Interaction of gamma quanta with matter. Total linear attenuation coefficient of gamma quanta in matter Calculation of characteristics of interaction of gamma quanta with air
The interaction of gamma quanta with matter is fundamentally different from the interaction of charged particles.
First of all, the concept of slowing down is not applicable to gamma quanta. Their speed does not depend on energy and is equal to approximately 300,000 km / s. In addition, they have no charge and therefore do not experience the slowing down Coulomb interaction.
Nevertheless, for r-quanta, effective interaction can manifest itself already at a distance of tenths of an angstrom (1A \u003d 10 -8 cm). Such an interaction occurs in a direct collision of an r-quantum with an atomic electron or nucleus. Gamma - quantum with its electromagnetic field can interact with the electric charges of these particles and transfer them all or part of their energy.
Figure: 7.2.
The specific ionization created by gamma quanta is approximately 5 × 10 4 times less than the specific ionization of alpha particles and 50 times less than the specific ionization of beta particles. Accordingly, the penetrating power of gamma radiation is greater. The interactions of photons with matter can be classified according to two main features:
- 1) by the type of particle with which the photon interacts (atom, electron, atomic nucleus),
- 2) by the nature of the interaction (absorption, scattering, formation of pairs).
In the energy range from 0.5 to hundreds of MeV, the main role in the loss of energy of r - quanta is played by 4 processes that cause a weakening of the intensity of r - radiation: coherent scattering, photoelectric effect, Compton scattering and the formation of electron-positron pairs (Fig. 5.2).
Let us dwell in more detail on the consideration of the main processes accompanying the passage of gamma radiation through matter.
Interactions of gamma quanta with matter
PHYSICAL BASIS OF WELL RADIOMETRY
PART 2. Nuclear physics methods
In nuclear geophysics, only the most penetrating radiation is used - neutrons and gamma quanta, which "transilluminate" the well-formation system through the steel casing and cement stone. The reactions caused by neutrons in rocks are much more diverse than those caused by gamma quanta. For this reason, stationary and pulsed neutron methods are widely used in oil, gas and other mineral deposits to determine reservoir properties of rocks, identify productive objects, control field development, elemental analysis of rocks and mineral raw materials, and solve many other important problems. ...
A measure of the interaction of gamma quanta (as well as other particles) with matter is the effective interaction cross sections - microscopic and macroscopic. Microscopic section s determines the probability of interaction of one particle with another target particle (nucleus, electron, atom). Macroscopic section Σ - ϶ᴛᴏ measure of the probability of interaction of a particle with a unit volume of matter; it is equal to the product of the microsection and the number of targets per unit volume. According to the historical tradition, the macro-treatment for gamma quanta is usually called linear attenuation coefficient and denote m (not Σ). The value 1 / Σ determines the mean free path for a particular type of interaction.
Gamma radiation is attenuated in matter due to: photo effect; Compton effect; pair formation; photonuclear interactions.
When photo effect (Fig.7.1a) gamma quanta interact with the electron shell of the atom. The resulting photoelectron carries away part of the energy of gamma radiation E=hv-E 0, where E 0 is the binding energy of an electron in an atom. The process takes place at energies not exceeding 0.5 MeV. The photoelectric effect also produces characteristic X-rays.
The microscopic cross section of the photoelectric effect depends on the energy of the gamma quantum and the serial number Z element
s f \u003d 12.1 E –3,15 Z 4.6 [barn / atom].
Strong dependence on Z allows the use of the photoelectric effect for the quantitative determination of the contents of heavy elements in rocks (X-ray radiometric and selective gamma-gamma methods).
When compton effect, gamma radiation interacts with electrons, transferring part of the energy to them, and then propagates in the rock, experiencing multiple scattering with a change in the original direction of motion. This process is possible at any energies of gamma quanta and is basic at 0.2<E<3 МэВ, т. е. именно в области спектра первичного излучения естественно-радиоактивных элементов.
Fig. 7.1a, b. The main types of interactions of gamma radiation with matter ( and) and the ranges of energies and atomic numbers in which they appear ( b) (IAEA, 1976 ᴦ.):
1 - photo effect; 2 - Compton scattering; 3 - the effect of the formation of electron-positron nap
The process of formation of electron-positron pairs arising from photons in the field of atomic nuclei is most likely for rocks containing heavy elements (see Fig. 7.1b) at energies not less than 1.02 MeV.
Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, at different energies, gamma quanta interact mainly with various targets: atoms, electrons, atomic nuclei.
In the energy range where the Compton and photoeffects are most significant (Fig. 7.1b), the total macroscopic interaction cross section (also called the linear attenuation coefficient)
m \u003d m f + m k \u003d m k (1 + m f / m k) (7.1)
where m to \u003d n e s k - macrosection of the Compton effect; n e is the number of electrons per unit volume.
The electron density of media consisting of elements with a ratio of Z / A \u003d 1/2 is strictly proportional to the bulk density (such media are called "normal"). Due to the presence of hydrogen, for which Z / A \u003d 1, rocks differ from "normal" environments; the measure of this difference is the "normalization factor".
The effective atomic number of a complex composition medium is the ordinal number of such a monoelement medium, the photoelectric absorption cross section of which is the same as in a given multielement medium.
For mono-element environment n e\u003d d N A Z/Awhere N A - Avogadro's number; AND and Z - mass number and serial number; d - density. Elements that make up rock-forming minerals Since the condition for the stability of atomic nuclei (the condition for saturation of nuclear forces) requires that A=N+P» N+Z»2 Z, (N» Z) (where N and R Is the number of neutrons and protons in the nucleus), then Z/A\u003d 0.5 regardless of the type of element (the only exception is hydrogen).
Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, with Compton scattering, the macrosection m k is determined by the density (the value 2d Z/A called electron density). This fact serves as a strict physical justification density modification of the gamma-gamma method (GGM) ... In the energy region of the Compton effect, m »d, and the value
does not depend on density (Fig.7.2b); this value is commonly referred to as the “mass attenuation coefficient”.
