. achieved.

This has the following technical features. That is, the detector on period of the detection cycle is such that the number of gamma rays in the background and in the hole is minimized relative to the gamma rays in tissue, and the quality of the gamma ray spectrum is optimized to yield more meaningful results when analyzed for tissue components. It is something to do. Another technical advantage is as follows. The data collection period of the detection cycle is shifted as the tissue changes, since the effect of the hole becomes larger and smaller at different times during the detection cycle depending on the neutron capture cross-section of the tissue.

In practice, these objects and advantages are achieved by an apparatus that performs the following operations. That is, by irradiating the tissue with a pulse of neutrons, detecting the time distribution and energy of gamma rays resulting from the interaction of neutrons with tissue neutrons, and measuring the neutron radiation according to a predetermined measurement of the thermal neutron decay time or capture cross section. 1st
The duration and location of the data collection time following the pulse is controlled to generate a gamma ray energy spectrum and this spectrum is analyzed.

If the device properly controls the duration and location of the data collection time, the resulting spectrum will contain a small amount of unwanted background and gamma rays from the holes.

In this method, the thermal neutron decay time or the capture cross section of the tissue is either known in advance or determined from the time distribution of the gamma rays detected by the detector.

An object of the present invention is to perform neutron logging spectral analysis to obtain information on the composition of rocks without being influenced by boreholes.

Another object of the invention is to avoid the problems caused by intermittent compositions in which boreholes with completely different neutron capture cross sections are taken, which if not taken into account would adversely affect the measurement operation. The goal is to perform neutron logging spectrum analysis.

These and other objectives are achieved by a neutron spectroscopy logging device that accepts gamma rays that are indicative of rock composition, eliminating borehole effects without compromising measurement speed (count rate) or introducing undue undesirable background components. and method. These objects are achieved by providing an apparatus and method that automatically adjusts the time position of a data collection gate in response to measurements of the thermal neutron decay time of a composition.

These objectives are achieved as follows.
That is, the neutron composition characteristics (thermal neutron decay time) are measured to form a series of bursts of neutrons, the gamma rays between these bursts are differentiated from the gamma rays generated in the detected borehole, and the count rate is maximized. This is accomplished by an apparatus and method for extracting detection events with respect to time as a function of the measured thermal neutron decay time to make measurements with an optimal ratio of signal to interfering background. This apparatus and method improves the spectral analysis of gamma-ray spectra obtained since the effects of borehole and background on the spectra can be minimized without adversely affecting the count rate during the detection process. Moreover, in the present invention, since the count rate is maintained as high as possible, the neutron capture gamma spectrum logging operation can be performed at a fairly high speed.

For example, the measured neutron properties are the thermal neutron absorption properties of the formation, i.e., the thermal neutron decay time τ or the macro absorption (capture) cross-section Σ, and the analyzed gamma-ray energy spectrum is the thermal neutron absorption (capture) gamma-ray spectrum. . By selecting the portion of the gamma-ray time distribution to be analyzed according to the measured value of the neutron absorption property, the selection of the captured gamma-ray energy spectrum is made appropriate for the particular formation being explored. Therefore, regardless of the fact that the thermal neutron absorption properties of the formation, which fundamentally determine the duration and shape of the captured gamma ray time distribution following the neutron pulse, differ between the formations penetrated by the borehole, the selection The resulting spectrum represents the composition of that stratum.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

As mentioned above, the present invention broadly relates to the discovery of oil and gas reserves in geological formations;
The purpose is to obtain useful information for evaluation. The present invention is particularly useful in logging cased holes, for example, for workover and identification of producing wells and unproducing oil zones;
Open holes are also useful. A representative apparatus for carrying out the invention in a manner with particularly advantageous results is shown in FIG. In particular, this device simultaneously takes τ and Σ measurements of a formation of interest and analyzes selected portions of the formation's captured gamma-ray spectrum primarily in accordance with the curve-fitting technique disclosed in the aforementioned U.S. Pat. No. 3,521,064. Configure the device that will be used.

Referring first to FIG. 1, a representative embodiment of the present invention shows a water-resistant, pressure-resistant and heat-resistant borehole measuring instrument 10.
including. This measuring instrument 10 is connected to a borehole 12 by an armored cable 14 in order to investigate underground strata 16.
suspended inside. Borehole 12 is shown including a borehole fluid 18, a steel casing 20, and cement 22 outside the casing. Although no piping is shown in borehole 12, meter 10 can be dimensioned to use through-piping if desired.

The measuring instrument 10 includes a pulsed neutron source 24 and a radiation detector 26 provided apart from the neutron source 24 . In order to prevent neutrons from directly entering the detector 26 from the neutron source 24, between the neutron source 24 and the radiation detector 25,
A neutron shielding material 28 of a normal composition is preferably provided. The neutron source 24 is adapted to generate discrete pulses of fast neutrons, for example 14 MeV, and can be of the type described in detail in US Pat. Nos. 2,991,364, 3,546,512. The radiation detector 26 may have any structure as long as it detects gamma rays and generates a pulse signal in response to the gamma rays having an amplitude representing the energy of the detected gamma rays. Radiation detector 26 includes a scintillation crystal 30. This scintillation crystal 30 is optically coupled to a photomultiplier tube 32. The crystal preferably uses the thallium-activated sodium iodide form, although other suitable crystals such as thallium- or sodium-activated cesium iodide may also be used. Alternatively, solid state detectors with germanium (lithium) crystals can also be used. Electric power is supplied to the measuring instrument 10 from a ground power source (not shown) through a cable 14 . Neutron source 24 and radiation detector 26
The meter 10 may also include a suitable power source for driving the power supply and other measurement equipment.

