Note: Descriptions are shown in the official language in which they were submitted.
:~Z36934
METHOD FOR QUALITY CONTROL OF COURTING LOGS
Background of the Invention
m e present invention relates to hydrocarbon well
logging systems and methods; more particularly, it relates
to a computer-based system and method for acquisition
presentation, processing, and recording of nuclear
hydrocarbon well logging data.
Well logging systems have been utilized in
hydrocarbon exploration for many years. Such systems
provide data for use by geologists and petroleum engineers
in making many determinations pertinent to hydrocarbon
exploration. In particular, the systems provide data for
subsurface structural mapping, defining the lithology of
subsurface formations, identifying hydrocarbon productive
zones, and interpreting reservoir characteristics and
contents. Mbny types of well logging systems exist which
measure different formation parameters such as conductivity,
travel time of acoustic waves within the formation and the
like.
Still another class of systems seeks to measure
incidence of nuclear particles on the well logging tool from
the formation for purposes well kncwn in the art. These
systems take various forms, including those measuring
natural gamma rays the formation. Still other systems
measure gamma rays in the formation caused by bursts of
neutrons into the formation by a neutron source carried by
the tool and pulsed at a preselected interval.
In these nuclear well logging systems, reliance is
made upon the physical phenomenon that the magnitude of
gamma rays given off by a nucleus resulting from natural
radioactive decay or induced nuclear radiation is indicative
of the presence of certain elements within the formation.
In other words, formation elements will react in predictable
ways, for example, when high energy neutrons on the order of
14.2 MeV collide with the elements' nuclei. Different
elements in the formation may thus be identified from
characteristic gamma ray energy levels released as a result
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of this neutron bombardment. Thus, the number of gamma rays
at each energy level will be functionally related to the
quantity of each element present in the formation suah as
the element carbon which is present in hydrocarbons. The
presence of gamma rays at a 2.2 MeV energy level may for
example, indicate the presence of hydrogen, whereas
predominance of gamma rays having energy levels of 4.43 and
6.13 MeV, for example, may indicate the presence of carbon
or oxygen.
Also, in these nuclear well logging systems, it is
frequently useful to obtain data regarding the time spectral
distributions of the occurrence of the gamma rays. Such
data can yield extremely valuable information about the
formation, such as identification of lithologies which are
potentially hydrocarbon producing. Moreover, this desired
spectral data may not only be limited to that of natural
gamma rays for example, but also may be desired for the
gamma ray spectra caused by bombardment of the formation
with the aforementioned pulse neutron sources.
Prior art well logging systems for conducting time
spectral analysis of nuclear particles have conventionally
included a subsurface well logging instrument to traverse a
well borehole. The instrument typically includes a gamma
spectrometer including a thallium activated sodium iodide
crystal optically coupled to a photomultiplier tube. A high
voltage supply accelerates deuterons into a tritium target,
generating a large number of 14.2 MeV neutrons, this pulsed
neutron source being activated at repetition rates of 1,000
bursts per second. Subsequent gamma radiation from the
formation incident upon and detected by this high resolution
scintillation crystal generates a pulse of light which in
turn causes the photomultiplier tube to generate electrical
pulses each proportional to the gamma ray energy causing the
pulse. The scintillation spectrometer, comprised of the
detector-photomultiplier tube, is maintained at a low
temperature in thermal isolation in a Dewar-type flask.
~69;~4
As the photonultiplier tube generates these
electrical signals, a downhole electronic amplifier provides
voltage amplification and transmits the detector v~l~age
pulse signals in analog form uphole on a single or multi-
conductor logging cable to surface instrumentation foranalysis and storage. At the surface, this pulsed
information is tiffed and routed to an analyæer system
for deriving the desired time spectra. The surface
analyzer provides a total pulse count and selects pulses
within prescribed time windows for separate counting. In
one variation on the aforementioned systems, rather than
sending the actual analog vDltage pulses to the surface from
the dcwnhole spectrometer, in some instances systems are
provided wherein each pulse is first digitized downhole, and
the digitized value of each pulse is transmitted to the
surface for analysis.
Wèll logging systems for measuring neutron
absorption in a formation use a pulsed ncutorn source
providing bursts of very fast, high-energy neutrons.
Pulsing the neutron source pernits the measurement of the
macroscopic thermal neutron absorption capture cross-section
of a formation. The capture cross-section of a reservoir
rock is indicative of the porosity, formation water
salinity, and the quantity and type of hydrocarbons
contained in the pore spaces.
