RU2690095C1 - Device for neutron porosity determination - Google Patents

Abstract

FIELD: soil or rock drilling; mining.SUBSTANCE: use: to measure neutron porosity of rock formations in wells. Summary of invention consists in the fact that device for determination of neutron porosity includes pulsed fast neutron source, neutron detector arranged in cylindrical guard housing, wherein as the neutron monitor, the epithermal neutron detector, as well as the thermal neutron detector, one thermal neutron detector is used, located coaxially with the pulsed fast neutron source and the cylindrical guard housing, thermal neutron detector is connected to amplifier-integrator, integrating amplifier is connected to processor, processor is connected to telemetry system, wherein pulsed fast neutron source is connected to control unit, control unit is also connected to processor.EFFECT: technical result: assurance possibilities detection of fast neutrons emitted pulse neutron source during its pulses, proportional to the He-3 counter under the condition of applying electric signals at the output of the proportional counter.1 cl, 3 dwg

Description

The invention relates to the field of geophysical studies of parameters of geological formations by the method of pulsed neutron-neutron logging and can be used in downhole devices designed to measure the neutron porosity of rock formations in wells.

Pulsed neutron-neutron logging is used in cased wells for lithological dissection of cuts and separation of reservoirs, identification of water and oil and gas saturated reservoirs, determination of the positions of water-oil contact, determination of gas-liquid contacts, evaluation of rock porosity, quantitative assessment of initial, current and residual oil saturation, monitoring the testing process and development of wells ("Technical instructions for conducting geophysical studies and work with instruments on the cable in oil and gas kvazhinah RD 153-39.0-072-01 ", Moscow - 2002).

It is known “A device equipped with a neutron generator for measuring neutron porosity, which is highly sensitive to porosity”. The device includes: a fast neutron source, a near neutron detector and a long-range neutron detector located at a greater distance from the neutron source than a near neutron detector, a fast neutron source made in the form of an electronic neutron generator, an electronic neutron generator is a 14 MeV neutron generator emitted Neutrons into the rock have energy higher than the energy of neutrons emitted by an AmBe source, the near neutron detector is a thermal neutron detector, the detector is pilaf neutrons comprises ³ He, the active region thermal neutron detector proximate to an electronic neutron generator, located on it at distances of less than about 7 or 9, or 10 inches, the active region thermal neutron detector, far in relation to an electronic neutron generator ranges from at a distance of more than 15 inches, a screen is installed between the thermal neutron detector and the electron neutron generator. Application for US patent No. 2011/0297818 A1, G01V 5/10. 12/08/2011. Analog.

The disadvantage of analog is the relatively low accuracy of measurement of rock moisture in the well in the presence of crystallization (bound) water in it, since the thermal neutron flux is determined by the total water content, and not only the water contained in the pore space. The relatively low accuracy of moisture measurement can also be caused by the presence of impurities in the rock, which noticeably absorb thermal neutrons.

The well-known device for determining neutron porosity, characterized by increased accuracy and a decrease in lithological effects, is known. The device includes: a pulse generator of 14 MeV neutrons, a neutron monitor, the first and second neutron detectors and a data processing circuit. Moreover, the first neutron detector, or the second neutron detector, or both neutron detectors are located at a distance from the pulse generator, ensuring the minimal effect of lithology. Application for US patent No. 2011/ 0260044 A1, G01V 5/10. 10/27/2011. Analog.

The disadvantage of analog is the relatively low accuracy of measurement of rock moisture in the well in the presence of crystallization (bound) water in it, since the thermal neutron flux is determined by the total water content, and not only the water contained in the pore space. The relatively low accuracy of moisture measurement can also be caused by the presence of impurities in the rock, which noticeably absorb thermal neutrons.

The well-known “downhole devices for determining absorption and porosity cross-sections, equipped with neutron monitors” are known. The devices include: a pulsed neutron source, a neutron monitor located near a neutron source, a gamma detector located from a neutron source at a distance of about 8-40 inches, a protective screen between a gamma detector and a neutron source, an epithermal neutron detector, located between the neutron source and the gamma detector at a distance of 9 to 14 inches from the neutron source, a thermal neutron detector, located next to the epithermal neutron detector, additionally one or more children The incidents of epithermal and thermal neutrons located at a distance from the neutron source are greater than the distance between the gamma detector and the neutron source, and the distance between the additional epithermal and thermal neutron detectors and the neutron source is 24 inches or more. U.S. Patent No. 7,365,307 B2, G01V 5/10. 04/29/2008 Prototype.

