FR2489534A1 - METHOD FOR SIMULTANEOUSLY MEASURING NEUTRON LIFE DURATIONS IN A SURVEY AND IN FIELD FORMATIONS EMPLOYING ITERATIVE IDENTIFICATION TECHNIQUE - Google Patents

Abstract

THE INVENTION CONCERNS DIAGRAPHY TECHNIQUES. SURVEY AND SURROUNDING FIELD FORMATIONS ARE IRRADED WITH 14MEV NEUTRONS EMITTED BY A 21 NEUTRON SOURCE. DURING THE INTERVAL BETWEEN NEUTRON PULSES, THE GAMMA CAPTURE RADIATION IS MEASURED IN AT LEAST FOUR INTERVALS AND NEUTRON CONTIGUS TIME. NOT OVERLAPPING. THE MEASUREMENTS TAKEN IN THE FOUR INTERVALS OF TIME AT EXPONENTIAL CURVES, USING THE LESS-SQUARE TECHNIQUE, TO SIMULTANEOUSLY DETERMINE THE COMPONENTS OF THE THERMAL NEUTRON DECREASE TIME RELATED TO SURVEYING AND FIELD FORMATIONS. APPLICATION TO OIL PROSPECTION.

Description

The present invention relates to the measurements

  killed in a survey and on the field formations crossed by a survey. The invention relates more particularly to the measurement of the decay time of thermal neutrons (or the lifetime of neutrons)

  field training near a survey.

  The observed decay rate of popula-

  thermal neutrons near a sounding, following a high-energy impulse or burst of neutrons, can be

  by the sum of exponential terms of the formation of ter-

  and increased probing of a substantive term that may vary depending on the conditions of the training and the

  survey. In typical conditions that we encounter

  in the field, the component of the service life or

  decay time of thermal neutrons which is relative

  survey-corresponds to faster decay

  that for the life-time component of thermal neutrons

  relating to field training. The main parameter of interest is t, that is, the average lifetime of thermal neutrons in the formation. Another interesting parameter is _CB ', that is, the duration of

  average life of thermal neutrons in the sounding. Linen-

  vention provides processes and a device for

  the simultaneous determination of these two interesting parameters.

sants.

  The device and methods of the invention

  read a source of fast neutrons operating in

  impulses. Fast neutrons are slowed (or moderated) rapidly to the level of thermal energy by

  interaction with the nuclei of the elements in the sound

  in the field formations surrounding the survey

  and in the fluids contained in the pores of such

  mation. The service life or the decay time of

  thermal neutrons for the formation of ground is deter-

  to a large extent by the salt or chlorine content of the field formations. Hydrogenated materials

  in the pores-and in the survey attenuate or slow down

  rapidly weave the fast neutron flux that is

  emitted by a fast neutron source under a

  sions. When fast neutrons are slowed down to the level of thermal energy, they are said to be thermalized, and they can then be captured by the nuclei of the elements.

  constituting the matrix of training and flui-

  filling the matrix of the formation, as well as the materials present in the sounding, among which the sounding fluid, the logging instruments and

  possibly the casing of the well. The chlorine element

  a very high capture cross-section for thermalized neutrons and this element is found in

  high-salinity probing fluids and fluids

  field formations contained in the pores of

  field training near a survey, in pre-

  high salt content (NaCl). Thus, a measurement of the decay time of thermal neutrons or the

  Neutron lifespan of the field formations

  killed near a survey can provide information on the amount of saline fluids present in the pores of formations. Combining this measurement with formation water salinity measurements, porosity measurements and shale content measurements of the formation, one

  gets a combination that can be used to distinguish

  fight the salt water oil filling the pores to

near a poll.

  There are currently two commercially used techniques for measuring the thermal neutron lifetime or the thermal neutron decay time of near-field formations.

  a survey. In these two techniques used commercially

  In general, it is assumed that the survey materials have a

  cross section of thermal neutron capture nota-

  significantly higher than that of the

  ronnantes. According to this supposition, it is possible

  be an impulse or a burst of neutrons from a logging instrument located in the borehole, and after

  sufficient time for the thermal neutrons pre-

  Most of the probes in the survey itself are captured by survey nuclei with a high capture section, and the survey decay time component can be ignored. It is then possible to measure the decay rate of the thermal neutron population in the field formations. These commercial measurements of neutron lifetime (or thermal neutron decay time) have been found to be of great value in evaluating the productive potential of field formations located near boreholes equipped with casing. In these two trade techniques that are available on time

  current, a logging instrument that crosses the sound

  using a high energy neutron source

  (14 MeV), operating in pulse mode, these

  usually produced in an accelerated tube

deuterium tritium ester.

  The first commercially available technique currently available is the so-called technique

  "fixed window". In this technique, we operate

  the source of neutrons by repetitive pulses and

  for each neutron pulse a fast neutron cloud

  is injected into the field formations around

  nantes, with a configuration that generally

  a spherical symmetry with respect to the source. The fast neutron cloud propagates from the tool in the well through the drilling mud, the borehole casing, and the cement between the casing and the ground formation surrounding the borehole.

  enters the field formations. In this technique

  that each of these fast neutron pulses has

  characteristic of an intensity approximately

  aunt and has a characteristic duration ranging from 20 to 50 Ps.