Neutrons leaving the pulsed sosurce interact with
the surrounding environment and are slowed down. In a well
logging environment, collisions between the neutrons and the
surrounding fluid and formation atoms act to slcw these
neutrons. Such collisions may Lmpart sufficient energy to
these atoms to leave them in an exited state, from which
after a short time gamma rays are emitted as the atom
returns to a stable state. Such emitted gamma rays are
labeled inelastic gamma rays. As these neutrons are slowed
to the thermal state, they may be captured by atoms in the
surrounding matter. Atoms capturing such neutrons are also
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caused to be in an exicted state, and after a short time
gamma rays are emitted as the atom returns to a stable
state. These emitted gamma rays are labeled capture mamma
rays.
The number of capture gamma rays present at any
time is directly proportioned to the number of thermal
neutrons, i.e., the thermal neutron population. The decay
rate of this neutron population is an exponential function,
and is defined by specifying the time required for the
thermal neutron population to decrease to one-half. m is
time is referred to as a neutron "half-lifetime." While it
is actually the neutron lifetime that is measured, the more
useful parameter is the capture cross-section. Capture
cross-section and neutron lifetime are inversely related,
with capture cross-section being a measure of the rate at
which thern21 neutrons are captured in the formation.
Analysis of formation in this manner is referred to as
"neutron decay analysis."
The measurement of neutron population decay rate
is made cyclically. The neutron source is pulsed for 40-60
microseconds to create a neutron population. Since neutron
population decay is a time-related function, only two tome
referenced gamma ray count measurements are necessary. The
capture gamma rays are normally detected from time intervals
that are 400-600 microseconds and 700-900 microseconds
after each neutron burst. As the neutron source is pulsed
and the measurements made, the subsurface well logging
instrument is continuously pulled up the borehole.
The recorded log consists of four curves or tracks
on a plotter. The capture gamma rays measured during the
first measurement time period are recorded on one track.
The capkure gamma rays measured during the second
measurement time period are recorded on a second track. On
the third and fourth tracks, there are recorded a monitor of
the neutron source output and the calculated capture cross-
section. Capture cross-section is continuously calculated
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from the measurements made during the two measurement time periods.
Along with the thermal neutron log, &n epithermal neutron
log may be simultaneously reoorded. Also, casing collars may be
recorded.
Detailed discussion of ~uoh a digital well logging system as
well as general theoretical background as to su¢h logging operation
may be found in U. S. Patent Nos. 3,379,882 and 3,379,884 which issued
to Arthur H. Youmans and each of which i8 assigned to the assignee of
the present invention.
qhe prior art nuclear well logging systems just described,
though proving to be a very valuable tool in oil and gas exploration,
have suffered from numerous deficiencies. First, with respect to the
analog systems which transmitted analog voltage pulses form the
downhole spectrometer to the surface corresponding to each detected
gamma ray, serious problems were encountered in pulse distortion and
degradation due to limited band width on the conventional logging
cables. Even with the previously described systems incorporating
downhole digitization of each spectrometer pulse in an effort to avoid
this pulse distortion, the system atill transmitted the digital values
for each pulse uphole, resulting in extremely slow system throughput.
Due to the downhole instrumentation constraints of high temperature
environments, low power availability, logging tool size constraints,
and low signal-to-noise ratios, the approach of deriving downhole
spectra was largely thought to be impractical if not impossible.
Nevertheless, a well logging system and particularly a nuclear well
logging system was highly desired which not only solved the pulse
distortion and throughput problems, but provided better logging cable
utilization which did not require the dedication of logging cable
conductor time to sending the actual parameter values for each
detected gamma
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ray pulse. It was further highly desirable to provide a
nuclear well logging system wit improved resolution,
statistical accuracy, calibration and calibration
maintenance characteristics. Still further, such a system
was highly desirable which could, at the same time, provide
for programmed dcwnhole system flexibility as well as the
opportunity for operator adjustment of parameters such as
those affecting spectral generation including discriminator
levels, gate positions, source tracking, and temperature
correction, as well as data manipulation under control from
the surface or subsurface. Again still further, such a
system was highly desirable which cDuld accumulate both
inelastic and capture gamma radiation and utilize such
accumulations in the determunation of formation parameters.
The present invention is directed to achieving these ends
and in the promotion of consistent reproducible well logging
spectral data at the surface.
Summary of the Invention
In accordance with the present invention, a
computer-based well logging system and method is provided
for acquiring nuclear well logging data, including
derivation in a downhole logging instrument of spectral
information relating to the time-distribution of nuclear
particles detected within a subsurface earth formation.
The system includes a subsurface well logging
instrument suspended within and adapted to traverse a well
borehole and a surface system interconnected to the
instrument by a suitable communication link such as a single
or multi-conductor logging cable.