Fig. 7.2a, b... Dependences of the mass attenuation coefficient m / d on the energy of gamma quanta ( and) and atomic number Z element ( b). Curves code - energy of gamma quanta, MeV
For convenience in comparing the influence of the photoelectric effect and Compton scattering, the photoabsorption cross section per electron is used
s f / Z = P e × 10 –2 ( E/132) –3,15 , (7.3)
where the value P e ("Photoelectric absorption index") is equal to ( Z/ 10) 3.6. The ratio of the cross sections m f / m to \u003d s f / Z s to " P e/ s k. Effective atomic number Z eff is expressed as follows (for a multi-element environment):
where Z i, A i, P i -serial number, atomic weight and weight (mass) fraction ith element, respectively, and the summation is extended to all elements in the natural mixture.
Attenuation and intensity dJ a wide beam of gamma radiation in a flat layer of a homogeneous substance with a thickness dx is described by a differential equation similar to the law of radioactive decay:
in integral form
J(x) = J 0 exp (–m x). (7.6)
If the density of the medium depends on x("Barrier" geometry), that is μ = μ (x), then
J(x) = J 0 exp [–Λ ( x)], (7.7)
where Λ is the optical thickness of the layer x, or
where T (x) is the mass thickness of the layer x; - mass attenuation coefficient.
For an isotropic point source, the exponential attenuation law (7.7) is superimposed on the geometric divergence law 1 / (4p r 2) in spherical geometry ("inverse square law"):
J(r) \u003d J 0 exp (–m r) / (4p r 2). (7.9)
This expression describes the spatial distribution of unscattered (neutron or gamma) radiation. The spectrum of multiply scattered radiation (Fig.7.3) from a monoenergetic source includes scattered radiation, but with decreasing energy, multiply scattered radiation makes an increasing contribution. While the photoeffect cross section is small, the determining factor is the electron density of the substance, which, in turn, is determined by the density of the medium. With an increase in the photoelectric absorption cross section (in accordance with a decrease in the energy of gamma quanta), the amplitude of the spectrum decreases, and is determined not only by the density, but also by the effective atomic number of the substance (the index of photoelectric absorption). For this reason, spectrometric registration makes it possible to determine not only the density of the rock, but also its effective atomic number (lithological type of rock). This modification of the HGM is usually called “selective”.
Figure 7.3. The spectrum of multiply scattered gamma radiation in rocks of the same density, but different composition (according to I.G. Dyadkin, 1978 В.; V. Bertozzi, D. Ellis, J. Volu, 1981 ᴦ.):
1 -3 - atomic numbers Z respectively small, medium and large; 4 - area of \u200b\u200bphotoelectric effect and Compton scattering; 5 - Compton scattering region, S - soft part of the spectrum; H - hard (Compton) part of the spectrum
When selective modification of GGM (GGM-S) use sources and detectors of soft gamma radiation. The GGM-S readings depend both on the Compton scattering of gamma quanta (hence, on the density of the medium) and on their absorption, ĸᴏᴛᴏᴩᴏᴇ is determined by the concentration of heavy elements in the rock. The interpretation parameter of the method is the photoelectric absorption cross section - P e [barn / electron]. The macroscopic absorption cross section per unit volume of a substance is denoted by U, it is customary to call parameter of photoabsorption [barn / cm 3] and is determined by the expression:
where e e is the electron density. The U parameter has a linear petrophysical model. This allows GGM-S data to be included in the system of petrophysical equations to determine the lithological composition and porosity of polymineral deposits. For example, for a two-component model of the medium (skeleton and fluid filling the capacitive space), the photoelectric absorption index is determined by the expression:
U \u003d K p U fl + (1-K p) U sk, (7.10)
where U fl, U sk are the corresponding parameters of the fluid and skeleton, respectively.
It is believed that a nuclear gamma quantum is a quantum of electromagnetic radiation with an energy in the range of 10 keV - 10 MeV, emitted by the nucleus. A gamma quantum can be viewed as a particle without mass and charge, moving at the speed of light. Despite the lack of charge, gamma quanta are able to interact with matter, mainly with electrons in atoms. There are three types of interaction of gamma quanta with electrons: photoelectric effect, Compton scattering and the formation of electron-positron pairs.
Photoeffect is an interaction in which the energy of a gamma quantum completely (excluding the binding energy of an electron in an atom) is converted into the kinetic energy of an electron. In this case, the gamma quantum disappears, and the electron loses its energy to ionize atoms, forming a certain amount of free charges. It is essential that it is the entire energy of the gamma quantum (with the exception of a very small part of it) that is transferred to the electron, and then converted into the energy of free charges. The amount of free charges is proportional to the energy of the electron, and, therefore, to the gamma quantum. Therefore, by measuring the charge formed in the substance, it is possible to determine the energy of the gamma quantum.
Unfortunately, the other two types of interactions are much more complicated. With Compton scattering of a gamma quantum on an electron, a gamma quantum transfers only a part of its energy to an electron and does not disappear. Thus, a gamma quantum of lower energy and an electron are obtained. Part of the energy transferred by a gamma quantum to an electron depends on the scattering angles of a gamma quantum and an electron after interaction.
This means that knowledge of the electron energy after Compton scattering does not give any information about the initial energy of the gamma quantum.
The formation of electron-positron pairs occurs if the energy of the gamma quantum exceeds 1.022 MeV. In this case, an electron and a positron are formed, and the gamma quantum disappears. The electron then loses its energy in the medium, and the positron annihilates, emitting two gamma quanta with an energy of 0.511 MeV. In turn, the emitted gamma quanta participate in the processes of photoabsorption and Compton scattering. Thus, during the formation of pairs, it is also impossible to obtain information about the energy of the primary gamma quantum.
An ideal detector should convert all the energy of a gamma quantum into an electrical pulse, the magnitude of which is directly proportional to the energy of the quantum, therefore, of all three processes of interaction of gamma quanta with matter, the photoelectric effect is the most informative.
To obtain good results when measuring activity, it is necessary to maximize the number of interactions passing through the photoelectric effect channel, reducing the number of the other two types that interfere with registration. Since the probability of the photoelectric effect, depending on the average charge of the atoms of the substance (Z), increases in proportion to the (Z4) - (Z5) degree, it is necessary to use substances with the maximum Z in detectors.