  The number of thermal neutrons constituting this cloud or population then decreases exponentially because the capture of neutrons thermalised by the nuclei

  field training and survey.

  After an initial period following the burst of neutrons (typically about

  300 to 400 ps), after which it is considered that

  resulting tribution of gamma-ray capture in the

  sounding, mud and casing virtually disappeared

  In this case, the number of thermonized neutrons is measured near the tool in the well for two successive time intervals, or windows, of fixed duration. These measurements can then be used during the constant time windows or intervals

  successive times, to define a decay curve

  approximately exponential for the population

  thermal neutrons in the surrounding terrain formation.

the survey.

  It is hypothesized that the time that has elapsed since neutron boiling is sufficient

  so that almost all the thermalized neutrons pre-

  in the vicinity of the sounding itself were captured by the nuclei of the sounding elements. This hypothesis

  is tantamount to considering that the decreasing time component

  thermal neutrons or the lifetime of the

  thermal neutrons relative to the sounding is generally

  shorter than the decay time component.

  this thermal neutrons or the life of the neu-

  thermal trons relative to the formation of ground. This is usually the case when drilling fluids with a high content of chlorine or salt water are encountered in the borehole. However, this relationship is not always verified in soundings containing air, gas, fresh water or oil. As a result, a particular advantage of the invention over this prior art "fixed window" thermal neutron lifetime measurement technique is that no assumptions about the characteristics are made. relating to the decay times of the thermal neutrons of the fluid of the

  survey, in relation to the time characteristics of

  growth or lifetime of thermal neutrons from

  field formations surrounding the survey.

  The population of thermal neutrons in the formation near the sounding is deduced,

  during the two fixed duration window intervals

  each neutron impulse or burst, by measuring the capture gamma rays that result from the capture of neutrons thermalised by the nuclei of the materials con-

  soil formations and fluids

  in the pores of these formations. For exemple,

  the two most frequent time intervals or windows

  used in the fixed window technique for

  measuring the decay times of thermal neutrons

  can be between 400 and 600 read after the

  puff of neutrons and between 700 and 900 ps after

  fairy of neutrons. These values are used in

  characteristics, regardless of the salinity of the fluid present in the sounding. Because

  these fixed time windows are provided for a sound-

  In general, regardless of salinity, they are not optimized to give a rate of

  maximum count. Windows being delayed by one

  relatively long after the puff, the count rate during windows is less than optimum in soundings filled with saline fluid. This can lead to statistical uncertainty about the measurement of X

  If we ignore the diffusion effects of neu-

  trons, the relation defining the decay of a population of thermal neutrons in a homogeneous medium having a capture cross-section I for thermal neutrons, at the macroscopic level, can be expressed in the manner indicated by the equation (1) N2 = N e- (vt (1) where N1 is the number of thermal neutrons at

  a first moment t1, N2 is the number of thermal neutrons

  present at a later time t2; e is the base of the natural logarithms; t is the time between two

  measures (t2-t1); and v is the speed of thermal neutrons

  c. The macroscopic capture cross-section of thermal neutrons v of a reservoir rock (obtainable from equation (1)) depends on its porosity, matrix composition, shale content, the salinity of the water of the formation of ground, and the quantity and the type of the oil contained in the pores of this rock. This quantity is therefore an interesting physical measure or parameter that can be obtained

for the formation.

  The second commercial technique of the prior art

  currently available for measuring thermal neutron decay time or thermal neutron lifetime uses a reciprocal relationship of the macroscopic thermal neutron capture cross section, E, which is defined by absorption time of thermal neutrons, d-. A relation analogous to equation (1) but expressed by means of Vs' written in the form N = N and / V (2) o

  with 1 = / vE. In equation (2), N represents the den-

  sity of thermal neutrons at any instant t;

  No represents the density of thermal neutrons at one

  as initial t0; e represents again the constant which constitutes the base of the natural logarithms; and IC is the

  necessary time for the thermal neutron population to

  decreases to the fraction 1 / e of its value at t0. When measuring the decay time of thermal neutrons using the second technique of the prior art, called the "sliding window" technique,

  the logging instrument emits into the formation an im-

  impulse or a burst of fast neutrons whose duration is

  effectively ordered and linked to the previously measured values

  for the field formations. For example, the

  duration of the neutron pulse can be of a 'd. We use

  gamma detectors to obtain

  gamma-ray capture during two successive time intervals following the generation of a neutron cloud

  near the sounding, in order to define the decay curve

  exponential. However, in this technique the two intervals used to measure the population of

  gamma rays in order to define the exponential decay curve

  nential do not have a fixed duration or a fixed starting point following the puff of neutrons. The value of r measured previously during the previous neutron burst cycle is used to establish the duration of the puff, neutrons for fast neutron generation, and

  waiting period until the opening of the first

  first time window following the puff, the duration of the first time window, the duration of the second

  time window and the interval, waiting between

  clashes of the first and second time windows.

  All of these times are set to satisfy a predetermined relationship with respect to Ad.

  duration of the second measurement window can be two 2j.

  A wait interval of two 2s can be used in the following

  the first puff of neutrons, before opening the first window. The first window can have a

duration of a 2 '.

