Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
- I 3~2;6~9
BACXGRO~ND OF THE INVENTION
This invention relate~ to in situ measurements of earth
formations traversed by a well borehole. More particularly,
the invention relates to the measurement of ~he thermal
neutron decay time (or neutron lifetime) of earth formations
in the vicinity of a wellbore.
The observed decay rate of the thermal neutron popu-
lation in the vicinity of a well borehole following a pulse
or burst of high energy neutrons can be approximated by the
sum of formation and borehole exponential terms plus a
background term which can vary according to formation and
borehole conditions. In typical field conditions the bore-
hole component of the thermal neutron lifetime, or decay
time, decays more rapidly than the formation component of
thermal neutron lifetime. The primary parameter of interest
iS TF, the mean lifetime of thermal neutrons within the
formation. Another parameter of interest is TB ~ the mean
lifetime of thermal neutrons in the borehole. The present
invention provides methods and apparatus for determining
both of these parameters of interest simultaneously.
The system and methods of the present invention employ
a pulsed source of fast neutrons. The fast neutrons are
slowed down ~or moderated) to thermal energy rapidly by
interaction with the nuclei of the elements in the borehole,
the earth formations surrounding the borehole, and fluids
contained in the pore spaces of such formations. The thermal
neutron lifetime or decay time of the earth formation is
largely determined by the salt or chlorine content of the
earth formations. The hydrogeneous matter in the pore
spaces and borehole rapidly attenuates or slows down the
fast neutron flux emitted by a source of pulsed fast neutrons.
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The fast neutrons when slowed to thermal energy are said to
be thermalized and may then be captured by the nuclei of
elements comprising the formation matrix and fluids filling
the formation matrix and the materials comprising the well-
bore, including the borehole fluid, logging instrument, and
possibly well casing. The element chlorine, which is found
in highly saline borehole fluids and earth formation fluids
in the pore spaces of earth formations in the vicinity of a
borehole when a high salt (NaCl) content is present, has a
very high capture cross-section for thermalized neutrons.
Thus a measurement of the thermal neutron decay time or
neutron lifetime of earth formations in the vicinity of a
well borehole can be indicative of the amount of saline
fluids present in the pore spaces of the formation. When
combined with formation water salinity, porosity measure-
ments and measurements of formation shaliness, thia results
in a combination which can be used to discriminate oil from
salt water filled pore spaces in the vicinity of a well
borehole.
BRIEF DESCRIPTION OF THE PRIOR ART
Two commercially available services for measuring the
thermal neutron lifetime or thermal neutron decay time of
earth formations in the vicinity of a well borehole are
presently available. Both of these commercial techniques
1 25 employ the assumption that the wellbore materials are of a
significantly higher thermal neutron capture cross section
than the surrounding earth formations. By making this
assumption, then a neutron burst or pulse may be emitted
from a well logging instrument situated in the borehole,
and after a time delay which is sufficient to allow the
thermal neutrons in the well borehole itself to all be
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substantially captured by the wellbore nuclei having a
high capture cross-section, the borehole decay time com-
ponent may be ignored. Then measurements of the rate
of decay of the thermal neutron population in the earth
formations may be measured. These commerical neutron life-
time (or thermal neutron decay time) measurements have
proven to be particularly valuable in evaluating the pro-
ducing potential of earth formations in the vicinity of
cased well boreholes. In both of these presently available
commercial techniques, a well logging instrument which
traverses the wellbore uses a pulsed source of high energy
(14 Mev) neutrons, usually produced in a deuterium-
tritium accelerator tube.
The first commerically available technique, at the
present time is known as the "fixed gate" technique. In
this technique, the neutron source is repetitively pulsed
and for each neutron pulse a cloud of fast neutrons is
injected in a generally spherically symmetric fashion about
the source into the surrounding earth formations., The fast
neutron cloud passes from the well tool through the drilling
mud, wellbore casing, cement between the casing and the
earth formation surrounding the wellbore and into the earth
; formations. In this technique, typically each such pulse of
fast neutrons has approximately a constant intensity and
lasts typically for a time duration of from 20 to 50 microseconds.
The number of thermal neutrons comprising this cloud or
population then decays exponentially due to the capture of
the thermalized neutrons by nuclei in the earth formations
and borehole.
After an initial time period following the neutron
burst (typically about 300-400 microseconds), during which
the resultant capture gamma ray distribution in the bore-
hole, mud and casing is assumed to be substantially dissipated,
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measurements of the number of thermalized neutrons in the
vicinity of the well tool are made during two successive
time intervals or gates, of fixed duration. These two
measurements made during the constant time gates or successive
time intervals can then be used to define an approximately
exponential decay curve for the thermal neutron population
in the earth formation surrounding the borehole.