Sleeve 34 impregnated with boron carbide
surrounds the part of the case of the measuring instrument 10 in which the neutron source 24 and detector 26 are housed. The sleeve 34 is provided with longitudinal grooves to allow borehole fluid to flow longitudinally along the case of the meter 10. The diameter of the sleeve 34 is sized to allow free movement of the meter 10 within the casing 20.
This sleeve 34 reduces the amount of unwanted gamma rays incident on the detector 26. Sleeve 30 removes borehole fluid 18 from the vicinity of meter 10, thereby reducing gamma rays resulting from interaction of fluid 18 with neutrons, and gamma rays resulting from interaction of neutrons with the casing, or the meter. It also acts as a neutron absorber, absorbing neutrons in the vicinity of the detector 26 to reduce gamma rays produced by activation of iron and other elements within.

The pulse from the photomultiplier tube 32 is sent to the preamplifier 3
6 and then applied via lead 38 to the input terminal of amplifier 40 for use in the differentiation of τ, and to the input terminal of amplifier 40 for spectral analysis.
is applied to the input terminal of amplifier 44 via. Separate detectors can be used to determine τ and spectroscopic features if desired. If separate detectors are used, the detectors associated with spectroscopy must be gamma-ray energy-responsive, but the detectors associated with τ need not be gamma-ray energy-responsive. For example, the detector associated with τ may be of the thermal neutron detection type, such as a proportional counter filled with helium-3. In any case, very high energy gamma rays are incident on the detector 26 during and immediately after the neutron pulse is generated by the neutron source 24. For this purpose, high rate pulses corresponding to the gamma rays are generated in the photomultiplier tube 32. Except for the time when gamma rays are measured, during the generation of the neutron pulse and for a short period immediately after generation ceases, the photomultiplier tube 32 and the amplifiers 40, 44 are protected from harmful effects due to abnormally high count rates. The operation of the photomultiplier tube 32 is stopped to protect other signal processing elements. As will be explained in detail later, the neutron source 24
The operation of is controlled by signals applied via lead 48 from programmer 46. These control signals can also be used to deactivate photomultiplier tube 32 for a desired amount of time.

A pulse is applied from amplifier 40 to discriminator 52 . The discriminator 52 passes only pulses above a predetermined amplitude level and converts the received pulses into standard size output pulses. These pulses are applied via conductor 54 to τ calculation circuit 56. This τ calculation circuit 56 is described in US Pat. No. 3,662,179.

As explained in this U.S. patent:
The τ calculation circuit 56 includes a gating circuit, a counting circuit, a comparator circuit, and an oscillator circuit, and is responsive to the pulse passed through the discriminator 52 during a variable length detection period following the selected neutron pulse. and solve the following equation.

N = 1/2 (N ₁ + N ₃ ) − N ₂ (1) where N ₁ is the number of counts during the first detection time, i.e. the duration starting at time 2τ after the end of the preceding neutron pulse. The number of counts during a gate of 1τ, N ₂ is the number of counts during a gate of length 2τ starting immediately after the second detection time, i.e. the first time, N ₃ is the number of counts during a gate of length 2τ, starting immediately after the second detection time, i.e. the preceding time. This is the number of counts during a gate whose duration is 3τ, starting when the time 6τ has passed after the end of the neutron pulse.

N ₃ is the main time interval with respect to the background gamma rays,
Since it is determined to correct the counts N ₁ and N ₂ from τ, it can be omitted when the detector related to τ is not sensitive to gamma rays.

If the solution to equation (1) is N=0, the τ calculation circuit 56
The apparent value of the collapse time obtained by can be taken as the actual collapse time of the stratum under investigation. If N is not zero, the adjustment of the variable frequency oscillator will generate an error signal in the τ calculation circuit 56, which will restore the device to the N=0 condition. As further described in U.S. Pat. No. 3,662,179, the variable frequency oscillator can be constructed from a digital oscillator that includes a 100 KHZ clock signal circuit and as many oscillating flip-flop circuits as desired. These flip-flops change states according to the 100KHZ clock signal, and the states of these flip-flops are determined by the corresponding number of comparators (τ calculation circuit 5).
6) is continuously compared with the corresponding number of flip-flop circuits in the detector signal counting circuit. When the counter and oscillator flip-flops are in the same state, a pulse is generated that resets all oscillator flip-flops. This technique actually converts a count rate signal into a time signal. The reason is that the oscillator flip-flops change their individual states until the state of the oscillator flip-flop circuit matches the state of the counter flip-flop representing the observed radiation recorded by the counter. Therefore,
The output signal generated by the oscillator represents the time it takes for the oscillator to count up to a predetermined level of radioactivity, and thus represents the thermal neutron decay time of the formation. The oscillator output signal is applied via conductor 58 as an error or control signal for controlling the operation of programmer 46.

By increasing the frequency of the clock signal,
Alternatively, by adding more stages to the array driven by the flip-flop's clock signal, the retrieved value of τ becomes more reliable.
For example, a frequency of 300KHZ can be adopted. Also, an array driven by a flip-flop clock signal can be configured to provide a high initial count rate that decreases exponentially with time after the neutron pulse has terminated. As a result, the τ calculation circuit 56
becomes more stable. The reason is that the change in τ caused by the generated error signal is always relatively small compared to the absolute value of τ. It also allows the use of small capacity counters.

Programmer 46 implements a counter with appropriate diode matrix logic. This kind of programmer has US Patent No. 3566116, 3609366,
Disclosed in No. 3662179. The programmer includes the conventional logic and signal combination circuitry necessary to generate the gate open signal during the τ detection time interval, , in response to the output signal train from the oscillator of the τ calculation circuit 56. These gate opening signals are applied via conductor 60 to the τ calculation circuit 56, which controls the operation of the gate circuit and pulse counting circuit as described above. Programmer 46 also includes circuitry that generates control signals that adjust the duration and repetition rate of the neutron pulses emitted by neutron source 24. Since the time of occurrence (start and duration) of the τ detection time interval, , is related to the time of occurrence of the associated neutron pulse, the τ gate opening pulse applied to conductor 60 is also applied to the neutron source 24 via conductor 48. Synchronized to control signal. The preferred pulse control operation order of the neutron source 24 and the gate control order of the τ calculation circuit 56 are as follows:
This will be explained later with reference to FIG.