  In the two systems of the prior art described above which are intended to determine the lifetime or the decay time of thermal neutrons, the neutron source and a detector are the only elements

  essential for measurement. However, in both techniques

  of the trade available, dual spaced detectors are used, and measurements of the thermal neutron capture gamma radiation taken by the detectors are used to generate approximations or

  measurements of the porosity of field formations close to

  survey. The device of the invention also employs

  two detectors and can perform porosity measurements.

  As previously envisaged, the two techniques

  commercial issues available at present for the

  measurement of the decay time of thermal neutrons used

  read the hypothesis that the decay time

  thermal neutrons from the sounding is significantly lower

  with the neighboring field formations of the

  ge and can therefore be separated by a timer over a period corresponding to the component of the survey. In sliding window techniques, a bottom time window can be used for a period of time generally following the two detection windows used for the Z: or 'measurements in order to

  to measure the background gamma radiation that is due to the events

  neutron capture in the sounding and drilling

  mations of land surrounding the survey. After proper standardization, these background counts are generally subtracted from the counts made during the two measurement windows in such a system, so

  to remove the influence of gamma radiation

  This background is present in the vicinity of the sounding and any background radiation the neutron source can induce in the gamma detectors and in the formation. It should be noted that the two commercial logging systems described above do not use all of the gamma ray counting information of which

  after every puff of neutrons you can have it.

  The time intervals during which the detectors are not validated so as to accept the information are lost in these two systems of the prior art. Ainisi, the techniques of the prior art do not make full use of the neutron flux emitted by the neutron generator. Similarly, the two techniques of the prior art involve the assumption that the

  lifetime of thermal neutrons or the decay time

  thermal neutron formation can be

  substantially complete separation from that of the

  the survey, by the establishment of temporary windows

  poral. This hypothesis is not entirely valid, even under ideal conditions. The invention uses techniques and devices that avoid each of these

  assumptions and limitations of the prior art.

  According to the invention, a diagrammatic tool is moved

  This tool has a fast neutron source operating in pulse mode and two radiation detectors. The neutron source

  generates a pulse of fast neutrons of appropriate intensity

  approximately constant, for a period of between 30 ps. These neutrons are introduced into the environments that constitute the sounding and the surrounding formations and they give rise to the generation of a population of thermal neutrons, under the effect of the slowdown of

  fast neutrons in the environments of earth formations

  rain and in the sounding. After a very short break

  to enable the moderation of fast neutrons, as a result of the neutron pulse, the detectors are validated and the gamma capture radiation that results from the capture of thermal neutrons in the sounding and the field formations located near the sounding

  is recorded in a practically continuous manner

  that the next puff of neutrons is on the

  point to start. During multiple time windows

  who appear during this practical interval

  In a continuous manner, the rate of capture gamma ray count is observed in six or more substantially contiguous time slots. Measurements of count rates over multiple time windows are transmitted to a thermal neutron lifetime calculator that calculates the neutron life-time components for the ground formation and sounding, by performing an analytical identification of curve to

  meaning of least squares on these cadence data.

  This count is taken from six or more time windows that are almost contiguous following each burst of neutrons. The life calculator of the thermal neutrons is validated so as to simultaneously calculate, at the same time, the component of life of the thermal neutrons relative to the sounding and the component

  the lifetime of thermal neutrons relative to the

  field. About once a second, and for about 5% of the one-second operating cycle, the

  source of neutrons is stopped and the detectors are

  to determine the possible presence of a cadence

  counting, corresponding to a lifetime of

  long-lasting, relative to a neutron-induced gamma radiation activity of the source in the gamma ray detector, formation, borehole or logging probe, or to natural gamma radiation in the vicinity of the sounding. This background gamma radiation information is then appropriately normalized and subtracted from the six or more gamma thermal neutron capture gamma ray measurements that are made in the time windows following each burst of neutrons. Electronic devices are placed in the tool located in the borehole as well as on the surface for

  produce the measurement sequence and the pulses of neu-

  trons as described. In addition, impulses

  synchronization are also generated in order to pro-

  to find a way to separate gamma ray counts

  representative of the capture of thermal neutrons during

  each of the six or more window portions of the measuring cycle, as previously described. There is also a calculator placed on the surface to determine the decay times, or lifetimes, of thermal neutrons corresponding to the component of the sounding and the component of the field formations,

  and this calculator is associated with a logger diagra-

  in which a recording medium can be

  moved according to the depth in the survey,

  when the logging tool is moved into the

  dage. The life components of thermal neutrons

  which correspond to field training and

  sounding can be plotted on this recorder

  depth in the survey. The invention thus

  techniques for simultaneous determination of

  the value of the decay time of thermal neutrons

  or the macroscopic capture cross-section of

  neutronz. thermal for the sounding. and for the envi-

  he's funny. The invention will be better understood when reading

  the following description of embodiments and in

  with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation

  of a logging device corresponding to the invention.

  Fig. 2 is a block diagram showing the electronic device of the logging device of the invention. FIG. 3 is a graph showing the decay curve of the composite thermal neutron population and time windows, in accordance with FIG.

  embodiment of the invention.