The assumption is made that enough time has passed
following the neutron burst for essentially all thermalized
neutrons in the vicinity of the wellbore itself to have been
captured by the borehole elemental nuclei. The assumption is
that the borehole component of the thermal neutron decay or
thermal neutron lifetime is generally shorter than the earth
formation component of thermal neutron decay or thermal
lS neutron lifetime. This usually occurs when borehole drill-
ing fluids having a high chlorine or salt water content are
encountered. ~owever, in boreholes containing air, gas,
fresh water or oil this relationship does not always hold.
Accordingly, one particular advantage of the present invention
over this "fixed gate" prior art thermal neutron lifetime
, measuring technique is that no assumption is made as to the
relative thermal neutron decay time characteristics of the
borehole fluid with respect to the thermal neutron decay
time or lifetime characteristics of the earth formations
surrounding the borehole.
The thermal neutron population in the formation in the
vicinity of the borehole is inferentially measured during
the two fixed time gating intervals following each neutron
burst or pulse by measuring the capture gamma rays resulting
from the capture of thermalized neutrons by the nuclei of
materials comprising the earth formations and fluids in the
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pore spaces therein. The two time intervals or gates most
frequently used, for example, in the fixed gate technique
for measuring thermal neutron decay times can occur between
400-600 microseconds following the neutron burst and between
700-900 microseconds following the neutron burst. These
values are used in typical earth formations regardless of
the salinity of the fluid present in the borehole.
Since these fixed time gates are designed for general bore-
hole use regardless of salinity they are not optimized as
to maximizing count rate. Because the gates are delayed
for a relatively long time after the burst, the count rate
during the gates is lower than optimum in saline fluid fill-
ed boreholes. This can lead to statistical uncertainty in
the measurement of ~.
If neutron diffusion effects are ignored, the relation-
ship for the decay of a thermal neutron population in a
homogeneous medium having a thermal neutron macrosaopic
capture cross-section can be expressed as in Equation 1.
-~(vt) ~1)
wherein Nl is the number of thermal neutrons at a first
point in time tl; N2 is the number of thermal neutrons
; present at a later point in t2; e is ~he Naperian logarithim
base; t is the time between two measurements (t2-ti); and v
is the velocity of the thermal neutrons. The macroscopic
thermal-neutron capture cross section ~ of a reservoir rock
(which can be obtained from Equation 1) is dependent upon
its porosity, matrix composition, shalineg~, the formation
water salinity, and the quantity and type of petroleum
contained in the pore spaces therein. This quantity thus
represents a valuable physical parameter or measurement of
the formation to be obtained.
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The second presently commercially available prior art
technique for measuring thermal neutron decay time or thermal
neutron lifetime uses a reclprocal relationship of the
macroscopic thermal neutron capture cross-section ~ which
is defined in terms of T the time constant for absorption of
the thermal neutrons. A relationship analagous to Equation
1 but defined in terms of T iS given by :
N = NOe / (2)
_ where T = l/v~. In Equation 2, N represents the thermal
neutron density at any time t; No is the thermal neutron
density at an initial time, to; e again represents the
Naperian logarithm base constant; and ~ is the time required
for the thermal neutron population to decay to l/e of its
value at to.
In measuring the thermal neutron decay time using the
6econd prior art technique known as the Hsliding gate"
arrangement, the well logging instrument emits a pulse or
burst of fast neutrons into the formation the duration of
which is actually controlled and related to previously
measured values of ~ of the earth formations. For example,
the neutron pulse duration may be one I duration. Gamma
ray aetectors are used to obtain counts of capture gamma
- rays during.two successive time intervals following the
generation of the neutron cloud in the vicinity of the ~ell
borehole in order to define the exponential decay curve. In
this technique, however, the two intervals used for measuring
the gamma ray population to define the exponential decay
curve are not fixed in duration or in starting time following
the neutron burst. The value of ~ previously measured on
the pr.evious neutron burst cycle is used to establish the
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neutron burst duration for the generation of the fast neutrons
as well as the waiting interval to the opening of the first
time gate following the burst, the duration of the first
time gate, the duration of the second time gate and the
- 5 waiting interval between the initiation of the first and
second time gates. All of these times are adjusted until
a predetermined relationship to T iS satisfied. For
example, the second measurement gate duration may be two T
in duration. A waiting interval of two T following the
neutron burst maybe used before the opening of the first
gate. The first gate may have a duration of one T .
