Background

[0001] In logging with a neutron porosity tool (chemical source) or a pulsed-neutron tool, neutrons emitted by a neutron source in the tool generate gamma rays in the formation that are detected by the tool. Further, some neutron sources generate gamma rays directly. It may be useful to isolate gamma rays generated by the formation from those generated directly by a neutron source. ³ He neutron detectors were useful for this purpose but ³ He has become expensive and difficult to obtain.

Brief Description of the Drawings

[0002] Fig. 1 illustrates a drilling system. [0003] Fig. 2 shows a schematic cross-section of a logging tool.

[0004] Fig. 3 shows a block diagram.

[0005] Fig. 4 shows energy spectra.

[0006] Fig. 5 shows temporal responses.

[0007] Fig. 6 shows the variation of light intensity produced by a crystal with temperature. [0008] Figs. 7-8 are flow charts.

[0009] Fig. 9 illustrates windows in a spectrum. [0010] Fig. 10 illustrates an environment.

Detailed Description

[0011] In one embodiment, a drilling system 100, illustrated in Fig. 1, includes a derrick 105 from which a drill string 110 is suspended in a borehole 112. Fig. 1 is greatly simplified and for clarity does not show many of the elements that are used in the drilling process. Further, while Fig. 1 shows a land-based drilling system, the techniques described herein are also useful in a sea-based drilling system and in wireline and slickline systems and operations. In one embodiment, the volume within the borehole 112 around the drill string 110 is called the annulus 114. In one embodiment, the drill string 110 includes a bit 115, a variety of actuators and sensors, shown schematically by element 120, and a telemetry section 125, through which the downhole equipment communicates with a surface telemetry system 130. In one embodiment, the drill string includes a logging tool 135 to collect data from sub-surface formations, such as formation 140.

[0012] In one embodiment, a computer 145 receives data from the downhole equipment and sends commands to the downhole equipment through the surface telemetry system 130. In one embodiment the computer 145 includes input/output devices, memory, storage, and network communication equipment, including equipment necessary to connect to the Internet.

[0013] In one embodiment, the logging tool 135, illustrated in cross-section in Fig. 2, includes an outer shell 202 and an inner shell 203. The inner shell 203 forms a channel 204 through which drilling mud can flow down the drill string 110 to the bit 115. Devices that perform the functions of the logging tool 135 fit in the gap between the outer shell 202 and the inner shell 203.

[0014] In one embodiment, the logging tool 135 includes a neutron source 205 and a gamma ray source 210. In one embodiment, the neutrons produced by the neutron source 205 exit the logging tool 135 and penetrate the formation 140. Some of the neutrons scatter from the formation 140 back to the logging tool 135 where, in one embodiment, they pass through a low density window 235 to a detector 240. Some of the neutrons cause the formation 140 to radiate gamma rays that pass through low density window 235 to the detector 240.

[0015] In one embodiment, the gamma ray source 210 is placed very close to the detector 240. In one embodiment, the gamma ray source 210 is placed inside a housing of the detector 240. In one embodiment, some of the gamma rays emitted by the gamma ray source 210 reach the detector 240 directly. In one embodiment, the gamma ray source 210 is used in stabilizing the spectrum of the logging tool 135 and is not intended to penetrate to the formation 140 or to cause the formation to radiate back to the detector 240.

[0016] In one embodiment, the detector 240 is shielded by a shield 245. In one embodiment, an electronics package 250 processes signals produced by the detector 240. In one embodiment, a system includes the detector 240 and the electronics package 250.

[0017] In one embodiment, illustrated in Fig. 3, the detector 240 includes a scintillator 240A, discussed in more detail below, which generates photons as a result of being struck by neutrons and gamma rays. In one embodiment, a photomultiplier tube ("PMT") 240B is coupled to the scintillator 240A and produces a current pulse for each neutron or gamma ray that strikes the scintillator 240A, with the magnitude of the current pulse being related to the energy of the neutron detection process (i.e., the cascading neutron activity that results from a neutron impact) or of the impacting gamma ray. In one embodiment, the PMT 240B is replaced by another type of photodetector that produces a current pulse (or, with appropriate adjustments to the other circuitry described herein, a voltage pulse) for each neutron or gamma ray that strikes the scintillator 240A.