  Figure 4 is a graph showing a sequence

  telemetry as a function of time in accordance with the

  vention. FIG. 5 is a graph showing the decay curve of the composite thermal neutron population and time windows in accordance with a

  second embodiment of the invention.

  FIG. 6 is a schematic representation of a telemetry sequence as a function of time for the

  window configuration of Figure 5.

  FIG. 7 is a flowchart showing an embodiment of a method allowing a calculator

  surface to obtain interesting parameters of the forma-

field conditions.

  The techniques of the prior art envisaged

  for the purpose of determining the service life or

  decay times of thermal neutrons can

  to counter two main difficulties. These two difficulties

  The main ones are: (1) under certain conditions

  the field formations and the survey, the component

  the survey did not decrease to a negligible level.

  before the beginning of the validation sequence of the

  to determine the lifetime of the neutrons, this

  which leads to an erroneous measure of ^ i; and (2) the accuracy

  The statistical significance of ZF is sometimes very poor because the decay rate samples must be taken after relatively long time intervals after the neutron burst, in order to minimize the

effects of the survey component.

  A third difficulty relating to techniques

  neutron lifetime logging of the prior art

  was initially considered by Mills, et al. in an article entitled "Pulsed Neutron Experiments in a Borehole Model", Mills, et al., Nuclear Science and

  Engineering, Vol. 21, pp. 346-356 (1965). The article

  cited shows that even if one calculates V F from data

  counting time taken with sufficient delays.

  In order for the survey component to decrease to a negligible level, the calculated value of X F remains a function of rB2, that is, the lifetime of

  thermal neutrons inside the sounding. We can consider

  that this is due to continual backscatter of

  neutrons thermalized towards the sounding, from the

  even after the "original" population of thermal neutrons in the survey decreased by capture to a low level. Thus, the two techniques of the prior art do not take into account the effect that is produced by this third difficulty. However, the invention takes into account the effects corresponding to all three difficulties and leads to a measurement of the lifetime or the decay time of thermal neutrons.

  much more reliable than those that could be obtained

than now.

* To make accurate determinations of saturation

  hydrocarbon logging based on the duration of the

  life or decay time of neutrons emitted by impulse

  the following three criteria must be met: (1) The observed service life 'ZF of the component

  relating to training must be calculated on the basis of

  counting cycles which do not contain any

  bution from neutron capture within the survey; (2) The statistical precision of tr must be as large as possible; and (3) the intrinsic average life of the

  component relating to training,%: Fi 1 must be deter-

  before carrying out the water saturation calculations.

  carbides. According to the above-mentioned article by Mills, et al., The measured lifetime can be related to the intrinsic lifetime alone, if ZB is known. It is therefore desirable to simultaneously and simultaneously measure the formation-related component, UF 'and the sounding component, ZB, of the lifetime or decay time of the thermal neutrons to obtain the

maximum accuracy.

  As previously discussed, the observed decay rate of the thermal neutron population near a sounding, following a high energy burst of neutrons, can be expressed as the sum of a component relating to training, a survey component and a component

  background. This can be expressed mathematically by the equation

  (3) C (t) = Ae t 'B + Be / t F + CE (3) In equation (3), C (t) is the counting rate at any instant t measured from a moment of reference

  ence. A and B are constants that can be inter-

  in accordance with Figure 3, where A represents the initial component of the sounding at the reference time equal to 0, while B represents the initial component of the formation at the reference moment equal to 0. These components are represented on the figure 3 by

  points of intersection on the y-axis of a graphi-

  than plotted against time. In equation (3), represents the sounding component of the composite thermal neutron lifetime. The quantity TB can be considered as the slope of the curve of the sounding component in FIG. 3. Similarly,

  F represents the life component of the training

  the composite neutron lifetime and can be

  consider it as the slope of the curve of the

  posture of the formation of Figure 3. Finally, CB represents

  This is the component of the count rate that is due to long-lived radiation and can be considered as a constant component, as shown by the horizontal line marked "BACKGROUND" in Figure 3. Composite decay curve thermal neutrons

  shown in Figure 3 corresponds to the result of

  the summation of the curve of the sounding component, the curve of the component of the formation and the curve of the substantive component that are represented

in this figure.

  In the technique of the invention, the background component CB is measured during a separate part of the operating cycle, as shown in FIG.

  will now consider Figure 4 which shows the

  time dependence of a telemetry signal stream from an instrument in the sounding

  and will be described in more detail later.

  A synchronization pulse starts each cycle of

  operation of the instrumentation located in the survey.

  This synchronization pulse is immediately followed

  life of a puff of neutrons of approximate intensity-

  constant, with a duration that will be described later

  in more detail. Six window intervals

  or more, follow each puff of neu-

  trons and during these intervals count rate measurements are made in a remote detector of

  the source and are transmitted to the surface. Interactions

  Multiple time window valleys are practically contiguous and last for a total of about 1 ms following each sync pulse. This cycle of

  Repetitive operation is repeated approximately 1000 times during

  within one second. At the end of an interval of 945 ms, we use a bottom window represented on

  Figure 4 to count the background radiation correspon-

  Figure 4. During this interval of - 55000 ps or 55 ms, the neutron generator does not work. Therefore, the measurements made during this time interval, after about 5 ms to allow the

  thermal capture radiation following the last bouff-

  sequence of neutrons to reach a negligible level, contain only counting information

  on radiation attributable to the substance. This information

  The background radiation meter is transmitted by telemetry to the surface by the device in the sounding and is processed in the same manner as

  will describe in more detail later.