The surface system desirably includes a master
controller or computer with associated storage or m~fYIry,
one or mDre forms of visual display such as a plotter, an
input/output device for communicating with the controller,
and a conventional modem for oommunications interface
between the surface system and the instrument. The surface
system serves the purpose of aoquisition, storage, and
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display of data generated by the instrument as well a3 providing data
and command oontrol function to the instrument via the communioation
link.
As the subsurfaoe instrument traverse the well borehole, a
depth indicator provides signals indicative of the depth and rate of
travel of the instrument within the borehole. In response thereto,
the oontroller produoes periodio depth command signals at prescribed
depth intervala suoh a every quarter of a foot (four samples per foot
or in the alternative twenty aamples per meter) whioh may be used as
oommand signals oonveyed to the inatrument for purposes of retrieving
data generated by the instrument within eaoh auoh depth interval.
The ~ubsurfaoe inatrument inoludea a long-spaoed (LS) and
short-apaoed (SS) deteotor for deteoting natural or induoed gamma ray
emissions from subsurface formationa which produoe eleotrioal pulses.
Each pulse oorresponds in time with the incidence of a oorreaponding
gamma ray on the deteotor and has an analog voltage amplitude
correlative of the gamma ray. If the aystem i8 employed for spectral
analysis of neutron-induced gamma rays, the inatrument will further
include a neutron source for repeatedly inducing bursts of neutron
into the formation at a preselected frequenoy suoh as 1,000 KHz.
A multi-channel scale (MCS) section is provided within the
instrument for accumulating these indicationa of the time of occurence
of detectsd gamma ray pulses ocouring during presoribed time intervals
and oonveyed from the deteotor to the seotion. A memory within the
analyzer iB divided into one or more pluralities of memory looation~,
eaoh memory location uniquely corresponding sequentially to a
different tims window or channel having a preaeleoted diaorete time
width and employed to accumulate a count o$ gamma rays oocurring
within the particular time window during a preselected time interval.
In the preferred embodiment, these windows will be referenoed to the
time of firing of the aforementioned neutron aouroe.
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lZ36934
The MCS section inoludes a channel generator, spectrum
accumulator and central prooessor unit (CPU). In its overall
operation, the MCS section accumulates spectral data in the spectrum
accumulator by using a channel number generated within the channel
generator and associated with a pulse as an address for a memory
location in the speotrum aooumulator. After all of the channels have
had their data accumulated, the CPU reads the spectrum, or oollection
of data from all of the channels, and wends the data to a modem which
further transmits suoh data to the surface apparatus. The channel
generator further generates synchronized signals which control the
pulse frequency of the pulse neutron source. The CPU further
generates and communioates oontrol oommands whioh define oertain
operational parameters of the system.
The MCS seotion thus automatioally aooumulates oounts for
each of these speotra by the aforementioned address oode generation.
A high speed first-in-first-out buffer interposed between the ohannel
generator and speotrum aooumulator temporarily stores this arrival
time data for later aooumulation in the analyzer memory to enhanoe
data aoquisition rate of the analyzer. A direot memory aooess is
provided between the CPU and the memory of the speotrum accumulator.
In this manner, the CPU aooesses the spaotral data thus being aoquired
by the memory of the speotrum accumulator, either under downhole or
surface oontrol as desired, suoh as upon ooourenoe of depth interrupt
oommands, without affeoting aoquisition of the speotral data.
The CPU will periodioally aoquire this spectral data from
the spectrum accumulator memory as desired for transmi3sion to the
surface, etorage in CPU memory, or downhole analysis, also as desired.
A feature of the present inYention is the flexibility
provided by the instrument CPU, either along or in response to
surfaoe-generated commands from the surfaoe
~34
CPU, in controlling and defining the various spectra being generated
downhole as w911 a the related parameters for ac¢ompli~hing this
function.
further feature of the present invention i9 the ability of
such instrument to provide time-distribution spectra over the entire
time interval between ~ouroe firings, suoh ~peotra including both the
gamma radiation produoed from the inelastio soattering and capture of
neutrons from said neutron souroe. Tha instrument iB further oapable
of measuring baokground gamma radiation as part of the data oolleotion
proce~. The ~peotra 80 generated by the instrument may be analyzed
and processed to derive many parameters indi¢ative of formation and
borehole conditions. For example, the ratio of inela~tic~ in the
short to long spaced detectors (RIN) may be derived from such spectra.
RIN is primarily sensitive to changes in the borehole geometry. The
ratio of the inela~tio~ to the oaptures in the short-~paced deteotor
(RICS) may also be derived from suoh ~peotra. RICS is primarily
sensitive to formation porosity. An indioation of the presence of gay
in a fluid filled formation oan be determined from a oompari~on of
RICS and RA~0, the ratio of captures in the short to long ~paoed
deteotor~. Still further, the macroscopio thermal neutron absorption
oapture cross-section may be derived from ~uoh speotra. From the
capture-cross section a quantitative measure of the repeatability of
suoh measures may be derived (MSD), from which the weight of a filter
for the ¢apture cros~-~ection values may be controlled.