[0018] In one embodiment, the PMT 240B is coupled to a voltage divider 240C. In one embodiment, the voltage divider 240C is coupled to a high voltage source 240D. In one embodiment, the high voltage source 240D is coupled to the PMT 240B through the voltage divider 240C so that the high voltage source 240D can control the gain of the PMT 240B. That is, by adjusting the high voltage source 240D, it is possible to adjust the response of the PMT 240B to radiation striking the scintillator 240A.

[0019] In one embodiment, the PMT 240B is coupled to a preamplifier 320. In one embodiment, the preamplifier 320 converts current pulses in the PMT 240B arising from radiation entering the scintillator 240A to voltage pulses. In one embodiment, an amplifier 325 is coupled to the output of the preamplifier 320 and provides adjustable amplification of the signal from the preamplifier 320. In one embodiment, the adjustable amplification combines an adjustable attenuator with an amplifier. In another embodiment, amplification of the amplifier is directly adjustable. [0020] In one embodiment, the output of the amplifier 325 is a series of voltage pulses. In one embodiment, each voltage pulse corresponds to the impact of radiation on the scintillator 240A. In one embodiment, the amplitude of each voltage pulse corresponds to the energy deposited (or generated) in the scintillator 240A by the radiation that initiated the pulse.

[0021] In one embodiment, an analog to digital converter ("ADC") 330 is coupled to the output of the amplifier 325. The ADC 330 samples the signal produced at the output of the amplifier 325 and converts it to a series of numbers representative of the sampled signal.

[0022] In one embodiment, a processor 335 receives the numbers produced by the ADC 330, processes those numbers as described below optionally under direction received through a telemetry module 340, and controls the gain of the system that includes the detector 240 and the electronics package 250, which in one embodiment includes the preamplifier 320, the amplifier 325. the ADC 330, the processor 335, and the telemetry module 340.

[0023] In one embodiment, the scintillator 240 A includes a device (such as a crystal) that produces signals in response to being impacted by radiation such as neutrons or gamma rays. In one embodiment, signals produced by scintillator 240A in response to being impacted by a gamma ray are distinguishable from the signals produced in response to signals produced by scintillator 240A in response to being impacted by a neutron. In one embodiment, the signals are distinguishable using pulse shape discrimination, as discussed below. In one embodiment, the signals are distinguishable using another distinguishing factor.

[0024] In one embodiment, the scintillator 240A includes a crystal formed of Cs ₂ LiYCl6 (often doped with Ce), which is commonly referred to as "CLYC." CLYC is a replacement for ³ He detectors, which have become scarce and expensive because of the limited supply of ³ He. In addition, CLYC has the added advantage of being able to both detect neutrons and gamma rays and distinguish between them. In one embodiment, the scintillator 240A includes a device that, as of the date this application was written, had not yet been developed but produces signals in response to neutron impacts that are distinguishable from signals in response to gamma ray impacts by a characteristic other than pulse shape.

[0025] CLYC detects neutrons in a similar fashion to lithium glass, through interactions with ⁶ Li. Neutrons are primarily detected when they interact with the isotope ⁶ Li, which makes up 7.5% of natural lithium, although the CLYC crystal may be grown using lithium that has a higher concentration of ⁶ Li. Symbolically, the interaction is

⁶ Li + n→ ³ H + a.

[0026] The resultant tritium ( ³ H) and alpha particle have a combined energy of 4.78 MeV, which registers in the scintillator as an apparent energy of 3.2 MeV, as shown in Fig. 4, which is taken from Jack Glodo, et al., "Cs ₂ LiYCl ₆ : Ce Scintillator for Nuclear Monitoring Applications," IEEE TRANSACTIONS ON NUCLEAR SCIENCE at p. 1258 (Vol. 56, No. 3, June 2009). [0027] A lithium glass detector is an alternative to a ³ He detector, but while it is sensitive to both neutrons and gamma rays, it cannot differentiate between them. However, with CLYC, the manner in which the neutron energy is converted to light is different than that for gamma rays. As a result, it is possible to differentiate between neutrons and gamma rays. Thus, in one embodiment, it is possible to use pulse shape discrimination to differentiate between gamma ray and neutron events. The pulses from gamma rays rise and fall much faster than those from neutrons, as shown in Fig. 5, which is taken from Glodo p. 1260.