Programmer 46 may be configured to generate a signal representing τ (or Σ) for transmission to the neutron source, and may also be configured in accordance with the disclosure of the aforementioned US Pat. No. 3,662,179. Alternatively, the number of times the neutron source 24 is pulse-controlled and the time at which the τ detection time is sequentially triggered can be counted for a predetermined period of time. Since the repetition rate of the neutron pulses is controlled as a function of τ, the number of pulses generated in a given time is proportional to Σ and therefore τ.
The counting time must of course be of sufficient duration to make statistically reliable measurements and must be repeated at a frequency that produces the desired vertical resolution in the τ-Σ log. A convenient circuit for this purpose includes a programmer 46 synchronized with a control signal for the particular neutron pulse to be counted.
from programmer 46 to conductor 63 to pass the pulses generated by and applied to conductor 65.
AND circuit 62 responsive to an open signal applied to
can include. The gated pulses are applied via conductor 67 to a binary counter 69. At the end of the counting period, a delay shot 71, energized, for example, by the trailing edge of the gate open signal on conductor 63, signals a binary encoded parallel signal over conductor bundle 73 indicative of the stored pulse count. The counter 69 is commanded to first output to the processing and cable drive circuit 64 and then reset to zero.

Next, returning to the spectroscopic operation of the present invention, the output pulse from the amplifier 44 is passed through the conductor 66 to the pulse height analyzer 6.
Added to 8. The pulse height analyzer 68 can be of the conventional type, such as a single lamp (Wilkinson rundown) type;
is operative to select the pulses according to their amplitude and apply them, e.g. in binary coded parallel form, to the signal processing and cable drive circuit 64 via corresponding channels of the output conductor bundle 70. do. It is understood to include the usual low-level and high-level discriminators for selecting the energy range to be analyzed, and a direct gating circuit for controlling the time portion of the pulses generated by the detector to be analyzed. There will be. To this end, appropriate signals are generated by programmer 46 which are applied via conductors 72, 74, 76 to the discriminators to adjust them and open the linear gate circuits. The portion of the time distribution of the signal following each neutron pulse selected for analysis corresponds to any desired portion of the time distribution of gamma rays resulting from the neutron pulse, but preferably thermal neutron capture with the core of the formation. This corresponds to the gamma rays generated by the interaction. Therefore, the programmer 46
is preferably configured to produce a gate open signal on conductor 76 such that the linear gate circuit is opened during the time following each neutron pulse when formation-trapped gamma rays are predominant. In accordance with features of the invention described below, the time at which this gating (detection) time occurs is controlled in relation to the preceding neutron pulse as a function of τ measured in circuit 56. To that end, programmer 46 includes appropriate logic and signal combination circuitry to generate the desired gate opening signal in synchronization with the generation of the associated neutron pulse.

The part of the total energy spectrum of the gamma rays that corresponds to the gated detector signal to be analyzed can also be selected as desired, e.g.
Can be expanded from 1.5MeV to 7.5MeV. The number of channels used over the energy range of interest will of course depend on the desired precision of the analysis and the resolution of the scintillation crystal used. For thallium-activated sodium iodide crystals, in the energy range of 1.5-7.5 MeV
It has been found that around 200 channels can provide satisfactory spectral analysis. However, if desired, a smaller number of channels, such as about 50 channels, can be used. In general, the number of channels, channel width, total energy range and other characteristics of analyzer 68 can be determined in accordance with the techniques disclosed in the aforementioned US Pat. No. 3,521,064.

Processing and driver circuit 64 may be of any conventional configuration for encoding, time division multiplexing, and other processing of data bearing signals applied thereto and for providing those signals to cable 14. , the special circuitry used for these purposes is not part of the invention. circuit 64
A preferred structure for the system is described in U.S. patent application Ser.
Data).

On the ground, the Σ-related signal from the binary counter 69 and the count per channel signal from the pulse height analyzer 68 are amplified, decoded and otherwise processed in circuit 78 for application via wire bundle 80 to computer 82. undergo the necessary processing. Computer 82 generates values of one or both of Σ and τ and desired spectral output values, such as values indicative of things such as water saturation, shale content, rock properties, porosity, and water salinity. do. A preferred form of spectral output will be discussed later. Digital representations of these values are applied to tape recorder 86 and digital-to-analog converter (DAC) 88 via leads 84A-84H. DAC 88 generates analog signals proportional to their respective inputs and provides those analog signals to visual indicator 90.
Monitor data (not shown) such as the average count rate or head voltage during the first τ detection interval may also be recorded. Tape recorder 86 and visual recorder 90 are conventional and record the logging signal as a function of instrument depth. A cable-following mechanical link, indicated at 92 in FIG. 1, is provided for this purpose.

Computer 82 can be of any structure suitable for the calculation of Σ and τ, performance with spectral matching, component ratio determination, and ratio formation processes. For example, computer 82 is a PDP manufactured by Digital Equipment Corporation located in Massachusetts, USA.
- A general purpose digital computer such as an 11-inch computer can be used. computer 82
can be installed inside the borehole as shown in Figure 1, or it can be installed at a distance from the borehole.
such as by recording the decoded signals from processing circuitry 78 on magnetic tape.
It is also possible to process the recorded representation τ and Σ data of the counts per channel.