  When measuring the background counting rate CB in the manner described and transmitting it to the surface by telemetry, it can be subtracted from the composite count rate C (t) of equation (3), in order to obtain a resulting count rate C '(t) according to equation (4): C' (t) = C (t) -CB = Ae-t / VB + Be t / IF (4)

  In equation (4), all the symbols have been defined

cédemment.

  In the process of the invention, such as

  figures 3 and 4, six (or more)

  measured during the six time windows that follow

  neutron burst, which are designated T1, T2, T3, T4, T5 and T6, are combined by a technique

  curve identification in the least squares sense.

  Count rate measurements made in the six

  time windows can give rise to an identifica-

  curve in real time in a calculator located in

  surface area, for example, in order to obtain the parameters

  in equations (3) and (4). The identification procedures give 72 T B, A and B, as previously indicated. It will be observed that the six (or more) approximately contiguous time slot intervals shown in Figure 3 have a minimal or negligible delay between each window. Therefore, all of the cadence information of

  counting (after a short interval of moderating

  between the end of the neutron burst and the opening

  of the first time window, this duration of modera-

  typically being in the range of 20 to 30 ps) is used in the process of the invention. No counting information is lost due to waiting for the decay of a survey component. In addition, because this technique simultaneously determines> F

  and CBs the criterion of the Mills article, et al. men-

born previously is satisfied.

  We will now consider Figures 5 and 6 which schematically represent another implementation

  temporal windows using the techniques of the invention

  tion. FIG. 5 shows an interval corresponding to a puff of neutrons with a duration of 15 to 20 ps, a moderation time interval of 20 to 30 seconds following the puff, and then a time window, bearing the designation "WINDOW" 1 "which is open for a relatively short time. We use a time window 2

  slightly wider or-of slightly longer duration.

  The following time windows 3, 4, 5 and 6 each have

  does not last longer than the window that pre-

  gives way, in the sequence of time windows. This

  configuration of time windows is intended to optimize

  statistically, the counting rates

  in each of the windows. As and when

  of the composite decay curve of popula-

  In the case of thermal neutrons, the time windows of successively increasing width allow the appearance of a higher number of shots at the lower counting cade of the temporal windows which appear later. Tables 1 and 2 below indicate the actual times that are considered for the time window configurations shown in Figures 3 and 5 (in Tables 1 and 2, all times are measured

  compared to the reference moment O at the beginning of the bou-

fairy of neutrons).

TABLE 1

  CONFIGURATION OF TEMPORAL WINDOWS OF THE

FIGURE 3

  NI Initial Instant Instant Final Time Window 1 50 ps 195 ps 145 ps 2 200 ps 345 ps 145 ds 3 350 ps 495 ds 145 us 4 500 is 645 ps 145 ps 650 ds 795 ds 145 ls 6,800 ls 945 ps 145 ds

TABLE 2

  CONFIGURING TEMPORAL WINDOWS OF FIGURE 5

  Initial Instantaneous N Instantaneous Final Time Window 1 60 pis 90 s 30 ps 2 90 ps 140 ps 50 ps 3 140 ps 200 ps 60 ps 4 200 ls 300 ps 100 rs 300 ls 500 ps 200 ps 6 500 is 998 ps 448 ps The short time intervals (5 ps) between the time windows in Table 1 are set to take into account the time required to transfer the contents of a

  counter to a buffer in the electronic circuits

  of the tool in the survey, which will be described later.

  térieurement. Similar short intervals would be required for the time windows of Figure 2, but they are not shown in the table for the sake of simplicity. Note that we want to ensure that the time windows in Tables 1 and 2 correspond to periods as contiguous as possible, taking into account the

  time limitations of electronic circuits.

  Figure 6 shows a signal train of

  telemetry resulting from the configuration of temporary windows

  As shown in FIG. 5, a synchronization pulse is emitted to the surface by the electronic circuits in the borehole. This impulse

  followed by the burst of neutrons and the moment of reference

  ence coincides with the end of the puff of neutrons. Linen-

  A short time period of 10 to 30 μs then elapses, then a number representing the count in time slot 1, designated G in Figure 6, is telemetrically transmitted to the surface. Similarly, numbers representing the counts in windows 2 to 6 are transmitted to the

  area. This sequence is extended for 945 ms. Linen-

  The 50 ms bottom window time is then triggered in the manner previously described in connection with FIG. 4. In this manner, the counting rates C (t i), with i = 1 to 6, are transmitted by telemetry to the surface. o they are used in a calculator of

  surface (which will be described in more detail later

  to implement a least squares curve identification technique, in order to extract the

interesting parameters.

  Since equation (4) is non-linear, it is necessary to use an identification procedure

  iterative for identification in the least squares sense.

  A particular identification procedure will be considered later and will be described in more detail in relation to Figure 7. It is however sufficient to say for the moment that the parameters of interest are obtained using the surface calculator, by means of an iterative identification procedure in the least squares sense. The

  values of ^ IrFi rBe A and B can then be recorded

  according to the depth in the sounding, of a

classic way.