In both of the above described prior art systems for
determining thermal neutron lifetime or decay time, the
neutron source and one detector are all that is essential
for the measurement. However, in both of the commercially
available techniques, dual spaced detectors are employed and
measurements at the detectors of the capture gamma radiation
due to thermal neutrons are used to generate approximations
or measurements of the porosity of the earth formations in
the vicinity of the borehole. The system of the present
invention also employs two detectors and can make porosity
measurements.
As previously discussed, both of the commercially
available techniques for measuring thermal neutron decay
time at present employ the assumption that the borehole
thermal neutron decay time is substantially less than that
of the earth formations in the vicinity of the borehole and
may thus be discriminated against by "timing out" the bore-
hole component. In the sliding gate techniques at a time
interval substantially following both of the detection gates
used for ~ or ~ measurements, a background time gate can be used to
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measure the background of gamma radiation due to thermal
neutron capture events in the borehole and earth formations
surrounding the wellbore. These background counts are,
after appropriate normalization, generally subtracted from
the counts made during the two measurement gates in such a
system so as to remove the influence of natural gamma ray
background which occurs in the vicinity of the well borehole
and any background which may be induced within the gamma ray
detectors and formation by the neutron source. It should be
noted that both of the previously described commercial well
logging system~ do not use all of the possibly available
gamma ray count information following each burst of neutrons.
The time intervals during which the detectors are not gated
to accept information are lost in both these prior art
systems. Thus, the full utilization of the neutron output
from the neutron generator is not made in the prior art
schemes. Similarly, both of the prior art techniques
assume that the formation thermal neutron lifetime or thermal
neutron decay time may be essentially completely separated
from that of the borehole component by time gating. Even
under ideal conditions, this assumption is not completely
valid. $he present invention utilizes techniques and~systems
whlch avoid each of these prior art assumptions and limita-
tions.
- BRIEF DESCRIPTION OF THE INVENTION
In the present invention a well logging tool is moved
through the borehole and includes a pulsed source of fast
neutrons and two radiation detectors. The neutron source
generates a pulse of fast neutrons of approximately constant
intensity for a duration of between 10 and 30 microseconds.
These neutrons are introduced into the media comprising the
well borehole and surrounding formations and result in a
thermal neutron population being generated from the slowing
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down of the fast neutrons in the earth formation media and the
borehole. After a very short pause to allow moderation of the
fast neutrons ollowing the neutron pulse, the detectors are
gated on and the capture gamma radiation resulting from the
capture of thermal neutrons in the borehole and earth forma-
tions in the vicinity of the borehole are recorded essentially
continuously until the next neutron burst is about to begin.
During multiple time gates which occur during this essentially
continuous interval, the capture gamma ray counting rate is
observed in six or more essentially contiguous time gates.
The multiple time gate measurements of the counting rates
are supplied to a thermal neutron lifetime computer which
computes the formation and borehole neutro~ lifetime components
by means of least squares fitting of this count rate data
lS taken during six, or more, essentially contiguous time gates
following each neutron burst. The thermal neutron lifetime
computer is enabled to calculate both the borehole thermal
neutron lifetime component and the earth formation thermal
neutron lifetime component, simultaneously. Approximately
i 20 once per second, and for approximately five percent of the
one second operating cycle, the neutron source is turned off
and the detectors are used to establish any relatively long
lived background counting rate due to source neutron induced
gamma ray activity within the gamma ray detector, the
formation, borehole, logging sonde, or natural gamma radiation
in the vicinity of the borehole. This background gamma ray
information is then properly normalized and subtracted from
the six or more time gate measurements of thermal neutron
capture gamma rays made following each neutron burst.
Electronic systems are provided in the downhole tool
and at the ~urface for producing the measurement sequence
and neutron pulses as described. Additionally, synchronization
_ g _
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or sync pulses are also generated to provide a means for
separating the counts of gamma rays representative of
thermal neutron capture during each of the six or more
gating portions of the measurement cycle, as previously
S described. Moreover, a surface computer for deriving the
thermal neutron decay times, or lifetimes, of the borehole
component and earth formation components is provided and is
attached to a well logging recorder in which a record medium
may be moved as a function of borehole depth, while the
logging instrument is moved through the borehole. The
formation and borehole components of thermal neutron lifetime
may be plotted as a function of borehole depth on this
recorder. Thus, the system of the present invention includes
, techniques for determining the value of thermal neutron
decay or macroscopic thermal neutron capture cross-section
of the borehole and the surrounding media simultaneously.