The disadvantage of the prototype is the impossibility of registering fast neutrons emitted by a pulsed neutron source during its pulses, proportional to the He-3 counter, subject to the imposition of electrical signals at the output of the proportional counter.

 The prototype contains at least three neutron detectors: a neutron monitor, a detector of epithermal neutrons, and a thermal neutron detector, which are located at different distances from a pulsed neutron source. This leads to an increase in the length of the downhole tool and the deterioration of the conditions for the unobstructed wiring of the well tool through the well.

The technical result of the invention is to provide registration of fast neutrons emitted by a pulsed neutron source during its pulses, proportional to the He-3 counter, subject to the imposition of electrical signals at the output of the proportional counter. The result is a reduction in the number of neutron detectors in the downhole tool and the length of the downhole tool, which improves the conditions for the unobstructed wiring of the downhole tool in the well.

This is achieved by using instead of several neutron detectors: a neutron monitor, an epithermal neutron detector, and a thermal neutron detector of one thermal neutron detector located one distance from the neutron source, and it registers separately fast, epithermal and thermal neutrons by measuring the time dependence the signal arising at the output of a proportional counter during and between neutron pulses, using an amplifier-integrator.

The technical result is achieved by the fact that the neutron porosity detection device, which includes a pulsed source of fast neutrons, a neutron detector placed in a cylindrical security case, serves as a neutron monitor, epithermal neutron detector, and thermal neutron detector coaxially located with a pulsed fast neutron source and a cylindrical security case, a thermal neutron detector is connected to an amplifier-integrator, an amplifier The integrator is connected to the processor, the processor is connected to the telemetry system, while the pulsed source of fast neutrons is connected to the control unit, the control unit is also connected to the processor.

The invention is illustrated in FIG. 13.

FIG. 1 schematically shows the composition and mutual arrangement of the main elements of one of the possible structures of the downhole tool, where:

1 - cylindrical security case,

2 - pulsed source of fast neutrons,

3 - thermal neutron detector,

4 - amplifier-integrator

5 - processor

6 - control unit

7 - telemetry system.

FIG. 2 shows the time dependence of the specific energy release in the detector 3 of thermal neutrons when using it as a proportional ³ He counter and the components of the specific energy release calculated for calcite with neutron porosity (humidity) of 14.9% when it is irradiated with 14 MeV neutrons with a neutron pulse 1 duration μs, where:

8 - dependence of the specific energy release in the detector 3,

9 - dependence of specific energy release in the detector 3, caused by fast neutrons with energy of 14 MeV - 40 keV,

10 - dependence of specific energy release in the detector 3, caused by epithermal neutrons with an energy of 40 keV - 0.414 eV,

11 - dependence of the specific energy release in the detector 3, caused by thermal neutrons with an energy of less than 0.414 eV.

FIG. 3 shows the calculated dependences of the specific energy release in the detector 3 of thermal neutrons when using it as a proportional ³ He counter for various calcite neutron porosity when it is irradiated with 14 MeV neutrons with a neutron pulse duration of 1 μs, where:

12 is the dependence of the specific energy release in the detector 3 for calcite with a neutron porosity of 0.8%,

13 is the dependence of the specific energy release in the detector 3 for calcite with a neutron porosity of 14.9%,

14 is the dependence of the specific energy release in the detector 3 for calcite with neutron porosity of 36.4%,

15 - the dependence of the specific energy release in the detector 3 for fresh water.

The downhole device according to the claimed technical solution contains a cylindrical security body 1, a pulsed source of 2 fast neutrons, a thermal neutron detector 3, an amplifier-integrator 4, a processor 5, a control unit 6 and a telemetry system 7.

The cylindrical security body 1 serves as a strong device body and is made of steel with a thickness of about a few millimeters.

The pulsed source of fast neutrons 2 can be made in the form of a neutron generator with an energy of 2.5 MeV or 14 MeV, is located coaxially with the guard casing 1 and serves to irradiate the rock with pulses of fast neutrons. The pulse source 2 is electrically connected to the control unit 6.

The thermal neutron detector 3 serves to register neutrons coming from the pulsed source 2 and from the environment. As a detector of 3 thermal neutrons, a proportional counter filled with ³ He can be used, the length of which usually ranges from 8 cm to 15 cm and a diameter of about 30 mm. The detector 3 may be made in the form of a cassette containing several proportional counters. The thermal neutron detector 3 is usually located with respect to the pulsed source 2 at a distance of L <15 cm and coaxially with the guard casing 1.