Of course, all interaction processes can take place even for one gamma quantum. For example, having formed a pair, the gamma quantum disappeared, the positron annihilated, producing two 0.511 MeV gamma quanta, one of which was scattered by Compton, and the other was absorbed by the photoelectric effect. If the energy of a gamma quantum is less than 100 keV, then the main process is the photoelectric effect; at an energy of more than 100 keV, the fraction of scattered gamma quanta increases, and at an energy greater than 1.022 MeV, the formation of pairs begins to contribute.
Figure 1.6.1 shows the probabilities of all processes depending on the energy of gamma quanta for NaI - a crystal used in scintillation detectors.
So, in order to determine the energy of a gamma quantum, it is necessary to measure the charge generated in the detector when the gamma quantum is completely absorbed.
There are 12 types of interaction of γ-quanta with matter. Of these, in the energy range 0.05-5-1.5 MeV, characteristic of isotope sources used in geophysics, three are significant: the photoelectric effect, the Compton effect, and the formation of pairs.
The total microscopic cross section for the interaction of quanta with matter is equal to the sum of the cross sections of the listed processes:
Photoelectric effect (photoelectric absorption) is called such an interaction of a quantum with an atom, in which the quantum is absorbed, and its energy is partially spent on the detachment of an electron, and partially transferred to the latter in the form of kinetic energy.
An atom that has lost an electron as a result of the photoelectric effect is in an unstable state. An electron from a more distant level fills the almost instantaneously released shell. An excess of energy, equal to the difference between the energies of these levels, is released in the form of quanta of the characteristic X-ray radiation, which has a certain energy for a given element.
Compton effect called elastic scattering of y quanta by electrons of atoms. As a result, the quanta change direction and transfer part of their energy to the electrons. For Eg\u003e Ei, atomic electrons can be considered free and at rest. Their connection with the atom has practically no effect on the scattering patterns.
(Eg is the energy of gamma quanta, Ei is the energy of the total electron, Z is the ordinal number of the element).
Pairing effect consists in the formation of an electron and a positron by a quantum at an energy equal to the sum of the rest energies of these particles \u003d 1.02 MeV.
The positron annihilates almost instantly as a result of a collision with a free electron of matter. In this case, two g-quanta with an energy of 0.51 MeV are formed.
Sources of gamma quanta and neutrons are the most important elements of downhole radioactive logging equipment. If the change in the flux density of the particles under study in time is associated only with statistical fluctuations, the source is called stationary. If the change is caused not only by statistical fluctuations, the source is called non-stationary. Usually non-stationary sources work
in pulse mode.
Fluctuation - Random deviation of a physical quantity from its mean value; cyclical fluctuations, instability.
Sources of g-quanta are metal ampoules containing, as a rule, (b-active preparations. As a result of b-decay, g-radiation arises. Radiation of b-particles is quenched in the ampoule body or with the help of special filters
trov. The type of preparation that determines the -g activity, radiation energy, and other parameters of the source depends on the type of the problem being solved (Table 3). Ampoule sources are stationary.
Radiation detectors subdivided into gas-filled, scintillation and semiconductor. The principle of their work is based on the registration of electrons and ions or light photons arising from the interaction of radiation with matter.
Gas-filled detectors are a glass or metal tube filled with an inert gas and having two electrodes. In the absence of ionizing radiation, no current flows between the electrodes. Gamma quanta are absorbed in gas with the formation of electrons, neutrons - with the formation of a-particles and protons. The charged particles ionize the gas, resulting in pulses of electric current.
Scintillation counters made from optically active substances - scintillators. When ionizing radiation interacts with an optically active substance, atoms and molecules are excited, from which they are freed by emitting photons. When registering quanta, monocrystals of sodium iodide NaJ or cesium iodide CsJ, activated to increase the light yield with thallium T1, are used as scintillators. For registration of thermal neutrons
crystals of lithium iodide, activated with europium, enriched with the 6Li isotope, or crystals based on zinc sulfide, activated with silver are used.
Semiconductor detectors are based on the generation of free charge carriers in a solid under the influence of ionizing radiation. The range of particles in a solid is approximately 103 times less than in a gas, and the probability of ionization
much higher.
Semiconductor detector (PPD) is a crystal of a semiconductor silicon or germanium material with small p- and n-regions, characterized by a high concentration of impurities, and an extended uncontaminated region L located between them. The width of region i can be brought to 8-12 mm by means of compensation of impurities with lithium ions. Therefore, existing PPDs are usually silicon-lithium or germanium-lithium. When the i-region is ionized,
no current pulse, the strength of which is proportional to the energy
ionizing particle.
PPD is used mainly for registration of quanta. The relatively small working volume leads to the fact that the efficiency of the SPD is low - most quanta pass through it, avoiding absorption.
Question
physical foundations of yfm - see above (top 31). Plus it!
Detectors - see above (31).
Elastic neutron scattering is a nuclear reaction in which the internal energy of the nucleus does not change and the sum of the kinetic energy of the system before and after the collision is conserved.
The deceleration continues until the thermal equilibrium of neutrons with the medium, that is, until their energy becomes, on average, commensurate with the energy of the thermal motion of atoms and molecules. That is why such neutrons are called thermal.
Question
Density GGK
Density gamma-gamma-ray logging (GGK-P) is used to determine the density of rocks and assess the quality of well casing. Accordingly, there are gamma-gamma density meters and gamma-gamma cement meters.
Physical foundations of GGK-P can be understood by analyzing the phenomena that arise when a substance is irradiated with hard y-quanta. With the geometry implemented in borehole conditions, the sources and detectors are on one side of the object under study (Fig. 94). Therefore, through special collimation holes in a screen made of metal with a large Z (lead, tungsten), only scattered
quanta. Consequently, the type of interaction of gamma quanta with matter is regulated by the kamton effect.
Compton effect called elastic scattering of y quanta by electrons of atoms.
The ratio of the value of Z, number of protons to A-
the rate of decrease in the number of identical nuclei with time \u003d 0.5.