The invention is best understood by reference to the
f following detailed description thereof when taken in con-
junction with the accompaning drawings, in which:
BRIEF DESCRIPTION OF DRAWINGS
Figure l is a schematic drawing showing a well logging
system in accordance with the present invention.
~, Figure 2 is a schematic block diagram depicting the
electronic systems of the well logging system of the present
invention.
Figure 3 is a graphical relationship illustrative the
composite thermal neutron population decay curve and time
gates according to one embodiment of the present invention.
Figure 4 is a schematic graphical illustrating a tele-
metry sequence as a function of time in the present invention.
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Figure 5 is a graphical relationship illustrating the
composite thermal neutron population decay curve and time
gates according to a second embodiment of the present invention.
Figure 6 is a schematic representation of a telemetry
sequence as a function of time for the gating arrangement of
Figure 5.
Figure 7 is a flow chart diagram illustrating an embodi-
ment of one method for obtaining parameters of interest of
earth formations by a surface computer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The previously discussed prior art techniques for
determining thermal neutron lifetime or decay time can en-
counter two major problems. These two major problems are:
(1) Under certain formation and borehole conditions, the
borehole component has not decayed to a negligible level
prior to the beginning of the gating of detectors sequence
for determining the neutron lifetime. This results in an
erroneous measurement of TF, and; (2) The statistical accuracy
of TF is sometimes quite poor because the decay rate samples
have to be taken at relatively long intervals of time after
the neutron burst in order to minimize the effects of the
' borehole component.
A third problem in prior art neutron lifetime logging
techniques was firs~t discussed by Mills, et al in a paper
entitled "Pulsed Neutron Experiments in a Borehole Model",
Mills, et al in Nuclear Science and Engineering, Vol. 21,
Pages 346-356 (1965). The Mills, et al paper shows that even
if TF is computed from count rate data taken at time delays
sufficient for the borehole component to decay to a negligible
level, that the computed TF is still a function of TB,
the lifetime of thermal neutrons within the borehole. This
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may be thought of as being due to thermalized neutrons con-
tinuously diffusing back into the borehole from the formation
even after the "original" borehole thermal neutron population
has decayed by capture to a low level. Thus, the two prior
art techniques completely dismiss the effect caused by this
third problem. The present invention however takes into
account the problems of all three effects and results in a
much more reliable measurement of the thermal neutron lifetime
or decay time than heretofore has been available.
In order to obtain accurate hydrocarbon saturations from
pulsed neutron lifetime or decay time logs. The following
three criteria should be met:
~ ) TF, the observed lifetime of the formation component,
i should be computed from count rate data which contains no
contributions from neutron capture within the borehole;
(2) TF should be statistically as accurate as possible;
and
(3) The intrinsic mean lifetime of the formation
component TFi should be determined before hydrocarbon satura-
tion calculations are made.
According to the previously mentioned Mills, et al paper,
the measured lifetime can be related to TFi, the intrinsic
lifetime only if TB iS known. It is therefore, desirable to
measure both the formation component TF and the borehole
component TB of the thermal neutron lifetime or decay time
simultaneously for maximum accuracy.
As previously discussed, the observed decay rate of the
thermal neutron population in the vicinity of a well borehole
~ following a burst of high energy neutrons may be described as
: 30 the sum of a formation component, a borehole component and a
,
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background component. This may be expressed mathematically as
in Equation 3.
C(t) = Ae t/TB ~ Be t/TF + CB (3)
In Equation 3, C(t) is the counting rate at any time t measured
from a reference time. A and B are constants which may be
interpreted according to Fig. 3 of the drawings with A re-
presenting the initial borehole component at the reference
time = O, and B representing the initial formation component
at the reference time = O. These components ar~ shown in
Fig. 3 as intercepts on the ordinate axis as a function of
time. TB in Equation 3 represents the borehole component of
composite lifetime of thermal neutrons. TB may be thought
of as the slope of the borehole component curve of Fig. 3.
, Similarly, TF represents the formation lifetime component of
the composite neutron lifetime and may be thought of as the
slope of the formation component curve of Fig. 3. Finally,
CB represents the component of the count rate due to the
long lived radiation and may be thought of as a constant
component as shown by the horizontal line labelled back-
ground in Fig. 3. The composite thermal neutron decay curve
shown in Fig. 3 is the resultant or summation of the bore-
hole component, formation component and background component
curves illustrated therein.