For analysis of geologically captured gamma ray spectra,
The so-called "borehole effect" that occurs from the interaction of neutrons with materials in the immediate vicinity of the neutron beam 24 and the detector 26 (measuring device housing, borehole fluid 18, casing 20, cement 22, etc.) causes spectroscopic detection. The time of occurrence of the period is reduced relative to the time of occurrence of the associated neutron pulse by controlling it according to the measured value of τ for a particular formation to be analyzed. FIG. 2 shows controlling the generation time of the neutron pulse and the length of the detection period based on the tissue decay time. Figure 3 shows a typical example of how the captured gamma ray count rate varies with time after a fast neutron burst 98, represented by the decay time τ. At the left end of the time distribution curve 100 there is an early borehole effect, i.e.
There is a region of rapid decay due to the high absorption of thermal neutrons by the medium immediately surrounding the neutron source and detector in the borehole. Next, there is the almost straight section. This straight line section is useful for determining the thermal neutron decay time according to the technique disclosed in U.S. Pat. Responding to collapse. Finally, as you head to the right, curve 1
00 is flat, and the count rate in this part corresponds to the background radioactivity in the strata and borehole.

To calculate τ, the US Pat. No. 3,566,116
2τ of the neutron pulse before the start of the detection period.
After the time delay most undesirable borehole effects are eliminated, placing the initial portion of the first detection interval in the exponential decay region of curve 100 and controlling the τ calculation circuit 56 gate circuit accordingly. . Such a τ gating order (for the case where the overall detection period is 7τ, a background time interval of 3τ separated by 1τ from the time interval, with two adjacent main time intervals (1τ), (2τ) ) is shown in FIG. 2, although other total τ detection periods and other detection time intervals within the overall period can be employed, as disclosed in the above-mentioned US patents. In any case, the time setting of the τ detection period is automatically controlled according to the τ of the particular formation under investigation.

For the purpose of captured gamma ray spectroscopy, 1 is shown in Figure 2.
Leading neutron pulse 98, indicated at 02
2τ starts after a delay of 1τ after the end of
It has been found that a spectroscopic detection period of time satisfactorily eliminates the early borehole effect and at the same time increases the counting rate for improved statistics and better vertical resolution. Because the timing of the spectroscopic detection period is thus tied to the measured neutron properties of the formation, the spectroscopic period is automatically controlled in a correct manner from formation to formation. Other time settings for the detection period 102 can of course be used. However, in general, the timing of this period should be determined to maximize the formation capture gamma rate relative to the background count rate in a manner commensurate with suppressing borehole effects.

To further increase the efficiency of this device and achieve higher count rates, the duration and repetition rate of the neutron pulses are also controlled according to τ. According to the invention, the duration of each neutron pulse is preferably 1τ. In fact, in the detection period 102 of FIG. 2 following a single neutron emission, not enough gamma rays are detected to perform a spectral analysis of gamma ray energy. Therefore, it is necessary to use an operating cycle as shown in FIG. 3, where one cycle includes repeated neutron emissions and data collection time. Each gamma-ray spectrum is thus a collection of gamma-rays detected during a series of detection periods following a series of pulses in a series of elementary cycles. A predetermined number of neutron pulses spaced apart in time are generated preferably periodically during each successive time interval. FIG. 3 outlines a preferred elementary cycle with a duration of 31τ.
This cycle consists of a 20τ spectroscopic subcycle and a 10τ
, and a recycle period of 1τ. The spectroscopic subcycle consists of a total of five periods A, B, C, D, E, each period 4τ in length, each period containing neutron pulses 104A-104E of 1τ duration and 2τ duration. and certain associated spectral detection periods 106A-106E. Detection periods 106A-106E are timed with respect to the associated neutron pulses 104A-104E as described in connection with spectroscopy gate 102 of FIG. Each neutron pulse 104A-104E is preferably generated at the same time as the preceding spectroscopic detection period 106A-106E ends, such that neutron pulses occur at 4τ time intervals during a spectroscopic subcycle.

The decay time subcycle begins with the sixth neutron pulse 104E out of a total of 31 τ cycles. This pulse also precedes the spectral detection period 106
Try to make it happen as soon as possible after E. The main function of the decay time subcycle is to allow τ to be determined, and the sixth spectroscopic detection period 106E is also included within that subcycle. The decay time subcycle also includes, of course, the two main τ detection time intervals, and if the detector associated with τ is a gamma ray detector, the background detection time interval is set as described with reference to FIG. It will be destroyed. The last 1τ time interval is included within the 31τ cycle and the programmer 4
6, and as described later,
Perform spectral stabilization.

The above 31 τ cycles are repeated sequentially as the instrument is moved through the borehole, and the decay times are iteratively determined by τ calculation circuit 56 and under the control of programmer 46 neutron pulses 104A-104
E and associated spectral detection periods 106A-106E
The time of occurrence of and τ detection time interval, , is τ
continuously adjusted according to the measured value. Therefore, the spectral detection periods 106A-106E and the τ detection periods are defined as follows: placed automatically. Furthermore, the neutron source pulse control operation and detector gating operation, which constitute a cycle of 31τ, result in a large spectroscopic duty cycle (12τ from every 31τ) with a correspondingly high count rate for spectral analysis purposes. and frequent τ to quickly respond to changes in formation conditions.
Sampling (once during 31τ) is performed.

The resolution of the gamma ray energy spectrum provided by the meter 10 will of course depend on the stability of the energy response of the detector, analyzer, etc. Furthermore, if the spectral matching technique disclosed in the aforementioned U.S. Pat. It is important that they be approximately the same when detected. It is therefore desirable to have a device for iteratively examining the response of the system during use and for immediately compensating for any instability detected. Such instability may be due to, for example, the sensitivity of the detection crystal, photomultiplier tube, and electronics lowered into the borehole to changes in temperature within the borehole, as well as to other operating conditions. This is usually due to a change in the pulse amplification gain of the detector-analyzer/circuit, or a change in the pulse height/channel relationship of the pulse classification circuit of the pulse height analyzer. Detection and compensation for such errors may be performed at any suitable time and in any suitable manner.
However, it is convenient to carry out according to one of the following methods between 24τ and 31τ within each cycle of 31τ (shown as the stability period in FIG. 3).