In turn, at Z / A \u003d 0.5, according to the ratio mc is proportional to the bulk density of the substance d. Below is an explanation ..
shares and denoted by mk
For the campton effect:
The fulfillment of the condition Z / A \u003d 0.5 means that the bulk density of the medium is equal to its electron density 6e. The density of the solid phase btw of most rocks, in particular sandstones and carbonates, is practically equal to its electron density
nost be tv. At the same time, for the liquid phase (water, oil and some other formation fluids) Z / A \u003d 0.5 due to the influence of hydrogen. Therefore, for the liquid phase, the density dw and the electron density dw differ significantly. For example, for water:
For porous water-saturated rocks, you can write:
If we subtract one from the other and use equality 1 we get:
Thus, the error due to the influence of the hydrogen content is small, approximately constant and amenable to taking into account when interpreting.
Probes
There are single-probe (one source - one detector) and dual-probe (one source - two detectors) density meters. The full length of the probe Ls (probe) is the distance between the centers of the source and the detector, the length of the probe L is the distance
along the outer generatrix of the probe between the proximal edges of the collimation holes. The maximum length of the probe is limited by the safety-related activity of the source, the minimum - by the size of the screen. For most two-probe devices, the small probe has Ls \u003d 15-25 cm (L \u003d 10-18 cm), the large one has Ls \u003d 35-45 cm (L \u003d 30-35 cm).
GGMs have a shallow depth, and therefore their readings are large
the effect is exerted by clay cake and caverns. For the same reason, they cannot be used to determine the parameters of rocks in cased wells.
Tasks solved using gamma-gamma density measurement:
selection of rocks with different density in the sections of wells; isolation and quantification of the content of minerals, the density of which differs from the density of the host rocks; determination of the porosity coefficient.
Let us dwell briefly on each of them. Gamma-gamma density measurement allows separating rocks, the density of which differs by more than 0.03-0.05 g / cm3. It unambiguously distinguishes rock salts, anhydrites, in the terrigenous and carbonate sections - oil and gas reservoirs, characterized by low density.
With the help of gamma-gamma density measurement, it is possible to determine the depth, thickness and structure of coal seams (d \u003d 1.2-1.8 g / cm3). It is also used for the extraction of minerals, the density of which differs from the density of the host rocks. First of all, this concerns manganese and chromite ores (d \u003d 3.7-4.5 g / cm3). The presence of a correlation between the ash content of coals and their density, the density of ferruginous quartzites and the iron content in them makes it possible to use GGK-P for
calculation of reserves.
The porosity coefficient is determined by the formula:
Derived from formula 2) (above).
Question
NEUTRON LOGING
The well logging method based on the irradiation of rocks with a stationary flux of fast neutrons and registration of thermal neutrons, epithermal neutrons, or gamma rays of radiation capture is called neutron logging (NL).
NK modifications
There is epithermal neutron neutron logging (NNK-NT), thermal neutron neutron neutron logging (NNK-T), integral neutron gamma-ray logging (NGK) and spectrometric neutron gamma-ray logging (SNGK).
Downhole tools neutron methods are approximately similar (Fig.).
In general, they contain: a liner / with an ampoule source of fast neutrons 2 (during transportation and storage, the liner with the source is placed in a protective container); excluding direct exposure of the detector to a moderator screen 3 made of hydrogen-containing material and an absorber screen 4 made of lead; detector of 5 neutrons or 7-quanta; detector of 6 y-quanta of natural radiation; electronic circuit 7. Thus, the devices are designed for simultaneous NDT and GC.
The length of the detectors and the presence of screens in the downhole tool lead to
the fact that the detector 4 is located behind the inversion point. Therefore, environments with high con-
the centralization of retarders, for example, porous oil-bearing reservoirs, differ by
diagrams of neutron methods with reduced indicators, and the layers are dense, low
coporous - increased. Probes of neutron methods, detectors in which
are placed behind the inversion point, called over-inversion.
The NDT modification depends mainly on the type of the detector and the surrounding filters. In NNK-T measuring devices, helium is used, less often scintillation
tion counters. The recorded count rate is mainly due to the thermal neutron flux. In NNK-NT measuring devices, the counters are surrounded by cadmium filters that absorb thermal neutrons. Scintillation, rarely gas-filled detectors are used in the NGK equipment.
y-quanta, high-quality proportional scintillation detectors are used in the SNGK spectrometric equipment. In some cases, semiconductor detectors (SCDs) are used, which provide a significantly higher energy resolution. However, RPMs require cooling, which significantly complicates the design of the instruments and the measurement technology.
An important parameter of the NDT equipment is the length of the probe Ls - the distance from the middle of the source to the middle of the detector (for multi-probe instruments, to the beginning of the detector).
Physical fundamentals
Neutron readings depend on the moderating, absorbing and emitting properties of the rock. Consider the parameters determining these properties.
Neutron moderation length Ls... It can be seen that the retardation length is determined by the porosity coefficient of the rocks, that is, it is related to their hydrogen content; the influence of lithology is much less. For
for most rock-forming minerals that do not contain water of crystallization, the differences in Ls values \u200b\u200bare insignificant. Moreover, they are due not only to different retarding properties of the elements included in minerals, but also to the difference in density.
In rocks, the pores of which are saturated with water, oil and gas, the total hydrogen content is estimated by the hydrogen index (HI), which is equal to the ratio of the volume concentration of hydrogen in a given medium to its concentration in fresh water.
water under normal conditions. This parameter is also referred to as
equivalent humidity w. For fresh water, the equivalent
humidity ww \u003d 1. For oils wn ~ ww \u003d 1.