In the technique of the present invention, the back-
ground component CB is measured during a separate portion of
the operating cycle as illustrated in Fig. 4. Referring now
to ~ig. 4, a telemetry stream from a downhole instrument
which will be described in more detail subsequently is shown
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as a function of time. A synchronization pulse begins each
operating cycle of the downhole instrumentation. This
synchronization pulse is followed immediately by a neutron
bur~t of approximately constant intensity and having a
duration which will be described in more detail suhsequently.
Six or more time gating intervals follow each neutron burst
during which count rate measurements at a detector spaced
from the source are made and transmitted to the surface.
The multiple time gating intervals are essentially contiguous
and last for a total of approximately 1 millisecond follow-
ing the synchronization pulse. This repetitive operating
cycle is repeated approximately 1000 times during a one
second interval. At the end of a 945 millisecond interval,
a background gate shown in Fig. 4 is used to count background
lS radiation corresponding to CB in Fig. 3. During this 55,000
microseconds or 55 milliseconds interval, the neutron generator
is not pulsed. Therefore, the measurements made during this
time interval, after approximately 5 milliseconds to allow
thermal capture radiation following the last sequential
neutron burst to develop to a negligible level, will contain
only count information due to radiation attributable to
background. This background count information is telemetered
to the surface by the downhole system and processed as will
! be described in more detail subsequently.
When the background counting rate CB is measured in
the manner described and telemetered to the surface, it may
be subtracted from the composite counting rate C(t~ of
Equation 3 to obtain a net counting rate C'(t) as given in
' Equation 4.
~ 30 C'(t) = C(t)-CB = Ae t/TB + Be t/TF (4)
- In Equation 4, the symbols are all as previously defined.
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In the method of the present invention, as illustrated
with respect to Figures 3 and 4, six (or more) counting
rates measured in the six time gates following the neutron
burst and labelled Tl, T2, T3, T4, T5, and T6 are aombined
by a least-squares fitting technique. The counting rate
measurements in the six time gates may be fitted in real
time in a surface computer, for example, in order to obtain
the parameters of interest in Equations 3 and 4. The
fitting procedures yield ~F~ IB' A and B as previously
defined. It will be observed that the six (or more) approxi-
mately contiguous time gating intervals illustrated in
Figure 3, have negligible or minimal time delay between each
gate. Therefore, the full counting rate information (following
a short moderation time interval) from the end of the neutron
burst to the opening of the first time gate which moderation
time is typically of the order of 20-30 microseconds is
utilized in the method of the present invention. No count
; information is lost due to waiting for a borehole component
to decay. Additionally, since this technique simultaneously --
determines TF and ~B~ the criterion of the Mills, et al
paper previously referenced is met.
Referring now to Figures 5 and 6, an alternative time
gating scheme which employs the techniques of the present
invention is illustrated schematically. In Figure 5, a
neutron burst of 15 to 20 microseconds duration is shown, a
20-30 microsecond moderation time interval follows the burst
and then a time gate labeIled gate 1 is opened for a relatively
short duration of time. A slightly wider or longer duration
time gate 2 is used. Subsequent time gates 3, 4, 5 and 6 are
each of longer duration than their predecessor in the time
gating sequence. The aim of this time gating scheme is to
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statistically optimize the counting rates in each of the
gates. As the composite thermal neutron population decay
curve falls off, the successively wider time gates allow
more counts to occur at the lower counting rate of the later
time gates. ~he actual times contemplated for time gating
schemes shown in Figures 3 and 5 are given in Tables l and 2
which follows: (In Tables 1 and 2 all times are measured
with respect to the reference time = O at the beginning of
the neutro burst).
=.
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TABLE 1
TIME GATING SCHEME OF FIGURE 3
.
Gate No. Start Time Stop Time Duration
150~ sec. 195~ sec. 14S~ sec.
2200~ sec 345~ sec. 145~ sec.
~ 3350~ sec. 495~ sec. 145~ sec.
; 4500~ sec. 645~ sec. 145~ sec.
5650~ sec. 795~ sec. 145~ sec.
6800~ sec. 945~ sec. 145~ sec.
TABLE 2
TIME GATING SCHEME OF FIGURE 5
Gate No. Start Time Stop Time Duration
1 60~ sec. 90~ sec.30~ sec.
2 90~ sec. 140~ sec.50~ sec.
3 140~ sec. 200~ sec.60~ sec.
4 200~ sec. 300~ sec.100~ sec.
5 300~ sec. 500~ sec.200~ sec.
6 500~ sec. 998~ sec.448~ sec.