A natural gamma ray generator with a peak energy as low as possible than the peak energy range of the gamma rays being analyzed (e.g. 1.5 to 7.5 MeV) is
A pulse height analyzer 68 is provided in close proximity to the detector 26 and a designated channel or portion of the channel is located adjacent to the detector 26.
Assigned to the peak energy within. For example, a zinc 65 gamma ray source producing 1.1 MeV gamma rays can be used for this purpose. Detector in operation
The energy response of the analyzer system can be checked by determining whether it supports a particular energy/channel relationship established for the zinc-65 peak. It counts the number of pulses from Zn-65 that fall into a certain number of channels (energy bands) on either side of the channel location assigned to the 1.1 MeV energy level, compares each count to each other, and generate an error signal when one total count is greater than the other, and adjust the power supply voltage supplied to the photomultiplier tube 32, for example, to reset the energy level of 1.11 MeV at the specified channel location. This can be done by adjusting the response of the detector-analyzer system.
This method compensates for gain changes in any part of the detector-analyzer system.

A circuit for performing this gain control function in a borehole measuring instrument is disclosed in US Pat. No. 2,956,165, which is owned by the present applicant. Alternatively, the counting process and the comparing process can be easily performed by computer 82. In this case, an error signal of appropriate magnitude and polarity can be generated to control the power supply of the photomultiplier tube.

In another aspect, the counts representing a small range of gamma ray energies spanning the peaks, with or without subtraction of background counts, are
It can be configured to calculate the center of mass of zinc 65 with respect to the channel location, and then generate the required error signal to restore the threshold 1.11 MeV/channel relationship.

Zinc 65 to 100A to 100E during the spectroscopic detection period
In order not to constrain pulse height analyzer 68 due to the radiation source, its lower discrimination level is preferably set to pass only pulses higher than 1.11 MeV. For example, the lower discrimination level can be set close to 1.5 MeV and is typically kept at that level throughout each 31τ cycle, except for the stabilization period from 24τ to 31τ. Time 24τ during each gain control cycle
In , the lower discrimination level is adjusted to a level that allows the pulse corresponding to the gamma rays generated by zinc 65 to pass, and is maintained at that level until time 31τ, at which time it is returned to the normal high discrimination level. The control signals for this purpose are the programmer 46
is generated by Programmer 46 includes appropriate logic and signal combining circuits for its operation, which circuits are coupled to pulse height analyzer 68 via conductors 72. Similarly, a gate open signal is generated by programmer 46 and applied via conductor 76 to a linear gate circuit (not shown) of pulse height analyzer 68 so that the neutron pulses generated by zinc 65 are
Allows passage to the pulse classification circuit during the stabilization period.

In addition to the gain control described above, the pulse height/
It is also desirable to compensate for channel-related drift or offset. Pulsar circuit 77 is coupled to pulse height analyzer 68 via amplifier 44 and command signals from programmer 46 are coupled to pulse height analyzer 68 via conductor 79.
Small amplitude pulses and large amplitude pulses are alternately sent to the pulse height analyzer 68. The amplitude ratio of these pulses is approximately constant over a wide temperature range. Wave height analyzer 68
The transmission of pulses to can be synchronized with the transmission of pulses of zinc 65 in any convenient manner.

The analysis of the detected captured gamma ray spectra and the spectral output generated by computer 82 will now be described. Computer 82 performs essentially the same function as the ingredient ratio computer shown in US Pat. No. 3,521,064. That is, computer 82 calculates the magnitude of the detected captured gamma ray spectrum of the formation at a number of energy points or energy levels.
Determine the composition of an unknown formation by comparing it with a composite spectrum made up of weighted proportions of the individual spectra of several constituent materials or special combinations of materials believed to constitute the formation. The best possible match is then obtained, preferably by a "least squares method", and an indication of the relative proportions of the constituents that produce such a match is extracted. Factors included in a composite spectrum regarding the selection of certain constituent substances, how to measure the individual component spectra, the structure of the composite spectrum, and how to set the optimal conditions between the detected spectrum and the composite spectrum. Application of the least squares criterion and other techniques disclosed in the aforementioned U.S. Pat. Since it has been described in detail in , the details will be omitted here, and the explanation will be summarized to the extent necessary for understanding the present invention.

According to the above US patent specification, there are a certain number n of geological formations.
A postulated substance or element, such as hydrogen,
Assuming that the main constituent elements are chlorine, silicon, calcium, iron, and oxygen, and assuming that the composite spectrum to which the detected geological spectra match contains the individual captured gamma-ray spectra for those elements. (preferably including a secondary active gamma ray spectrum for oxygen). The function Gk of the gamma ray count amplitude at a selected number of energies (not less than n) in the detected gamma ray spectrum is defined by the amplitude coefficient αik
By combining 3a to 3 below,
A set of n linear equations is solved by a computer. The coefficients .alpha.ik are predetermined from known individual spectra and preset into the computer 82.

W _i=1 = α ₁₁ G ₁ + α ₁₂ G ₂ + α ₁₃ G ₃ + α _1o G _o (3a) W _i=2 = α ₂₁ G ₁ + α ₂₂ G ₂ + α ₂₃ G ₃ + α ₂ nG _o (3b) W _{i =o} =α _o1G1 +α _o2 G ₂ +α _o3 G ₃ +α _oo G _o (3n) Since the number of these equations is equal to the number n of constituent materials assumed to be included in the strata, the computer 82 calculates all The unknown component ratio coefficient W _i is automatically generated. Since these coefficients Wi represent the contribution of each component material to the composite spectrum, they also represent the relative proportions of the individual component materials in the formation. A representation of the coefficients for each of the n components is provided via conductor 84C to magnetic tape 86 and recorded as a function of instrument depth.