For clean, chemically bound water-free rocks saturated with water and oil with water:
that is, the VI of such rocks is equal to their porosity. For gas wg The average lifetime of thermal neutrons is t.With an increase in the content of elements with a high absorption cross section, t decreases. Abnormally low values t are typical for chlorides, low - for minerals of iron, sulfates, potassium feldspars, potassium and iron-containing clay minerals. Thermal neutron diffusion coefficient D depends primarily on Thermal neutron diffusion length- Ld. As a function of D and t, the value of Ld depends on both the decelerating and absorbing properties of the medium. With an increase in the hydrogen content and the number of elements with a high absorption cross section, the Ld value decreases. Emissivity of rocks n is the average number of g-quanta produced during the radiative capture of one neutron. Migration options- the total migration length of thermal neutrons Ln in the process of their deceleration and diffusion and the total migration length of neutrons and gamma-radiation of radiative capture Lnv are determined by the relations: the research radius of ННМ-НТ is less than that of ННМ-Т, and for ННМ-Т - than that of ННМ-Т. Neutron methods allow solving the following tasks: lithological dissection of the section; determination of porosity of rocks; determination of the position of the gas-liquid contact. The NNM-T and NGM methods allow to determine the location of the oil-water contact with significant mineralization of formation waters and a small zone penetration, as well as in cased wells based on observations of the rationing of the penetration zone. The NNM-NT and NNM-T methods are used when searching coal seams (coal contains up to 12% hydrogen) and for the extraction of rocks with a high boron content. Question With pulsed neutron methods, the rock is irradiated for a short time (with duration Δτ \u003d 1-200 μs) fluxes of fast neutrons, the following at intervals of time τ. Registration of the density of thermal neutrons or gamma quanta of radiation capture are carried out after a certain period of time no delay τz. There is a pulsed neutron gamma method (INGM) and im- pulse neutron-neutron method (INNM). Got more widespread pulsed radiation mode is achieved by using small-sized wells natural accelerators, in which ions are accelerated to high speeds in a magnetic a field of great tension. By bombing a special target, they knock out the strong neutrons with an energy of 14.1 MeV. This high energy provides the depth of research is up to 60-70 cm, which is more than when using a stationary local sources. In addition, when the power supply is disconnected, the the nickname does not radiate and is therefore safe. This does not exhaust the benefits impulse methods. With OSI, the processes of deceleration and diffusion occur as if sequentially in time and can be examined separately depending on the delay time registration. The intensity of the recorded radiation during deceleration (up to 10 2μs) characterizes the hydrogen content of rocks during diffusion (10 (2) 10 (4) μs) - concentration of absorbers. It is essential that the lifetime of thermal neutrons in a well is shorter than in a rock, and in formations saturated with saline water it is shorter than in oil-saturated formations. This allows, by applying the appropriate delay (more than 800 μs), get information independent of the influence well fluid and characterizing the type of filler. Determination of the of water-oil contact by pulsed neutron methods is possible when concentration of salts more than 30 g / l, while in stationary methods this value not less than 100 g / l. In principle, OSIs solve the same problems as stationary methods, however, the efficiency of the solution is higher. The disadvantages of OSI include the complexity equipment and low speed of logging. Look 35 37. Nuclear magnetic logging in the natural field of the Earth (NFL). Physical foundations. Magnetic properties of rocks. Nuclear magnetization vector. Longitudinal and transverse relaxation. PHYSICAL BASIS Nuclear magnetic logging (NML) is based on the study of the nuclear magnetic properties of hydrogen fluids filling the pores of the rock. The nuclei of hydrogen atoms, like other elements (fluorine, aluminum, carbon-13, etc.), have their own mechanical moment P (spin) and magnetic moment μ, the axes of which coincide. Spin (spinning) characterizes the intrinsic mechanical moment of the number of motions possessed by elementary particles. It can take only whole or half-integer values \u200b\u200b(0; 0.5; 1; 1.5), expressed in units of h / 2π, where h is Planck's constant (6.6261 · 10-34 J · Hz-1). The spins of the electron, positron, proton and neutron are equal to 0.5. This means that they take on a value of 0.5 h / 2π. When such nuclei are placed in a constant external magnetic field H, their magnetic moments μ tend to orient themselves in the direction of the vector of the given field, which leads to the appearance of nuclear magnetization. When the external magnetic field is removed, the acquired nuclear magnetization is destroyed due to the random thermal motion of atoms and molecules of the substance. Since this occurs in the Earth's magnetic field, the nuclei are oriented along this field, precessing (making damped rotations) around it like a top in a gravity field with the so-called Larmor frequency where Hs is the strength of the Earth's magnetic field (Hs≈40 A / m); γweight \u003d μ / Р is the gyromagnetic ratio (the ratio of the magnetic moment μ of precessing nuclei to their mechanical moment Р). The highest value of γgir is characteristic of hydrogen. This is the reason for the strongest expression of the effect of nuclear magnetism in hydrogen. In all other rock-forming elements, this effect is too small to be measured downhole. The main task of the NMR is to register the effects of free precession of protons of hydrogen nuclei in the earth's magnetic field. For this purpose, a downhole tool is lowered into the borehole, including an elongated rectangular coil, a switch, alternately connecting the coil leads to a DC source with a strength of 2-3 A, then to the amplifier output. When the coil is connected to a current source, a polarizing constant magnetic field is created in the environment. When the coil is connected to the amplifier, the EMF induced in it under the influence of the precession of hydrogen nuclei is amplified and transmitted through the cable to the surface to the ground equipment, where it is recorded (Fig. 79). When the polarization current Ip is passed through the polarizing coil for a time tp (Fig. 80, II, a), a constant magnetic field of strength Hp is formed in the medium under study. The vector of this field makes a certain angle with the vector of the Earth's field strength Hz and significantly (by about two orders of magnitude) exceeds it in magnitude. The nuclear magnetization vector M arising in this case during time tp is oriented according to the resulting vector Hav, which is the sum of two vectors of strength Hp and Hz (Fig. 80, I, b). where М0 is the vector of nuclear magnetization at tп → ∞; practically tp is taken equal to (3-5) T1 After switching off the polarizing current (by a stepwise decrease to the value of the residual current Ires and complete switching off after a time tres), only the Earth's magnetic field acts in the medium, and the nuclear magnetization vector