The slight time lapses (5~ sec.) between the time gates
of Table 1 are supplied to account for time necessary to
shift the contents of a counter into a memory buffer in the
downhole tool electronics to be described subsequently.
Similar short intervals would be required for the time gates
of Table 2 but are omitted from the table for simplicity. It
will be understood that it is meant to have the time gates
of Tables 1 and 2 as nearly contiguous in time as is possible
within the timing limitations of the electronics.
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Figure 6 illustrates a telemetry stream resulting from
the time gating arrangement illustrated in Figure 5. A
synchronization pulse is sent to the surface by the downhole
electronics. This is followed by the neutron burst and the
reference time begins at the ending of the neutron burst.
The short 10-30 microsecond moderation time interval elapses
and then a digital number representing the counts made in
time gate 1, labelled Gl in Figure 6, are telemetered to the
surface. Similarly, digital numbers representing the counts
in gates 2-6. This sequence is followed for 945 milliseconds.
Then the background gating interval of 50 milliseconds is
initiated as illustrated previously with respect to Figure
4. In either event, the counting rates C(ti) i=1-6 are
telemetered to the surface where they are employed in a
surface computer ~to be described in more detail subsequently)
to employ a least-square fitting technique for extracting
the parameters of interest.
Since Equation 4 is non-linear it is necessary to use an
iterative fitting procedure for the least-squares fit. A
particular fitting procedure i8 illustrated subsequently and
will be described in more detail with respect to Figure 7. It
will suffice to say, however, at this point that the para-
meters of interest are obtained from the surface computer by a
; least-square iterative fitting procedure. The values of
25 ~ ~F~ ~B~ A, and B may then be recorded as a function of bore-
hole depth in a conventional manner.
, Referring now to Figure 1, a well logging system in
, accordance with the concepts of the present invention is
,; illustrated schematically. A well borehole 10 is filled with
3~ a borehole fluid 11 and penetrates earth formations 20 to be
investigated. A downhole well logging sonde 12 is suspended
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in the borehole 10 via a conventional armored logging cable
13, in a manner known in the art, such that the sonde 12
maybe raised and lowered through the borehole as desired.
The well logging cable 13 passes over a sheave wheel 14 at
the surface. The sheave wheel 14 is electrically or mechani-
cally coupled, as indicated by dotted line 15, to a well
logging recorder 18 which may comprise an optical recorder,
or magnetic tape, or both, as known in the art. The record
of measurements made by the downhole sonde 12, may thus be
recorded as a function of the depth in the borehole of the
sonde 12.
; In the downhole sonde 12, a neutron generator 21 is
supplied with high voltage (approximately 100 kilovolts) for
its operation by a high voltage power supply 22. Control and
telemetry electronics 25 are utilized to supply control
signals to the high voltage supply and the neutron generator
21 and to telemeter information measured by the downhole
instrument to the surface via the logging cable 13.
~ Longitudinally spaced from the neutron generator 21 are
i 20 two radiation detectors 23 and 24. Radiations detectors 23
and 24 may comprise, for example, thallium activated sodium
iodide crystals which are optically coupled to photomultiplier
tubes. The detectors 23 and 24 serve to detect gamma radiation
produced in the surrounding formations 20 resulting from the
action of the neutron generator 21 in emitting neutrons. A
neutron shielding material 28 having a high density matter
content or large scattering cross-section is interposed
between the neutron generator 21 and the dual spaced detectors
23 and 24, in order to prevent direct irradiation of the
3a detectors by neutrons emitted by the neutron generator 21.
Shielding 29 may also be interposed between the detectors 23
and 24 if desired.
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Upon activation of the neutron generator 21, a burst,
or pulse, of neutron~ of approximately 10-30 microseconds
duration is lnitiated and is emitted into the well borehole
10, borehole fluid 11 and through the steel casing 26 and
cement layer 27 surrounding the steel casing into earth
formations 20 being investigated. The neutron burst is
moderated or slowed down by scattering interactions such
that the neutrons, are all essentially at thermal energy.
The thermalized or thermal neutrons then begin capture
interactions with the elemental nuclei of constitutents of
the earth formations 20 and pore spaces contained therein.