[0028] In one embodiment, the processor 335 performs type discrimination to distinguish events, such as pulses, caused by an impact of a first type of radiation (such as a neutron impact) on the scintillator 240A from events, such as pulses, caused by an impact of a second type of radiation (such as a gamma ray impact) on the scintillator 240A. In one embodiment, type discrimination is performed using pulse shape discrimination, such as the technique described in Glodo at p. 1260:

First, the background was estimated and subtracted from each trace. Second, each trace was aligned on the time axis to a common zero. Traces of different amplitude are typically slightly misaligned. Third, parts of each trace were integrated according to two preset windows. Each window setting was characterized by its position and width. We have tested only a few (position, width) combinations by hand. The selection presented in this paper is somewhat arbitrary but seems to provide very good results. The first window [grey area 505 in Fig. 5] started at the onset of the trace and had a short 80 ns width. This window was focused on the rising part of the trace and the fast component (CVL/Ce ³⁺ ). The second window [grey area 510 in Fig. 5] started 160 ns from the onset and had a 3200 ns width. This window was focused on the slow remainder of the trace. Basically, the procedure compared the amount of signal on the onset of the trace with the other selected part of the trace. In this case the focus was on the slow tail. The two integrals were used to produce a ratio (window2/windowl) that was later plotted against one of the values.

[0029] The ratio of window2/windowl generally falls into two ranges: a high range, which corresponds to neutrons, and a low range, which corresponds to gamma rays. In one embodiment, sorting the pulses into the low range or the high range causes the pulses to be sorted into neutron impacts and gamma ray impacts. [0030] In the logging application described herein, the CLYC detector 240 (i.e., detector 240 manufactured to include a CLYC crystal) is subjected to high temperatures. As with most crystals, this causes the amount of light produced by the crystal for a given event to change. A plot of the sensitivity of CLYC to temperature is shown in curve (a) of Fig. 6 for x-rays. As can be seen, the light output for x-rays drops by a factor of three in going from 250 K (-23 °C) to 400 K (127 °C). As of the writing of this application, the effect for neutrons is not known, but it is expected to be similar.

[0031] If the CLYC detector 240 were detecting neutrons below 400 K (127 °C), it would be possible to use pulse shape discrimination and set the neutron threshold low, perhaps at the voltage that corresponds to 60 keV. At 400 K, when the light output has dropped by a factor of three, the threshold would be at the equivalent of 180 keV, which is still well below the neutron peaks. There would be no problem, and gain stabilization to account for the reduced light output would not be necessary.

[0032] However, if it is desired to make neutron measurements at temperatures as high as 175 °C (448 K) or 200 °C (473 K), then light from the neutron peak will be very much reduced, and the threshold may be too high. (Using a simple log-log extrapolation of the last three points in Fig. 6, the light output is predicted to be down to 1.4% of the room temperature output at 200 °C.)

[0033] Further, if gamma spectra are desired, then the gain of the system is kept stable, because gamma-ray processing almost always requires that the energy of the gamma rays be well known. Thus, in this circumstance, gain stabilization is useful.

[0034] In one embodiment, illustrated in Fig. 7, the processor 335, optionally under control of a computer on the surface, such as computer 145, selects from among several methods to adjust the gain of the system to compensate for the loss in light output (block 705).

1. In one embodiment of selection, when neutrons are guaranteed to be present in quantity ("Y" branch from block 710), pulse shape discrimination can be used to select only neutrons (block 715). The location of the neutron peak is determined, and the system gain (i.e., the gain of the system that includes detector 240 and electronics package 250) is adjusted to move this peak to its nominal position (3.2 MeV) (block 720). If neutrons are not guaranteed to be present in quantity ("N" branch from block 710), processing returns to block 705 and a different gain control method is selected. 2. In one embodiment of selection, the gamma ray source 210 with an output spectrum that contains an identifiable energy peak is placed near the second detector 240, as shown in Fig. 2. In one embodiment, if neutrons are expected to be present ("Y" branch out of block 725), pulse shape discrimination is used to select only gamma rays (block 730). In either case (i.e., "N" branch out of block 725 or proceeding from block 730), in one embodiment, the system gain is adjusted to move the gamma ray peak to its desired location. In one embodiment, the gamma source 210 is chosen to provide an energy peak at the best energy. In one embodiment, the gamma ray source 210 is a radionuclide such as cesium- 137. In one embodiment, the gamma ray source 210 is a material containing an isotope that undergoes a nuclear interaction with neutrons so as to produce an identifiable energy peak in the output spectrum. In one embodiment, the material contains boron- 10.