processes around the vector Hc with the circular frequency ω (VI.1), gradually returning to its original value (Fig. 80, I, c). The nuclear magnetization vector M with respect to Hg can be decomposed into two components: the longitudinal Mll, which coincides with the direction of the vector Hz, and the transverse M⊥, perpendicular to Hz. Under the action of the vector M вектора, an electric sinusoidal signal (variable EMF) is induced in the coil - a signal of free precession (SSP), corresponding to the Et amplitude of the SSP (in V) at the time moment t (in s), which has passed since the beginning of the precession, decaying exponentially with the time constant of transverse relaxation T2 (Fig. 80, II, c): The transverse relaxation time T2 characterizes the signal decay rate (T2 is usually taken as the time during which the initial amplitude E0 decreases by about 2.7 times, E0 is the initial amplitude of the SSP, which is proportional to the nuclear magnetization vector M). To prevent the influence of transient processes caused by turning off the residual current, the moment of connecting the coil to the amplifier is shifted by the amount of dead time τ (see Fig. 80, II, d). The EMF induced in the probe coil is amplified and transmitted through the cable to the day surface, where the recording device records the EMF amplitude Ut at time t. The amplitude Ut is the envelope of the free precession signal: Ut \u003d U0exp (-t / T2), where U0 is the initial amplitude of the free precession signal. Since the free precession signal decreases exponentially, it is sufficient to have two values \u200b\u200bof its amplitude U1 and U2 or U1 and U3, separated by some time intervals t1, t2, and t3 (35, 50, and 70 ms) after the start of precession, so that by extrapolation restore the signal amplitude U0, which is used to determine the free fluid index: YMK equipment allows simultaneous automatic registration of two or three Nuclear magnetic logging in the natural field of the Earth (NFL). The probe, the method for determining the free fluid index (ISF), the factors influencing the readings of the method, the depth and the scope of the NMR. Interpreting NMC diagrams Interpretation of the NMR diagrams consists in determining the values \u200b\u200bof the free precession signal and the longitudinal relaxation time T1. The transverse relaxation time T2, being distorted by the inhomogeneity of the Earth's field, is not used to study the borehole sections. Based on the interpretation of NMC diagrams, it is possible to solve the main problems: identification of reservoirs and assessment of their reservoir properties; assessment of the nature of reservoir saturation and the prospects for obtaining oil, gas or water from the reservoir. Highlighting collectors The study of reservoir properties of rocks is carried out using U0. The magnitude of the measured signal of free precession is influenced only by those hydrogen nuclei that are part of the molecules capable of moving in the pore space of the reservoir. Studies have shown that bound water and solid hydrocarbons (bitumen, kir, asphaltenes) containing protons of low mobility are not marked by the signal of free precession on the NMR diagrams. This is due to the fact that, due to the presence of the dead time τ (see Fig. 80), only those ERPs are recorded in the NMR, which are characterized by a time T2\u003e 30 ms. The U0 value is calibrated in units called the free fluid index (FFI) and characterizing the pore volume (in%) occupied by the liquid participating in the formation of the CSP. The free fluid index is conventionally considered to correspond to the effective porosity coefficient where kwo is the residual water saturation coefficient. The free fluid index is defined as the ratio of the initial ERP amplitude recorded on a rock sample whose pores are filled with fresh water to the initial ERP amplitude measured on distilled water occupying the same volume as the rock sample. Accordingly, the ISF varies from 0 to 100%. To establish the scale of the NMR curves in ISF units, the equipment is standardized. The character of the dependence of ISF on the content of free water is not influenced by lithological, structural and other features of the rock. Consequently, in formations that are an alternation of interlayers of reservoirs and non-reservoirs, only the reservoir interlayers contribute to the ISF value, and the remaining differences that do not contain free fluid do not create a free precession signal. Therefore, the effective porosity kp.eff, determined for a heterogeneous reservoir or a pack of reservoirs, makes it possible to determine the total capacity of the object under consideration. Accordingly, the product of kp.eff and the thickness of the object H gives the total effective capacity of all reservoir layers contained in it. In reservoirs with fractured porosity, included in the general pore system, the transition from ISF to kp.eff is carried out in the same way as for granular reservoirs. For reservoirs characterized by the presence of isolated caverns not associated with a common pore system, a comparison of kp.eff and ISF is inappropriate, since the total volume of isolated caverns is not included in the effective porosity, but is included in the ISF. In this case, it is necessary to exclude the volume of isolated caverns, taken into account according to the ISF curve, but not affecting the kp.eff. Homogeneous hydrogen-containing formations, the thickness of which is equal to the length of the probe or exceeds it, are marked on the NMR curves by symmetric maxima located in the middle part of the formation; the boundaries of the layers are drawn in the middle of the inclined lines (Fig. 82). If the thickness of the formation is less than the length of the probe, the ISF decreases in comparison with the true values \u200b\u200band the maximum expands; determination of the boundaries of thin layers by NMR curves is difficult. Their average values \u200b\u200bare taken as essential (characteristic) quantities (ISF) k. Determination of the nature of rock saturation This determination is made from the longitudinal relaxation time T1. To measure T1, the instrument is installed at a predetermined depth in the intervals, characterized by the ICF curve as reservoirs containing free liquid. The longitudinal relaxation time T1 can be determined using Utp without taking into account a number of factors affecting the amplitude of the RCC, such as the diameter of the well, the thickness of the mud cake and the spatial orientation of the well. T1 measurement is performed at the depth of the studied formation in two ways: in a strong field - T1c. n and in a weak field - T1sl.p. To determine T1c. p a series of measurements of the amplitudes Utp (in V) is carried out for different times tp (in s) and the polarizing magnetic field Hp (in A / m). One of the measurements is performed with a sufficiently long time tp → ∞, which ensures the equilibrium state of the nuclear magnetization vector М∞сп (in A / m) (see Fig. 81, II, a and b). This vector corresponds to the amplitude of U∞.p and T1c. n can be calculated: Longitudinal relaxation time in a weak field T1s. n is determined by the duration of the residual polarizing field Host. To do this, measurements of the amplitudes of the SSP are performed at a fixed polarization time tp, but with a sequentially changing action time tres and, accordingly, the residual current Ires (see Fig. 80, II, c, d). In practice, to determine T1 from the measurement results, direct dependences of the amplitudes Utp and Utres on the times tp and tres are not used. The T1 values \u200b\u200bare found graphically. For this, the measurement results are used to calculate the values \u200b\u200bof the so-called longitudinal relaxation functions Fc. п (tп) and Fcl.