The control unit 6 is electrically connected to the pulse source 2 and the processor 5 and serves to control the operation of the pulse source 2.

The thermal neutron detector 3 is electrically connected to the input of the amplifier-integrator 4, which serves to integrate, amplify, and digitize the charge formed in the thermal neutron detector 3 as a result of the interaction of different energy neutrons with them.

A processor 5 is also connected to the amplifier-integrator 4. The processor 5 is used to program the modes of operation of the amplifier-integrator 4, the control unit 6 and the transfer of digitized data in the telemetry system 7.

Telemetry system 7 is used to transmit data to ground equipment (not shown in Fig. 1).

The signal taken from the output of the detector 3 is proportional to the specific energy release in the detector 3 (dependence 8 in Fig. 2).

When a substance is irradiated by a pulsed source of 2 fast neutrons in a substance at different times from the beginning of the pulse, fast neutrons of various energies, epithermal and thermal neutrons are present. The ratio of their flux depends on the time and neutron porosity of the rock.

The amount of energy release (charge) arising in the detector 3 under the action of fast neutrons is determined by their flux and average energy transmitted by ³ He due to the elastic scattering of fast neutrons.

The amount of energy release (charge) arising in the detector 3 under the influence of epithermal and thermal neutrons is directly proportional to the flux of these neutrons to it, because when they are captured by the ³ He nucleus, the same energy is released, equal to 0.76 MeV / neutron.

During a source 2 pulse and some time after it, the detector receives mostly fast neutrons directly from the source as well as from the surrounding substance (dependence 9 in Fig. 2 for 14 MeV neutrons - 40 keV). Due to the slowing down of fast neutrons in matter, the average energy of these neutrons is constantly decreasing. The delay time of fast neutrons strongly depends on the neutron porosity of the rock and decreases with its increase.

From dependence 9 it can be seen that the signal arising in the detector 3 of thermal neutrons at t ≈ 0.1 μs from the beginning of the neutron pulse can be used to monitor the output of the pulsed source 2.

A few microseconds after the start of the neutron pulse, epithermal neutrons begin to flow into the detector (dependence 10 in Fig. 2 for neutrons with an energy of 40 keV - 0.414 eV). In the case of a short ~ 1 μs neutron pulse, the maximum density of their flux to the detector is reached approximately in t≈2-3 μs and then quickly drops from a constant decrease of not more than several tens of microseconds. Thus, the signal arising in the detector of 3 thermal neutrons at t ≈ 2-3 µs is mainly caused by epithermal neutrons.

Thermal neutrons begin to flow to the detector several tens of microseconds after the start of the neutron pulse (dependence 11 in Fig. 2 for neutrons with an energy <0.414 eV). In the case of a neutron pulse with a duration of about 1 μs, the specific energy release reaches a maximum by the time t ~ 10–20 μs. The signal arising in the detector 3 thermal neutrons t> ≈20 μs, mainly caused by thermal neutrons.

The constant decay of the thermal neutron flux to the detector depends on the neutron porosity of the rock and practically does not exceed 1 ms. Therefore, when the pulse repetition frequency is less than 100 Hz, by the time of the next pulse arrival, thermal neutrons in the rock die out and with the arrival of the next pulse the process repeats completely.

Currently, epithermal and thermal neutrons are used to measure the neutron porosity of a rock in a well. For their registration, proportional ³ He or ¹⁰ V meters are used.

The collection time of the charge formed in the proportional counter as a result of neutron capture is about 1-4 μs [D. Mazed, S. Mameri, R. Ciolini. Design parameters and technology optimization of ³ He-filled proportional counter for thermal neutron detection and spectrometry applications. Radiation Measurements 47 (2012) 577-587]. The corresponding dead time for proportional counters is <10 µs [GP Manessi. Development of advanced radiation monitors for pulsed neutron fields. PhD thises. (2015) 1-147, p.16]. The indicated dead time inevitably leads to the miscalculation of neutrons at a frequency of recorded events of more than (5-10) kHz. Such a frequency can occur when neutrons are recorded during a relatively short and powerful neutron pulse and some time after it.