3. In one embodiment of selection, both techniques may be used. In that case, in one embodiment, the gamma peak is chosen to be at a low energy, and a linear calibration is used to adjust the energies. In one embodiment, pulse shape discrimination is used to sort neutrons from gamma rays (block 740). In one embodiment, the system gain is chosen to keep both the gamma and the neutron peaks in their nominal position (block 745). In this case, in one embodiment, part of the process of adjusting the system gain involves adjusting the system offset. In one embodiment, the gain and offset are chosen so that the gamma peak and the neutron peak are in the desired locations.

[0035] In one embodiment, the system gain can be changed in several ways. The output of the high voltage sources 230D and 240D can be adjusted, the electronic gain can be changed (i.e., by controlling the gain of amplifier 325), the measured spectra (discussed below) can be digitally shifted, or the ranges used to condense spectra into energy windows (discussed below) can be adjusted.

[0036] In one embodiment of a gain stabilization technique, illustrated in Fig. 8, the processor 335 receives digital representations of each pulse produced by the detector 240, converted to voltage by preamplifier 320, amplified by amplifier 325 and sampled and digitized by ADC 330 (block 805). In one embodiment, the ADC 330 samples the voltage output by the amplifier 325. In one embodiment, the processor 335 optionally distinguishes between pulses produced by gamma rays and pulses produced by neutrons, using a type discrimination technique such as the pulse shape discrimination technique described above, and rejects either the pulses produced by neutrons or the pulses produced by gamma rays (block 810). In one embodiment, no such rejection occurs and all of the pulses are passed through.

[0037] In one embodiment, the processor 335 uses well-known signal processing techniques to identify and store the maximum amplitude of each pulse. In one embodiment, the processor stores the maximum amplitude of each pulse in a spectrum. In one embodiment, the spectrum includes a number of channels, where each channel represents small a small range of maximum pulse amplitudes. In one embodiment, the number of channels is a multiple of 2. In one embodiment, the spectrum includes 256 channels. In one embodiment, each channel has an associated event counter. In one embodiment, for each received pulse the processor increments by one the event counter of the channel having the amplitude range corresponding to the maximum amplitude of that pulse (block 815). In one embodiment, the spectra are condensed to obtain count rates that are used to compute formation or borehole properties.

[0038] In one embodiment, the spectrum is referred to as a "voltage distribution."

[0039] In one embodiment, the processor 335 receives (block 805), optionally rejects (block 810), and sorts data (block 815) for a period of time that depends on the expected count rate, that is optionally adjustable from the surface through the telemetry module 340, and that is on the order of 5 seconds. In one embodiment, the processor 335 analyzes the spectrum using conventional peak-locating techniques to determine the location of the relevant peaks (block 820). In one embodiment, the relevant peaks, illustrated in Fig. 4, are 477 keV, 662 keV, and 3.2MeV. Other embodiments may use fewer peaks, additional peaks, or different peaks.