п (tores), which in strong and weak fields, respectively, have the form: where U (tp) is the SSP amplitude at the polarization time tp; where U (tost) is the amplitude of the SSP at the time of the residual current; U (tores → ∞) - the amplitude of the RSC at tores → ∞, not directly measured, but calculated by the formula U (tores → ∞) \u003d U0 (Ires / Iп). Calculated values \u200b\u200bof the function Fc. p (tp) or Fcl.p (tres) correspond to real measurements of tp and tres and are used to graphically determine T1. For this purpose, the calculated functions are plotted on a form with a semi-logarithmic scale (Fig. 83). In a homogeneous water-saturated medium, the pores of which have the same size, the longitudinal relaxation function, even in the presence of bound water, is one-component. On a semilogarithmic scale, such a dependence has the form of a straight line with constant T1 and function values \u200b\u200bof about 0.37 (Fig. 83, a). In the presence of a mixture of fluids with different T1, the dependence is depicted as a curve, which can be decomposed into several straight lines. These lines are used to find T1 of each component (Fig. 83, b). The tangent of the angle of the obtained straight lines is equal to the time T1. As can be seen from the example shown in Fig. 83, straight lines representing the functions Fc. п (tп) or Fcл.п (tores), are transferred parallel to themselves so that they intersect the ordinate axis at a point equal to one. Time T1, corresponding to ordinate 0.37, is counted (in ms) on the abscissa axis. For an approximate estimate of T1, it is sufficient to make measurements at two values \u200b\u200bof the polarization time. With accurate determinations, up to 15 measurements are made for the values \u200b\u200bof tp or tres. In highly permeable formations, the greatest relaxation times (more than 1 s) are observed in water-saturated formations or oil-saturated formations containing light oil. However, the dispersion of these values \u200b\u200bis large: in addition to the nature of reservoir saturation, the value of T1 is also influenced by such factors as the specific surface of the reservoir, its hydrophilicity or hydrophobicity, the type of porosity, clay content, and fluid viscosity. With the difference in oil and water saturation of the formation, it is taken into account that the highly viscous (resinous) components of oil at low temperatures are characterized by rapidly decaying signals of free precession and are marked by low readings on the NMR diagrams. According to the experience of studying productive horizons with injected fresh water, the time T1 of the penetration zone in aquifers lies within 200-600 ms, and in oil and gas reservoirs - 700-1000 ms. In addition, oil and gas reservoirs, due to the presence of residual oil or gas in the invaded zone, are characterized by two components in the longitudinal relaxation characteristic. Nuclear magnetic logging is designed to isolate formations containing a mobile fluid, to determine their porosity and saturation. Integration of the NMR results with the data of other logging studies of wells allows expanding and clarifying the possibilities of quantitatively assessing the porosity of reservoirs, their effective thickness, saturation and industrial oil content. The NMR method is also used to separate oil-bearing and bituminous rocks. Limitations of the NMR method are related to the impossibility of measuring the ERP in a medium (clay solution, rock) with increased magnetic susceptibility, in rocks with low effective porosity (1.5-2%), including fractured reservoirs, if part of the fractures is filled with clay solution ... This method is inapplicable for very viscous oils - more than 600 mPa · s, in the presence of free fluid in the flushing fluid - water or oil, which creates an additional SCP. The disadvantages of the method are: the duration of measurements (the speed of movement of the YMK device is limited by the polarization time tp\u003e 3T1 and should not exceed 250 m / h); shallow depth of investigation (about 0.2 m), as a result of which the influence of the penetration zone on the NMR readings is great. Nuclear magnetic logging is applicable when examining open-cased borehole sections. Similar information. Study of a geological section of wells (lithological-geological section of a well) Study of the technical condition of wells Control over the development of oil and gas fields Shooting and blasting operations in wells Reservoir testing and wellbore sampling 8. Interaction of gamma quanta with matter, gamma logging, problems to be solved Radioactivity is the ability of some atomic nuclei to spontaneously decay with the emission of α, β, γ rays, and sometimes other particles. Gamma rays are short wavelength electromagnetic radiation. The path length of γ - quanta in rocks reaches tens of centimeters. Due to their high penetrating power, they are the main type of radiation recorded in the method of natural radioactivity. Particle energy is expressed in electron volts (eV). The impact of gamma radiation on the environment is quantified in X-rays. Of the natural radioactive elements, the most abundant are uranium U238, thorium Th232, and the potassium isotope K40. The radioactivity of sedimentary rocks, as a rule, is in direct proportion to the content of clay material. Sandstones, limestones and dolomites have low radioactivity, rock salt, anhydrite and coal have the lowest radioactivity. To measure the intensity of natural gamma radiation along the wellbore, a downhole tool containing a γ-radiation indicator is used. Gas-discharge scintillation counters are used as an indicator. Gas-discharge meters are a balloon in which two electrodes are placed. The cylinder is filled with a mixture of an inert gas with vapors of a high-molecular compound under low pressure. The meter is connected to a high voltage DC source - about 900 volts. The operation of a gas-discharge counter is based on the fact that γ-quanta, falling into it, ionize the molecules of the gas filler. This leads to a discharge in the meter, which will create a current pulse in its supply circuit. Gamma-ray logging. When passing through a substance, gamma quanta interact with electrons and atomic nuclei. This leads to a weakening of the intensity of γ-radiation. The main types of interaction of gamma quanta with matter are the formation of electron-positron pairs, the photoelectric effect, the Compton effect (the γ-quantum transfers part of its energy to the electron and changes the direction of motion). An electron is thrown out of an atom. After several acts of scattering, the energy of the quantum will decrease to a value at which it is absorbed due to the photoelectric effect. The photoeffect is reduced to the fact that the γ-quantum transfers all its energy to one of the electrons of the inner shell and is absorbed, and the electron is thrown out of the atom. The well has a significant impact on the GGC readings. It reduces the density of the medium surrounding the probe and increases the GHC reading in proportion to the diameter. To reduce the effect of the borehole, the HGS instruments have clamping devices and screens that protect the indicator from scattered γ-radiation of the drilling mud. Irradiation of the rock and the perception of scattered γ-radiation in this case is carried out through small holes in the screens, called collimators. The characteristic feature of scattered gamma ray diagrams is not direct, but feedback with density, which is due to the size of the probe. If the indicator were placed near the source, a medium with an increased density would also be noted with a high intensity of scattered γ-radiation. 