“The time spectrum of counting rates for domestic low-frequency equipment is greatly distorted by miscalculations, and the correction method used is limited to errors of up to 2-fold, which is clearly not enough. The main interpretational parameter is the measured time decrement of neutron or photon decay, which depends not only on the properties of the reservoir, but also on the measurement conditions - the design and filling of the well, the probe size. The obtained value of the decrement is also usually not provided by the assessment of its accuracy "(SG Borodin." Deep processing of data from pulsed neutron logging of oil and gas wells ", dissertation author's abstract for the degree of Candidate of Physical and Mathematical Sciences, Moscow - 2009).

The proposed device for the implementation of pulsed neutron-neutron logging contains an amplifier-integrator that provides the registration of neutrons of the whole spectrum (dependence 8 in Fig. 2), including the imposition of recorded events over the entire time interval, both during neutron pulses and between them.

The integrator amplifier provides a measurement of the amount of charge produced in a neutron detector, neutrons incident on it, both in the case of a high repetition rate of recorded events, and in the case of their partial overlap. In this case, the charge collected from the output of the counter is proportional to the number of registered neutrons and the energy released during this [I. Rios, J. Gonzalez, and R.E. Mayer. Total fluence influence of the detectors of proportional detectors. Radiation Measurement 53-54 (2013) 31-37; J. Moreno, L. Birstein, R.E. Mayer et al. It is a proportional counter. Meas. Sci. Technol. 19 (2008) IOPScience 087002 (5pp)].

The device works as follows.

The downhole tool is placed in the well. Install with the help of the processor 5, the mode of operation of the control unit 6 and the amplifier-integrator 4.

Include pulsed source 2 to generate pulses of fast neutrons. Fast neutrons exit pulsed source 2 and generally get into the flushing (well) fluid, casing string, and then into the rock around the well, in which fast neutrons interact with the nuclei that make up their chemical elements, as a result of which they lose energy become epithermal with time, and then heat. The number of epithermal and thermal neutrons formed depends on the neutron porosity of the rock and the time after the neutron pulse. The number of thermal neutrons and their lifetime also depends on the presence of chemical elements that absorb neutrons.

The fast neutrons of the pulsed source 2, as well as the fast neutrons of the source, scattered in the environment during the neutron pulse, epithermal and thermal neutrons partially fall into the detector of 3 thermal neutrons. The charge produced by the neutrons in the thermal neutron detector 3 enters the integrator amplifier 4, in which it is amplified and further digitized. The time dependence of the signal at the output of the amplifier-integrator 4 is determined by the number of interactions of certain neutrons with the matter inside the detector 3 thermal neutrons at the appropriate points in time and the energy released.

Signals from the output of the amplifier-integrator 4 during and between neutron pulses are transmitted using processor 5 and telemetry system 7 to ground equipment (not shown in Fig. 1), where they are stored in the memory of a personal computer (PC). The process is repeated for N≥1 neutron pulses, with each subsequent dependence of the signal on time for the detector 3 thermal neutrons are summed up with the previous one. The number of neutron pulses N is determined by the specified measurement accuracy.

In ground equipment, the obtained time dependence is compared with a set of dependencies from the database, calculated in advance for rocks of different neutron porosity, with different parameters of the well and casing, as well as flushing fluid and certified by measuring these dependencies by this method on geophysical rock models.

From the database, the calculated dependence is chosen which is closest, in accordance with the applied evaluation criteria, to the dependence registered by the detector of 3 thermal neutrons. The neutron porosity of the rock, as well as well parameters, are assumed to coincide with the neutron porosity and the parameters used in deriving the calculated dependence.

Thus, the claimed technical result: ensuring the registration of fast neutrons emitted by a pulsed neutron source during its pulses, proportional to the He-3 counter provided that the electrical signals at the output of the proportional counter are superimposed by using a pulsed source of fast neutrons 2 placed in a cylindrical security case 1 and connected to the control unit 6, which is also connected to the processor 5, use instead of several neutron detectors: neutr nnogo monitor detector epithermal neutron, and thermal neutron detector a single detector thermal neutron 3 connected in series to the amplifier-integrator 4, a processor 5 and 7 telemetry system.

Claims (1)

The device for determining neutron porosity, which includes a pulsed source of fast neutrons, a neutron detector placed in a cylindrical security case, characterized in that one thermal neutron detector located coaxially with the pulsed detector is used as a neutron monitor, epithermal neutron detector, and thermal neutron detector. a source of fast neutrons and a cylindrical guard case; a thermal neutron detector is connected to an amplifier-integrator; an amplifier-integrator is connected n to the processor, the processor is connected to the telemetry system, while the pulsed source of fast neutrons is connected to the control unit, the control unit is also connected to the processor.

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