[0040] In one embodiment, the statistical uncertainty of the counts in each channel is large for the data collection time, making it challenging to locate peaks using individual channels. In one embodiment, illustrated in Fig. 9, the spectrum is divided into windows (labeled Wl, W2, . . ., W9) that each span multiple channels. In one embodiment, four windows of equal width (i.e., W5, W6, W7, and W8) are positioned about the desired location of the peak 902 expected to be generated by the gamma ray source 210. In one embodiment, the peak 902 is expected to be at the border between W6 and W7. In one embodiment, the rest of the spectrum is divided into coarser windows (labeled Wl, W2, W3, W4, and W9). In one embodiment, the counts of each channel in the window range are summed, yielding a count for each window. [0041] In one embodiment, the processor 335 makes gain adjustments based on the difference between the spectrum (or determined voltage distribution) described above and a desired voltage distribution for one or more of the radiation types (e.g., neutrons or gamma rays) being processed. In one embodiment, the desired voltage distribution is the voltage distribution for the radiation type being processed that is expected if the gain of the detector 240 is adjusted correctly. In one embodiment, the processor 335 examines the windows to determine if the gain-affected peak (i.e., the peak 902 located incorrectly because of a gain miss-adjustment) is grossly out of range (block 825), e.g. by determining that the gain-affected peak is located much too high in the spectrum or much too low in the spectrum. For example, in one embodiment, if the sum of the counts in the seven highest windows (W3, W4, W5, W6, W7, W8, and W9) is zero, processor 335 determines that the peak is grossly out of range ("Y" branch out of block 825). In that case, in one embodiment, the processor makes a coarse gain adjustment (for example, increasing gain by a factor of two) (block 830) and processing returns to block 805.

[0042] If, in one embodiment, the processor 335 determines that the location of the relevant peaks is not grossly out of range ("N" branch out of block 825), the processor performs an analysis using the following equation:

where:

Ci is the count for window i.

[0043] In one embodiment, the processor 335 makes a fine gain adjustment (block 835), where the amount of the gain adjustment is related to Q: gain _new = gain _old ^■ f Q) where: gairinew is the gain after the adjustment, gairi _o i _d is the gain before the adjustment, and f(Q) is a function of Q.

[0044] In one embodiment, f(0) = 0; that is, if Q is 0, f(Q) is zero and no adjustment is made. In one embodiment f(Q) is linear. In one embodiment, f(Q) is non-linear. [0045] In one embodiment, after the fine adjustment is made (block 840), processing returns to block 805.

[0046] In one embodiment, the processor 335 is a circuit. In one embodiment, the circuit includes an event discriminator 345 that performs the functions of blocks 805, 810 (which optionally performs type discrimination 350, such as pulse shape discrimination), 815, and 820 in Fig. 8. In one embodiment, the circuit includes a gain adjustor 355 that performs the functions of blocks 825, 830, and 835.

[0047] In one embodiment, system gain is controlled by controlling the voltage of the detector 240. In one embodiment, system gain is controlled by controlling an attenuator in the amplifier 325. In one embodiment, system gain is controlled by controlling the amplification of the amplifier 325. In one embodiment, system gain is controlled by shifting the voltage spectrum. In one embodiment, system gain is controlled by adjusting the voltage ranges used to define the energy windows.

[0048] In one embodiment, the processor 335 executes software or firmware. In one embodiment, the software or firmware includes an event discriminator process 345 that performs the functions of blocks 805, 810 (which optionally performs type discrimination 350, such as pulse shape discrimination), 815, and 820 in Fig. 8. In one embodiment, the software or firmware includes a gain adjustor 355 that performs the functions of blocks 825, 830, and 835.

[0049] In one embodiment, shown in Fig. 10, the logging tool 135 is controlled by software in the form of a computer program on a non-transitory computer readable media 1005, such as a CD, a DVD, a USB drive, a portable hard drive or other portable memory. In one embodiment, a processor 1010, which may be the same as or included in the processor 335 or the computer 145, reads the computer program from the computer readable media 1005 through an input/output device 1015 and stores it in a memory 1020 where it is prepared for execution through compiling and linking, if necessary, and then executed. In one embodiment, the system accepts inputs through an input/output device 1015, such as a keyboard or keypad, mouse, touchpad, touch screen, etc., and provides outputs through an input/output device 1015, such as a monitor or printer. In one embodiment, the system stores the results of calculations in memory 1020 or modifies such calculations that already exist in memory 1020.

[0050] In one embodiment, the results of calculations that reside in memory 1020 are made available through a network 1025 to a remote real time operating center 1030. In one embodiment, the remote real time operating center 1030 makes the results of calculations available through a network 1035 to help in the planning of oil wells 1040 or in the drilling of oil wells 1040. [0051] The word "coupled" herein means a direct connection or an indirect connection.

[0052] The text above describes one or more specific embodiments of a broader invention. The invention also is carried out in a variety of alternate embodiments and thus is not limited to those described here. The foregoing description of an embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.