9. Allocation of perforation intervals by location of couplings The method of electromagnetic location of couplings is used: to establish the position of the tool joints of the stuck drill pipes; determining the positions of casing coupling joints; precise binding of readings of other devices to the position of the couplings; interconnection of readings of several instruments; specifying the depth of running tubing; determining the current bottom of the well; in favorable conditions - to determine the perforation interval and identify the places of disturbance (ruptures, cracks) of the casing strings. Physical basis of the method: The method of electromagnetic location of couplings (LM) is based on the registration of changes in the magnetic conductivity of the metal of drill pipes, casing and tubing due to violation of their continuity. Hardware: The detector (sensor) of the collar locator is a differential magnetic system, which consists of a multilayer coil with a core and two permanent magnets that create a constant magnetic field in and around the coil. When the locator moves along the string in places where the continuity of the pipes is broken, the magnetic flux is redistributed and the EMF is induced in the measuring coil. The active collar locator contains two coils, each of which has an exciting and receiving windings. Under the influence of an alternating magnetic field generated by applying an alternating voltage to the exciting windings, an alternating voltage arises in the receiving windings, which depends on the magnetic properties of the environment. An informative parameter is the voltage difference across the receiving windings, which depends on the continuity of the medium. Ticket 4 10. GIS complex in a cased well, tasks being solved A prerequisite for the successful use of logging to study the geological section of a well is the selection of an appropriate set (program) of geophysical studies. The program should ensure the solution of the tasks assigned to it with the smallest possible amount of measurements. Taking into account the similarity of geological and technical conditions for carrying out works in different regions, standard GIS complexes are installed. Typical complexes include general studies that are performed along the entire wellbore and legal studies of promising oil and gas intervals. In a cased well, all types of logging are carried out except for micro-logging and BKZ (since they are used in an uncased well, because these methods determine the thickness of the mud cake). 11. Neutron gamma-ray logging, physical bases, curves, problems to be solved Neutron logging is used in open and cased wells and is used to solve the following problems: for the purpose of lithological dissection of sections; determination of the position of the current gas-oil contact (GOC), intervals of gas breakthrough, cross-flow, degassing of oil in the reservoir and assessment of gas saturation; determination of the position of the OWC oil-water contact in wells with high salinity of formation waters. Neutron radiation has the highest penetrating power. This is due to the fact that neutrons, being uncharged particles, do not interact with the electron shells of atoms and are not repelled by the Coulomb field of the nucleus. Just like gamma quanta, neutrons are characterized by energy E, which in this case is related to their speed. There are fast neutrons with an energy of 1-15 MeV, intermediate 1 MeV - 10 eV, slow or epithermal 0.1-10 eV and thermal neutrons with an average energy of 0.025 eV. The interaction of neutrons with a thing is stuck in an elastic collision with a nucleus with a loss of part of the energy, i.e. in slowing down a neutron, and capturing a neutron by a nucleus. Day of neutrons with energies from several MeV to 0.1 eV, the main type of interaction is elastic scattering. In the case of elastic neutron scattering, the value of energy loss for collision is determined only by the mass of the nucleus: the smaller the mass of the nucleus, the greater the loss of energy. Naib. energy loss occurs when a neutron collides with the nucleus of a hydrogen atom. One of the main neutron parameters of the medium is the deceleration length L3. This is the average distance from the point where the neutron escapes to the point where it will slow down to thermal energy. The slowed down neutrons continue to move and collide with the nuclei of the elements, but without changing the average energy. This process is called diffusion. The average distance that a neutron travels from the deceleration point to the capture point is called the diffusion length. The diffusion length is usually significantly less than the deceleration length. The end result of the motion of a thermal neutron is its absorption by some atomic nucleus. When a neutron is captured by a nucleus, energy is released in the form of one or more γ - quanta. There are the following types of neutron methods: neutron gamma method NGM, neutron method for epithermal neutrons LMN, neutron method for thermal neutrons LMT. They differ from each other by the type of indicators used. Pulsed neutron methods. The essence of pulsed neutron logging lies in the study of non-stationary neutron fields and γ-fields generated by a neutron generator. The neutron generator operates in a pulsed mode with a frequency of 10 to 500 Hz. In pulsed methods, the rock is irradiated with short-term fluxes of fast neutrons with duration ∆t, following one after another at intervals of time t.
Question 36
A schematic representation of the processes occurring during studies by the NMR method and the vectors of nuclear magnetization arising in this case is given in Fig. 80. In the absence of an external artificial magnetic field, the magnetic moments of hydrogen nuclei μ are oriented in the direction of the Earth's magnetic field H3, precessing around it (Fig. 80, I, a).
The nuclear magnetization vector M is not established immediately after switching on the current Ip, but during the time T1 of longitudinal relaxation (equilibrium establishment), which characterizes the rate of increase in nuclear magnetization in the direction of the applied polarization field (Fig. 80, II, b):
log curves of changes with depth of amplitudes of the free precession signal U1, U2 and U3 at fixed times t1, t2 and t3 and constant values \u200b\u200bof tp and tres. From these data, the value of U0 is estimated (or directly recorded when using a calculating device), reduced to the moment when the residual polarizing current is turned off. Curves U1, U2, U3, U0, recorded as a function of depth, are called NMR curves (Fig. 81).
To obtain true values \u200b\u200b(ISF) and according to data (ISF) k, corrections are introduced for the influence of the well, mud cake, spatial orientation of the well, etc. For this, the corresponding palettes and nomograms are built.