  We will now consider Figure 1 which represents

  diagrammatically a logging device in accordance with

  principles of the invention. A sounding 10 is filled with a fluid

  11 and he goes through field formations to study. A logging probe 12 is suspended in the borehole 10 by means of a conventional logging cable 13, in a known manner, so that it is possible to raise and lower the probe 12 in the borehole in the desired way. The logging cable 13 passes over a pulley 14 on the surface. The pulley 14 is electrically or mechanically coupled, as indicated by the dashed line 15, to a log logger 18 which may include an optical recorder or magnetic tape, or both, as is known. The measurements made by the probe 12 located in the sounding can thus be recorded according to the depth of

the probe 12 in the sounding.

  In the probe 12 located in the sounding, a gen-

  Neutron generator 21 is powered by a high voltage power supply 22 so as to receive a high voltage (about 100 kV) for its operation. Electronic control and telemetry circuits 25 are used to apply control signals to the high-voltage power supply and the generator 21, and to transmit by telemetry to the surface by the cable

  13, the information resulting from the measurements made

  by the instrument in the survey.

  Two radiation detectors 23 and 24 are spaced

  longitudinally with respect to the neutron generator 21. The radiation detectors 23 and 24 may consist, for example, of sodium iodide crystals activated by

  thallium which are in optical coupling with

  multipliers. The detectors 23 and 24 have the function of detecting the gamma radiation produced in the surrounding formations which results from the action of the neutron generator 21 which emits neutrons. A shielding material for neutrons, 28, containing a high density or high scattering cross section body, is interposed between the neutron generator 21 and the two spaced apart detectors 23 and 24 to prevent direct irradiation of the detectors. by the neutrons that are emitted by the generator

  21. A shield 29 may also be

  stalled if desired between the detectors 23 and 24.

  When the neutron generator 21 is turned on, a puff or a neutron pulse with a duration of about 10 to 30 μs is generated and is emitted in the borehole 10 and the borehole 11, and passes through the steel casing 26 and the cement layer 27 surrounding the steel casing, to penetrate the field formations 20 that are studied. The burst of neutrons is moderated or slowed down by the diffusion interactions of

  so that all neutrons take practically

  corresponding to thermal energy. Thermal neutrons

  thermal or thermal conditions then begin to give rise to

  capture interactions with the nuclei of the constituent elements

  tituents of field formations 20 and contained pores

in these formations.

  Neutron capture by elemental nuclei

  constituting the field formations 20 and their pores pro-

  from the capture gamma rays that are emitted and reach

  the detectors 23 and 24. The photomultipliers of the detectors

  23 and 24 provide a voltage pulse for each gamma ray thus detected. These voltage pulses are applied to the electronic part 25, they are counted in a digital counter and their number is transmitted by telemetry to the surface by a driver

  16 of the logging cable 13. At the surface, an electrical box

  tronic surface 17 detects the information transmitted by telemetry from the probe 12 which is in the sounding and it implements the identification technique in the least squares sense to determine the parameters r Fy t ', A and B relating to the field formations which are studied. The surface electronic circuits then apply to the recorder 18 signals representative of the

  measured quantities and these signals are recorded as

  depth in the survey.

  We will now consider Figure 2 which is a

  schematic diagram showing the electronic parts of the dis-

  underground and surface electronic positives, this representation being more detailed but still schematic. The energy required for the operation of the underground electronic circuits is transmitted by a conductor of the logging cable 32 and is applied to a conventional low-voltage power supply 31 and to a high-voltage power supply 34. tension 34 may be of the Cockcroft Walton multi-stage type and provides approximately

  100 kV for the operation of the generator tube of neu-

  trons 33. The neutron generating tube 33 is of the deuterium-tritium accelerator type. An ion source 36

  which is maintained at a potential close to the mass is

  to generate deuterium ions from deuterium

  gaseous filling of the tube casing 33. A heating element

  regeneration 37 is impregnated with additional deuterium

  shut up and it maintains a pressure level of deuterium

  gaseous inside the casing of the tube 33 which is sufficient

  for providing the ion source 36 with deuterium gas for ionization. A target 35 is impregnated with tritium and is held at a negative potential

  relatively high 100 kV. The ion source is

  by an ion source pulse generator 41.

  When the ion source receives a voltage pulse of

  latively low, it causes the ionization of the gas contained in the casing of the tube 33 and accelerates the ionized gas towards

  the matter of the target 35. When the deuterium ions hit

  matter of the target 35 they give rise to an inter-

  thermonuclear reaction with the tritium ions contained in the target to produce neutrons which are then emitted in the sounding and the surrounding terrain formations with, in general, a spherical symmetry with respect to

  to the neutron generator tube 33.

  A regenerator control circuit 39 receives

  samples of the target current of the neu-

  trons, through a sampling circuit

  38 and he uses these samples to perform a comparison

  reason with a reference signal, in order to control the

  regenerator and thus the gas pressure in the

  neutron generator tube. 33. Circuits

  rhythm 42 which include a main rhythm oscillator

  palate operating at a relatively high frequency and an appropriate division chain, provide pulses to

  1 kHz to the ion source pulse generator 41, and

  Also, clock pulses at a rate of one second to the start control circuit 40 of the neutron generator are provided. In addition, the timing circuit 42 applies 2 MHz clock pulses to a microprocessor / data storage array assembly 44, and applies timing pulses to the background circuit 45.

  and counters 52 and 53. Timing signals are applied to

  similar to a pair of control circuits

of gain 48 and 49.