The capture of neutrons by nuclei of elements comprising
the earth formations 20 and their pore spaces produce capture
gamma rays which are emitted and impinge upon detectors 23 and
24. A voltage pulse is produced from the photomultipliers of
detectors 23 and 24 for each gamma ray so detected. These
voltage pulses are supplied to the electronics section 25,
counted in a digital counter, and are telemetered to the
surface via a conductor 16 of the well logging cable 13. At
the surface, a surface electronics package 17 detects the
telemetered information from the down hole sonde 12 and
performs the least-squares fitting technique to determine
the TF~ ~B~ A and B with respect to the earth formations 20
being investigated. ~he surface electronics then supplies
signals representative of the measured quantities to the
recorder 18 where they are recorded as a function of bore-
hole depth.
Referring now to Figure 2, a schematic block diagram
illustrating the electronic portions of the subsurface and
surface electronic systems are illustrated in more detail but
still schematically. Power for operation of the subsurface
electronics is supplied via a conductor of the well logging
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cable 32 to a conventional low voltage power supply 31 and a
high voltage power supply 34. The high voltage power supply
34 may be of the Cockcro~t Walton multiple stage type and
supplies approximately, 100 kilovolts for the operation of the
s neutron generator tube 33. The neutron generator tube 33 is
of the deuterium-tritium accelerator type. An ion source 36
which is maintained at a potential near ground is used to
generate deuterium-ions from deuterium gas filling the
envelope of tube 33. A replenisher heater 37 is impregnated
with additional deuterium and maintains a pressure lavel of
. .
l ~ deuterium gas inside the tube 33 envelope sufficient to supply
t
f ion source 36 wi~th deuterium gas for ionization. A target 35
~is impregnated with tritium and is maLntained at a relatively
~ high negative L00 kilovolts potential. The ion source is
controlled by an ion source pulser 41. When supplied with a
relatively low level voItage pulse, the ion source causes gas
'~ in the tube 33 envelope to become ionized and accelerated
,~ toward the target material 35. Upon impinging on the target
i
material of target 35, the deuterium ions interact thermo-
l~ 20 nuclearly with the tritium ions in the target to produce
l neutrons, which then are emitted in a generally spherically
~1 :
~'~ symmetrical fashion from the neutron generator tube 33 into
':
the borehole and surrounding earth formations.
~' A replenisher control circuit 39 is supplied with samples
~, ::
of the neutron generator target current by a sampling circuit
38 and utilizes this to compare with a reference signal to
' control the replenisher current and thereby the gas pressure
! ~ in the envelope of the neutron generator tube 33. Timing
j~ circuits 42 which comprise a master timing oscillator operating
at a relatively high frequency and an appropriate divider
chain, supplies 1 kilo hertz pulses to the ion source pulser
f
2 1--
~,:
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41, and also supplies one second clock pulses to the neutron
generator start-up control circuit 40. Moreover, timing
circuit 42 supplies 2 mega hertz clock pulses to a microprocessor
and data storage array 44 and supplies timing pulses to the
background circuit 45 and counters 52 and 53. Similarly,
timing signals are supplied to a pair of gain control circuits
48 and 49.
The interaction of thermalized neutrons with nuclei of
earth formation materials causes the emission of capture gamma
rays which are detected by detectors 46 and 47 (corresponding
to the dual spaced detectors 23 and 24 of ~igure 1). Voltage
pulses from the detectors 46 and 47 are supplied to gain
control circuits 48 and 49 respectively. Gain control cir-
; cuits 48 and 49 serve to maintain the pulse height output of
detectors 46 and 47 in a calibrated manner with respect to a
known amplitude reference pulse. Output signals from the gain
control circuits corresponding to gamma rays detected by
detectors 46 and 47 are supplied to discriminator circuits
50 and 51 respectively. The discriminator circuits 50 and
51 serve to prevent low amplitude voltage pulses from the
detectors from entering counters 52 and 53. Typically, the
discriminators are set at about 0.1-0.5 Mev to eliminate
noise generated by the photomultiplier tubes associated with
detectors 46 and 47. The discriminator 50 and 51 outputs
are supplied to counters 52 and 53 which serve to count
individual capture gamma ray events detected by the detectors
46 and 47. Outputs from the counters 52 and 53 are supplied
to the microprocessor and data storage circuits 44.
During the background portion of the detection cycle the
background circuit 45 is supplied with counts from the counters
52 and 53. This circuit also provides a disable pulse to the
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ion source 41 to prevent pulsing of the neutron generator
during the background counting portion of the cycle. The
background correction circuit 45 supplies background count
information to microprocessor and data storage 44. Back-
ground may be stored and averaged for longer periods than
capture data since at low discriminator threshold, most back-
ground is from gamma ray tNaI,) activation which has 27 minutes
half life. Better statistics in substracted signal results.