The computer 82 selects the selected component ratio coefficient.
It can be configured so that the ratio of Wi is a factor. For example, a spectral output is generated by a computer 82 and sent to a conversion circuit 88, a recorder 90, and a tape recorder 8.
6 via leads 84D-84H to provide formation properties such as salinity, porosity, rock quality, shale and water saturation. Indications of other characteristics can also be generated if desired.

An example of a ratio useful for retrieving salinity readings is
WCl/W _H , that is, the ratio of the weighting factor W for chlorine and the weighting factor W for hydrogen.
Therefore, the numerical value of this ratio can be recorded as an indication of salinity. This ratio can be quantified by correlation with a calibration curve obtained for strata of known salinity. If desired, this correlation can be performed in the computer 82 or in the recorder 90 by adding appropriate transformation coefficients and recordings made directly on the obtained values.

An appropriate ratio to indicate the perforation rate, e.g. aW _H /
It can take the form (bWsi+cWca). Here, Wsi and Wca are ratio coefficients calculated for silicon and calcium, respectively. Coefficient a,
b, c (and d described later), etc. are for various gamma ray radiation intensities of each element for the same neutron flux (various microscopic capture cross sections and various gamma ray/neutron capture interaction values τ), etc. The individual terms in which they appear, e.g. (bWsi + CWca), are chosen so that they are constant independent of the specific abundance of the element in the formation. In general, the porosity ratio represents the ratio of the amount of fluid in the formation to the amount of matrix material, and can take any other form appropriate for the purpose. For example, instead of the above ratio, a
A ratio of the form (bW _H +CWCl)/(dWsi+eWca) can be used. This form of ratio takes into account both hydrogen and chlorine, which are the main attributes of formation fluids.
This latter form of ratio is more accurate. but,
The former type of ratio is a good approximation because the proportion of hydrogen in water does not change rapidly with changes in salinity (chlorine). Like the salinity data described above, the perforation ratio values can also be retrieved through the use of calibration curves and are also provided by computer 82 or recorder 90.
It can be easily implemented.

To determine the rock condition of a formation, one or both of two ratios can be taken to indicate whether the formation is rich in limestone or sandstone. In other words, the appropriate ratio to represent limestone is Wsi/Wca, and the appropriate ratio to represent sandstone is Wsi/(aWsi+
bWca). Another form of the ratio that can be used to indicate limestone is
Wsi/(aWsi+bWca+CWu), where
Wu represents one or more components such as oxygen, iron, etc., which are taken into account in determining the ratio. Indications can also be given for rocks other than sandstone and limestone.

Experiments have shown that the ratios WFe/Wsi (where We is the iron ratio coefficient) and WFe/Wca are larger in shale formations than in non-shale formations. Therefore, values indicative of shale can be obtained by examining either of these ratios. Alternatively, the ratio W _F e/(aWsi+bWca)
can be formed. This ratio is advantageous over either the ratio W _Fe /Wsi or W _Fe /Wca because it takes into account that the shale host rock may contain silicon or calcium.

As a general matter, the interpretation of the τ-Σ log can be done qualitatively and quantitatively due to the presence of accurate data on formation properties such as lithology, shale content, porosity and salinity, etc. . Combining the above characteristics with spectral power in accordance with the present invention improves the knowledge of formation hydrocarbon content and productivity that can be obtained from collapse time logs. Furthermore, special benefits are realized in certain conditions that were previously a nuisance. For example, certain geological formations with significantly different properties, such as shale content and high-salinity sandstone, have similar values of τ and Σ, and are therefore not easy to distinguish using a single τ-Σ log touch. However, the additional information provided by the inventive representation of salinity and shale content allows such formations to be identified. Other specific improvements concern low salinity concentrations (e.g. in the 20,000 ppm range) where thermal neutron decay time logs are known to be unreliable. That is, according to the present invention, the spectral output for salinity concentration, eg, the Wcl/ _WH ratio, provides a more accurate salinity measurement. These measurements can be used in a known manner to obtain water saturation values instead of salinity values obtained from decay time logs. The spectroscopically measured salinity measurements can be used to calculate water saturation values in computer 82, and the resulting water saturation values can be plotted in recorders 86,90.

Although the present invention has been described above primarily with respect to the analysis of thermal neutron capture gamma ray energy spectra, it will be appreciated that gamma ray spectra obtained from other neutron interactions can be similarly analyzed. Such other gamma ray spectra include, for example, spectra generated by inelastic scattering of fast neutrons and spectra representing active gamma rays. In the case of an inelastically scattered gamma ray spectrum, the signal generated by the programmer 46 determines the appropriate length of time during the neutron pulse, or immediately after the neutron pulse, or during and immediately after the neutron pulse. can open the spectroscopic gate circuit (U.S. Patent No. 2991364)
(see issue). 31 in total as shown in Figure 3, depending on your wishes.
By changing the operating cycle of τ, the 20τ capture gamma ray spectroscopy cycle can be omitted, leaving only the 10τ thermal decay time. This allows extra time for decay of the trapped gamma rays during the neutron pulse so as to reduce the magnitude of the residual trapped gamma rays during the subsequent inelastically scattered gamma ray detection period.