  The interaction of the thermalized neutrons with the nuclei of the materials of the formation of the field causes the emission of capture gamma rays which are detected by the detectors 46 and 47 (corresponding to the two spaced detectors 23 and 24 of FIG. 1). Voltage pulses

  from detectors 46 and 47 are respectively

  to the gain control circuits 48 and 49. The cir-

  Control functions 48 and 49 have the function of maintaining the output pulses of the detectors 46 and 47.

  at a calibrated height with respect to a reference impulse

  of known amplitude. The output signals of the gain control circuits corresponding to the gamma rays detected by the detectors 46 and 47 are respectively applied

  discriminating circuits 50 and 51. Discriminatory circuits

  50 and 51 are intended to prevent low amplitude voltage pulses from the detectors entering the counters 52 and 53. The discriminators

  are typically set at 0.1-0.5 MeV for elimi-

  to reduce the noise generated by photomultiplicity

  associated with detectors 46 and 47. The output signals

  50 and 51 discriminators are applied to

  52 and 53, whose function is to count the events

  corresponding to the capture gamma rays that are detected by the detectors 46 and 47. The signals

  the output of meters 52 and 53 are applied to the

  microprocessor circuit / data storage circuits, 44.

  During the bottom portion of the detection cycle, the bottom circuit 45 receives the counter signals

  from counters 52 and 53. This circuit also applies

  an impulse pulse to the pulse generator.

  ion source ions, 41, to prevent pulse operation of the neutron generator during the game

  of the cycle which corresponds to the counting of the background radiation.

  The background correction circuit 45 applies informa-

  bottom-up count to the microprocessor / cir-

  data storage, 44. Information from

  background may be recorded and subject to a calculation of

  yenne over longer durations than those corresponding to the capture data, because for a low level of discriminator threshold, most of the background radiation comes from the activation of NaI by gamma rays, the latter having a period radioactive 27 minutes. This gives better statistical properties for

  the signal resulting from the subtraction.

  The digital counting information from the counters 52 and 53 and the background correction circuit 45 is applied to the microprocessor / circuitry assembly.

  44. These circuits 44 put the data into

  In a certain format, they are presented in serial form to the telemetry circuit 43 which is used to transmit by telemetry to the surface, by the logging cable 32, the digital information that comes from the meters and the background correction circuit. At the surface, a telemetry interface unit 54 detects the telemetry voltage signals, in analog form, from the drivers of the logging cable 32, and

  applies to a telemetry processing unit 55 which

  this in a certain format the information rate of comp-

  digital stage representing the counting rate that pro-

  comes meters 52 and 53 of the underground equipment, in accordance with the time window configurations

  which have been considered previously. The numbers represent

  the counting rates in each of the six windows.

  time, or more, and the counting rate of

  background are then applied to a computer 56.

  The computer 56 is programmed in accordance with the

  Figure 7, in order to interpret the information

  count rate rate corresponding to six or more time windows and counting of background radiation so as to express the thermal neutron decay time or the thermal neutron lifetime of the sounding and forming components. The signals of

  output representing the interesting parameters of the

  The data are applied by the computer 56 to a film recorder 57 and a magnetic tape recorder 58 for recording as a function of depth in the sounding. Surface computer 56 is programmed according to the flowchart shown in FIG. 7

  in order to extract the components of the decay time

  thermal neutrons r. F and 7 B which are

  field formation and sounding, and to determine intersections B and A in Figure 3 which represent the count rates at the end of the

  puff of neutrons which result respectively from

  of the formation and survey of the thermal neutron population. To accomplish this, we use an iterative identification technique in the sense of the least

  squares which is shown in Figure 7.

  The input information that is introduced into the program shown in Figure 7 includes the information

  count rate Ci, with i = 1 to 6 from each

  one of the six time windows, and a bottom count which is designated by BKG in Fig. 7. In a first control box 61, the count rates from each of the time windows are corrected to account for dead time in the detectors , using a

  the formula in box 61. In addition, the

  backgrounds are corrected for this time

  death. The corrected count rate information and the

  corrected background training are transmitted to a box of

  program control 62 in which the rate of

  background is normalized to account for different durations of time windows 1-6. Background count information is converted to a count rate

  background and subtracted from accounting rate information.

ge in each time window.

  The counting rates corrected for background radiation, Ci, are then applied to a program control box 63, assuming that ti

  corresponds to the middle point of each time window.

  The count rate data Ci, for i = 3 to 6, are then identified in the least squares sense with the expression in box 63, which gives preliminary values of B and ZOF 'as well as the effective value of the difference between the value obtained by least-squares identification and the cadences

  counting, for each value of the count rate.