The digital count information from counters 52 and 53 and
background correction circuit 45 are supplied to the micro-
processor and data storage circuit 44. These circuits 44
format the data and present it in a serial manner ~o the
, telemetry circuit 43 which is used to telemeter the digital
information from the counters and background correction
lS circuit to the surface via well logging cable 32. At the
surface, a telemetry interface unit 54 detects the analog
telemetry voltage signals from the logging cable 32 conductors
and supplies them to a telemetry processing unit 55 which
formats the digital count rate information representing the
counting rate from counters 52 and 53 in the subsurface
'~ equipment in terms of the time gating schemes as previously
i discussed. The digital numbers representative of the count
'~ rates in each of the six or more time gates and the back-
ground counting rate are then supplied to a digital computer
56.
The computer 56 is programmed in accordance with the
flow chart of Figure 7 to interpret the six or more time
gates and background counting rate information in terms of
the thermal neutron decay or thermal neutron lifetime of the
borehole and formation components. Output signals representing
formation parameters of interest are supplied from the
computer 56 to a film recorder 57 and a magnetic tape recorder
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1 ~ 62659
58 for recording as a function of borehole depth. ~he
surface computer 56 is programmed in accordance with the
flow chart illustrated in Figure 7 to extract the earth
formation and borehole components of thermal neutron decay
time ~F and ~B and the intercepts B and A of Figure 3 which
represent the counting rates at the end of the neutron burst
due to the formation and borehole components of thermal
neutron population respectively. In order to accomplish
this an iterative least squares fitting scheme is illustrated
in Figure 7 is utilized.
Input information to the program illustrated in Figure
7 comprises counting rate information Ci, i = 1,6 from each
of the six time gates and a background count labelled BKG in
Fig. 7. At a first control block 61, the counting rates
from each of the time gates are corrected for dead time in
the detectors by a formula illustrated in block 61. Additionally,
the background counts are corrected for this dead time. The
corrected count rate information and background information
are supplied to program control block 62 where the background
count rate is normalized to take into account the different
durations of the time gates 1-6. The background count is
converted to background count rate and subtracted from the
count rate information in each time gate.
The corrected for background count rates, Ci,, are then
supplied to a program control block 63 assuming ti to be the
mid point of each time gate. The count rate data Ci, for
i z 3-6, is then least-squares fitted to the expression
given in block 63, resulting in preliminary values of B,
TF and the RMS derivation from the least-squares fit to the
count rates for each value of count rate.
The esti~ates of ti are then supplied to a program
control block 64 in which the ti corresponding to the center
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of the time gate is corrected for the slope of the formation
thermal neutron decay time TF via an expression illustrated in
block 64. Slmilarly, the count rate information Ci for the
first two time gates i=1,2 which is more effected by the
borehole component is corrected for the effect due to the
formation component TF in a program control block 65.
The corrected counting rates C'i,i-1,6 are then supplied
to a program control block 66 which calculates a borehole
component of thermal neutron decay time TB and borehole
component count rate amplitude component A, according to
expressions given in program control block 66.
The center time coordinate for gates 1 and 2 is then
corrected for the slope of the borehole component TB
according to the expression given in block 67.
~ .
Control is then transferred to a program control block
68 where the expression for the counting rate Ci, i=3-6 in
time gates 3-6 are corrected for borehole component according
to the expression given in block 68.
Control is then transferred to program control block 69
where a test is performed to determine if the iterative
. .
process has converged. If convergance has not been achieved
; ~ as determined by the test at block 69, then an iteration
counter is incremented at block 71 and the corrected count
rate data C'i, i=1,6 as determined in program control block 68
25~ and 65 is substituted for the previous count rate data from
the last iteration and the program loops backs to program
control block 63 for the next iteration of least-squares
fittlng. When convergance has been achieved as determined by
the test at block 69, then output block 70 is entered and the
results output to the recorders illustrated in Figure 2 from
the control computer S6 of Fig. 2.
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1 ~ 62659
In this manner, the system of the present invention
simultaneously measures the thermal neutron decay time of
the earth formation component ~F' the borehole component IB
and the initial count rates amplitudes B and A due to formation
and borehole components of count rate. All of the previously
mentioned limitations of the prior art made by assuming the
; borehole component of thermal neutron decay time to be much
smaller than the formation component in prior art techniques
are thereby avoided.
The foregoing descriptions may make other alternative
embodiments of the present invention apparent to those skilled
in the art. It is therefore, the aim of the appended claims
to cover all such changes and modifications as fail within the
true spirit and scope of the invention.
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