The spectral output obtained from inelastically scattered gamma ray spectroscopy should preferably include output for carbon and oxygen. Their output is the inelastic gamma ray peak of carbon and oxygen, i.e.
4.4 MeV, and for oxygen it can be obtained by incorporating 6.9 MeV and 7.1 MeV signals (windowing). However, the analysis of inelastically scattered gamma ray spectra is
This is done using the spectral matching technique disclosed in No. Separate component spectra are then selected for inelastically scattered gamma ray analysis. These component spectra include, for example, spectra for carbon, oxygen, silicon, calcium, and hydrocarbons. It is also desirable to include a component spectrum for residual captured gamma rays and other background gamma rays. In any case, the purpose of the spectral matching technique is to obtain desired spectral outputs, and those spectral outputs include, for example, calcium,
Includes ratio factors for carbon, oxygen, silicon and hydrocarbons and their appropriate ratios. Given that both thermal neutron capture and inelastically scattered gamma ray spectroscopic outputs are desired, different types of spectra can be detected and analyzed simultaneously, i.e., during the same movement of the borehole instrument; The analysis can also be performed separately.

Simultaneous execution of detection and analysis is achieved by providing a linear gate circuit between the discriminator of the pulse height analyzer 68 and the pulse classification circuit, and opening the gate circuit at an appropriate timing with a signal applied from the programmer 46.
This can be done by passing a portion of the time distribution of the detection signal obtained from each neutron pulse or a selected neutron pulse that corresponds to inelastically scattered gamma rays. The captured gamma ray detection period can be timed as before. In this case, a computer 8 is configured to add an appropriate set of component spectral coefficients αik to each inelastically scattered gamma-ray spectrum and captured gamma-ray spectrum.
2 can be configured.

Detection and analysis are performed separately by a computer 82 that selects the appropriate detector gating order and adds the appropriate component spectral coefficients to enable analysis of each detected gamma-ray energy spectrum. This can easily be done using a switching circuit that can be operated from the ground.

If desired, the number of individual spectra that have to be provided in the composite spectrum can be reduced by subtracting certain individual spectra from the detected spectrum before assembling the detected composite spectrum. For example, the gamma-ray spectra for at least one of oxygen or iodine may be subtracted from the captured gamma-ray spectra generated during the spectroscopic period. Also, contributions to the detected gamma-ray spectrum from relatively long-lived neutron sources (such as oxygen) and other background neutron sources (such as calibrated neutron sources, natural gamma rays, radioactive salts in the borehole, etc.) can be taken into account by detecting such a contribution during a period following the spectroscopic period and subtracting the background spectrum thus obtained from the detected spectroscopic spectrum. In the captured gamma ray measurement cycle shown in FIG. 3, for example, the background spectrum can be measured during the stabilization period from 24τ to 31τ, and the spectrum is
is proportionally subtracted from the gamma-ray spectra generated between ~106E. This operation can be easily performed using the computer 82. The background corrected spectrum thus obtained is used to generate the desired spectral output.

Although the present invention has been described above with reference to specific embodiments, the embodiments can be modified in various ways without departing from the gist of the present invention. For example, the specialized neutron source used in the present invention can have a fixed pulse duration and pulse repetition rate, and may also have a fixed starting time of the τ detection period for the neutron pulse, or It can be controlled in some way other than automatically according to measurements of the property.

[Brief explanation of the drawing]

FIG. 1 is a schematic block diagram of one embodiment of the apparatus of the present invention, and FIG. 2 shows the preferred timing of the spectroscopic period and the decay detection period according to the present invention, and captures gamma rays after irradiation of the formation with fast neutron pulses. FIG. 3 is a graph of an example of the basic operating cycle for the combined decay time-spectroscopy function of the present invention. 10... Borehole measuring device, 12... Borehole, 24
...Neutron source, 26...Radiation detector, 32...
Photomultiplier tube, 42... Discriminator tube, 46... Programmer, 56... τ calculation circuit, 64... Processing and driver circuit, 68... Analyzer, 82... Computer, 86... Tape recorder, 92... ...Recorder.

Claims (1)