  The estimates of ti are then transmitted to

  a program control box 64 in which the t.

  corresponding to the center of the time window is corri-

  ge to take into account the slope of the thermal neutron decay time of the formation, r F by means of an expression that is carried in box 64. Similarly, the count rate information Ci for the first two time windows, i = 1, 2, which is

  more affected by the survey component, is cor-

  to account for the effect that is due to the component of the formation ZF, in a program control box 65. The corrected counting rates C'i, with i = 1 to 6, are then transmitted to a box order

  program 66 which calculates a component of the decreasing time

* Thermal neutron relative to the sounding, 'rB and a component of the count rate amplitude of the

  component of the survey, A, in accordance with the expressions

  in the program control box 66.

  The central time coordinate for windows 1 and 2 is then corrected to account for the slope

  of the survey component rB 'in accordance with the

  section 67.

  The control is then transferred to a program control box 68 in which the expression relating to the count rate Ci, with i = 3 to 6, in the time windows 3 to 6 is corrected to take account of the component of the survey, in accordance with the expression

which is shown in box 68.

  The order is then transferred to the

  program control 69 o a test is performed to de-

  end if the iterative process has converged. If the test carried out in box 69 determines that the convergence has not been obtained, an iteration counter is incremented in box 71 and the corrected clocking data C'i, with i = 1 to 6, which are determined at Program Control Boxes 68 and 65, replace the previous count rate data from the last

  iteration, then the program goes back to the control box.

  program 63 for the next iteration of the identifi-

  cation in the least squares sense. When the test of

  box 69 indicates that convergence has been achieved, the

  gram goes to the output box 70 and the computer com

  56 of Figure 2 transmits the results to the registers

  shown in Figure 2.-

  In this way, the device of the invention simultaneously measures the decay time of the neutrons

  of the component relating to the formation of

  rain t F 'and of the component relative to the sounding, Z B' and

  the values of the initial counting cycles B and A cor-

  corresponding to the components of the count rate due to the training and the survey. The invention thus avoids all the limitations of the prior art mentioned above which are due to the assumption that the polling component of the decay time of thermal neutrons is

  much lower than the field training component.

  It goes without saying that many modifications can

  can be made to the described device and process and

  represented, without departing from the scope of the invention.

Claims (10)

  1. Method for simultaneously determining the component of the sounding and the component of the formations   of the decay time of thermal neutrons   probes and field formations in the vicinity of the borehole, characterized in that pulses of relatively short duration of fast neutrons which are rapidly moderated by interaction are repeatedly emitted in a borehole (10). with cores of matter located in the survey and in formations   surrounding areas (20) and which are slowed down   than thermal energy, which creates bouf-   repetitive fairies of thermal neutron population   in the survey and in the field formations around   nantes; one detects, in the sounding, during an interval   time between the repetitive pulses of neu-   very fast, radiation representative of the population   thermal neutrons in the borehole and surrounding field formations, in at least four contiguous and non-overlapping time intervals, following a fast neutron pulse, and generating   at least four count rate signals representing   thermal neutron populations during the four, at least, contiguous and non-overlapping time intervals; detecting, during a separate time interval, between the repetitive fast neutron bursts, radiation attributable to a background radiation component of the radiation present in the sounding and generating a background count signal representative of that component; we correct the four, at least,   count rate signals to take account of the pre-   of the background radiation component in order to   at least four clocking rate signals corri-   to account for background radiation; we combine   the four, at least, count rate signals corri-   to account for background radiation, in accordance with an iterative identification technique that assumes the existence of an exponential relationship for a sounding component and a ground-forming component of the thermal neutron decay time,   in order to simultaneously obtain two measurement signals   the probing component and the field formation component of thermal neutron decay time; and we record these two signals from   measure according to the depth in the survey.   2. Method according to claim 1, characterized in that it further comprises the operation of   correct the four, at least, cadence signals of   step and the background count signal, to account for the duration of a dead time in the detection operation. of radiation.   3. Method according to claim 1, characterized in that at least six contiguous and non-overlapping time slots are used, following each   puff of neutrons, in the detection operation.   4. Method according to claim 3, characterized in that the combination operation involves a least-squares identification technique with an exponential relationship which is supposed to exist   for the components of the survey and the training of   the decay time of thermal neutrons,   in the iterative identification technique. -   5. Method according to claim 4, characterized in that each of the operations is performed at several different depths in a sounding and recording is performed of the two, at least, measurement signals for   each of the depths in the survey.   6. Method according to claim 1, characterized   in that the transmission and detection operations are repeated about 1000 times per second.   7. Method according to claim 1, characterized in that the operation of combining the four, at least, count rate signals, further comprises the operation of simultaneously producing, by the iterative identification technique, the minus two additional measurement signals representative of the initial thermal neutron population as a result of a repetitive neutron pulse in the sounding and in the media   field formations surrounding the survey.   Method according to claim 1, characterized in that the detection operations are triggered approximately 20 to 30 μs after each of the neutron pulses, and the four, at least, contiguous time slots and   not overlapping, extend practically over the whole   time interval up to the impulse   next among repetitive neutron pulses.   9. Process according to claim 8, characterized   in that the four, at least, continuous time intervals   that do not overlap have approximately equal durations.   10. Process according to claim 8, characterized   in that the four, at least continuous time intervals   gus and do not overlap have respective durations that grow successively with respect to the time interval previous.