Claims: 1. (a) irradiating tissue with a first pulse of neutrons; (b) determining the time distribution and energy of gamma rays resulting from the interaction of neutrons from this first pulse with neutrons in the tissue; (c) determining a texture property with respect to thermal neutron decay time; controlling the duration and position of the first time period such that the subsequent first time period is such that the number of gamma rays generated within the first time period other than due to interaction of neutrons with the tissue is minimized; , (e) a method for examining the texture of a geological formation, characterized in that the energy spectrum of the detected gamma rays occurring during said first time period is analyzed in order to measure other properties of the tissue. 2. The method of claim 1, wherein the step of measuring tissue properties related to thermal neutron decay time comprises: (a) irradiating the tissue with a second neutron pulse; (b) said second neutron pulse. (c) obtaining an indication of neutron interaction with neutrons of the tissue; and (c) deriving a measurement of the neutron properties from the indication obtained following the second pulse. 3. In the method according to claim 1 or 2, the step of controlling the timing of the first time includes the step of generating a control signal in accordance with the taken measurement value of the neutron characteristics; determining a time of occurrence of a first time in response to a control signal. 4. A method according to claim 3, in which the control signal is related to the time of occurrence of the first neutron pulse according to a function of the measured value of the neutron properties taken. 5. In the method of claim 3 or 4, the neutron property measured is the thermal neutron decay time of the formation, and the first time interval starts at a first selected time after the end of the second neutron pulse. How to generate control signals to cause 6. A method according to claim 5, wherein the first time period is ended at a selected second time after its start. 7. In the method according to any one of claims 1 to 6, the spectral analysis step converts the analyzed portion of the formation gamma energy ray spectrum into a component mainly assumed to constitute the irradiated formation. involves determining the proportion of the hypothesized components that will produce a composite spectrum that most closely matches the analyzed portion of the formation gamma-ray energy spectrum, as compared to a composite energy spectrum made of the weighted spectra. Method. 8. A method according to claim 7, in which an indication of the measured neutron properties and an indication of the determined ratio are recorded as a function of the depth of the formation. 9. A method according to any one of claims 1 to 4, wherein during each successive time interval having at least a first and a second time, the formation is exposed to at least the first and second pulses. The process of irradiating and retrieving measurements of neutron properties occurs within a second time interval, and the time of occurrence of each first time interval of a second time interval following the first time interval in the successive time intervals is equal to the first time interval.
A method controlled in response to neutron property measurements obtained during a second time of the time interval. 10. The method of claim 8, wherein the second time period follows the first time period in each time interval. 11. A method according to claim 9 or 10, in which the time of occurrence of each hour of the second time interval is measured in the first time interval with respect to the time of occurrence of the associated neutron pulse. A method controlled as a function of neutron properties. 12. A method according to claim 9, 10 or 11, wherein the time of occurrence of each neutron pulse in the second time interval is controlled as a function of the neutron properties measured in the first time interval. 13. A method according to any of claims 9 to 12, in which the time of occurrence of the second time interval following the neutron pulse of the second time interval is determined as a function of the neutron properties measured in the first time interval. and taking another measurement of the decay time in response to the signal generated during a second time of the second time interval. 14. The method according to any of the preceding clauses, wherein the gamma ray energy representation is obtained from thermal neutron capture interactions with the core of the irradiated formation and other borehole media, and by controlling the first time timing: The method by which gamma ray energy representations are selected that result primarily from the properties of the irradiated formations. 15. A method according to any one of claims 1 to 8, in which the time of occurrence of the first neutron pulse is controlled as a function of measured neutron properties. 16. The method of claim 9, wherein the measured neutron property is a thermal neutron decay time, and each of the first and second neutron pulses has approximately one measured decay time length. generating a neutron pulse control signal such that approximately two measured decay times for said first time begin approximately one measured decay time after the preceding neutron pulse has terminated; and a step of providing a length of approximately 7 measured decay times for a second time starting approximately 2 measured decay times after the termination of the preceding selected neutron pulse. and the first neutron pulse is approximately 4
separated by an interval of two measured decay time lengths,
A method in which each second neutron pulse is separated from an immediately following neutron pulse by an interval of at least 10 decay time lengths. 17. In the method according to claim 6, the first and second selected times are each:
A method of approximately one measured decay time and approximately two measured decay times. 18: a measuring instrument moving through the borehole; means supported by the measuring instrument for irradiating said formation composition with at least a first pulse of neutrons; and means for measuring the neutronic properties of said composition with respect to thermal neutron decay time. In an apparatus for investigating the composition of a stratum penetrated by a borehole, the detection means detects gamma rays generated from neutron interaction with atomic nuclei of the composition by the first neutron pulse, and forms a signal representing the energy of the detected gamma rays. means for analyzing at least a portion of the energy spectrum of said gamma rays; and gating means for providing to said analyzing means a signal generated in response to gamma rays detected during a first period following said first pulse; and control means for controlling the operation of the gate means in accordance with the value of the determined neutron characteristic. 19. The apparatus of claim 18, wherein the measuring device is responsive to an indication of the interaction of the neutrons with the core of the formation following irradiation of the formation with a second neutron pulse. 20. The apparatus of claim 19, wherein the neutron property measuring instrument is responsive to an indication of a thermal neutron capture interaction with a core of the formation, and the measured neutron property is a thermal neutron capture property of the formation. Device. 21. The device according to any one of claims 18 to 20, wherein the gate device is responsive to a control signal related to the time of occurrence of the first neutron pulse to control the time of occurrence of the first time. An apparatus including a first variable gating device, the controller including a device for generating a first gating control signal as a function of a measured value of a selected neutron characteristic. 22. The device according to claim 21, wherein the selected neutron characteristic is the thermal neutron decay time of the formation, and in at least one measured value of the decay time after the end of the first neutron pulse the selected neutron characteristic is a first hour so as to start one hour and end the first hour at at least two measured values of that decay time following the start of the decay time;
A device from which gate control signals are generated. 23. A device according to any of claims 18 to 22, in which the controller generates a control signal for controlling the time of occurrence of the first neutron pulse as a function of the measured value of the neutron property. Equipment including generating equipment. 24. Apparatus according to any of claims 18 to 23, wherein the neutron characteristic controller is adapted to the signal generated by the detector during a second time period following the neutron pulse after the second neutron pulse. a second variable gating device responsive to and responsive to a control signal related to a later neutron pulse generation time to control a second time generation time, the controller controlling the measured value of the selected neutron property; an apparatus for generating a second gate control signal as a function of a second gate control signal; 25. An apparatus according to any one of claims 18 to 24, comprising displaying the measured value of the selected neutron property and displaying the analyzed part of the detected gamma-ray energy spectrum. , as a function of the depth of the formation from the earth's surface. 26. In the apparatus according to any one of claims 18 to 25, the analyzed part of the detected gamma ray energy spectrum is
A composite spectrum that closely matches the analyzed part of the detected gamma-ray energy spectrum is compared to a composite energy spectrum that is composed of weighted spectra of components primarily assumed to constitute the irradiated formation. Apparatus further comprising apparatus for determining the proportion of the hypothesized component occurring. 27. Apparatus according to any of claims 18 to 26, in which a plurality of said neutron pulses are generated by an irradiator during each successive time interval to generate a calibration signal to determine their The apparatus further comprises means for applying a signal to the analyzer during predetermined times within each time interval. 28. The apparatus of claim 27, wherein the calibrator is configured to calibrate a gamma ray source of known energy and a signal generated by a detector in response to the calibrated gamma rays during one of the time intervals. a device for applying a first calibration signal train of small amplitude and a second train of high amplitude calibration signals to said analyzer during a predetermined time period within every successive time interval; apparatus for alternately applying the first signal train and the second signal train to the analyzer during predetermined times of a time interval; and forming a channel location ratio of the first and second calibration signal trains; a device, and
apparatus for generating a control signal relating the channel location ratio to the difference between said small amplitude and large amplitude amplitude ratios; and apparatus for correcting any errors in the disintegrator in response to said ratio; , wherein the amplitude ratio is substantially constant.