These and other features and advantage of the present
invention oan be understood from a reading of the following detailed
specification with reference to the following drawingss
Brief Description of the Drawings
Figure 1 is an overall schematio diagram of the nuolear well
logging system of the present invention,s
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~6934
g
Figure 2 is a representative display of time
distribution analysis data acquired by a well logging system
in accordance with that illustrated in Figure 1, and
Figure 3 is a sLmplified block diagram providing a
more detailed representation of the well logging inStrUmRnt
circuitry illustrated in Figure 1 and in particular, that of
the multi-channel scale section.
Detailed Description of the Preferred Embodiment
Referring now to the drawings in more detail, and
particularly to Figure 1, there is illustrated a nuclear
well logging configuration in accordance with the present
invention. Well 10 penetrates the earth's surface and may
or may not be cased depending upon the particular well being
investigated. Disposed within well 10 is subsurface well
logging instrument 12. Ihe system diagramed in Figure 1 is
a microprocessor-based nuclear well logging system using
multi-channel scale analysis for determining the timing
distributions of the detected gamma rays. Well logging
instrument 12 includes long-spaced (LS) detector 14, short-
spaced (SS) detector 16 and pulsed neutron source 18. Inthe preferred enbcdiment, LS and SS detectors 14 and 16 are
comprised of bismuth-germanate (BG0) crystals coupled to
photomultiplier tubes. qb protect the detector systems from
the high temperatures encountered in bo~reholes, the detector
system may be mounted in a Dewar-type ~X~u~. Aaso, in the
it preferred entodiment, source 18 comprises a pulsed neutron
source using a D'T reaction wherein deuterium ions are
accelerated into a tritium target, thereby generating
neutrons having ar. energy of approximately 14 MeV. qhe
filament current and accelerator voltage are supplied to
source 18 through power supply 15. Cable 20 suspends
instrument 12 in well 10 and contains the required
conductors for electrically connecting instrument 12 with
the surface apparatus.
The outputs from LS and SS detectors 14 and 16 are
coupled to detector board 22, which amplifies these outputs
and compares them to an adjustable discriminator level for
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passage to channel generator 26. Channel generator 26 is a
component of multi-channel scale (MCS) section 24 which
further includes spectrum accumulator 28 and central
processor unit (CPU) 30. As will be explained later, MCS
section 24 accumulates spectral data in spectrum accumulator
28 by using a channel number generated by channel generator
26 and associated with a pulse as an address for a memory
location. After all of the channels have had their data
accumulated, CPU 30 reads the spectrum, or collection of
data from all of the channels, and sends the data to modem
32 which is coupled to cable 20 for transmission of the data
over a communication link to the surface apparatus. Also to
be explained later is the further function of channel
generator 26 in generating synchronization signals which
control the pulse frequency of source 18, and further
functions of CPU 30 in commuicating control commands which
define certain operational parameters of instrument 12
including the discriminator levels of detector board 22, and
the filament current and accelerator voltage supplied to
source 18 by power supply 15.
The surface apparatus includes master controller
34 coupled to cable 20 for recovery of data from instrument
12 and for transmitting command signals to instrument 12.
There is also associated with the surface apparatus depth
controller 36 which provides signals to master controller 34
indicating the movement of instrument 12 within well 10.
Teleprinter 38 is coupled to master controller 34 to allow
the system operator to provide selected input into master
controller 34 for the logging operation to be performed by
the system. display unit 40, plotter 42 and mass storage
unit 44 are also coupled to master controller 34. The
primary purpose of display unit 40 and plotter 42 is to
provide visual indications of the generated logging data as
well as for retrieval of stored data and system operation
programs.
~236934
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In a well logging operation such as is illustrated
by Figure 1, master controller 34 initially transmits system
operation programs and oommand signals to be implenY~ted by
CPU 30, such programs and signals being related to the
particular well logging operation. Instrument 12 is then
caused to traverse well 10 in a conventional manner, with
source 18 being pulsed in response to synchronization
signals from channel generator 26. Typically, source 18 is
pulsed at a rate of 1000 bursts/second (1 KHz). This, in
turn, causes a burst of high energy neutrons on the order of
14 MeV to be introduced into the surrounding formation to be
investigated. In a manner previously described, this
population of high energy neutrons introduced into the
formation will cause the generation of gamma rays within the
fonmation which at various times will impinge on LS and SS
detectors 14 and 16. As each gamma ray thus impinges upon
the crystal-Fh~tomultiplier tube arrangement of the
detectors, a vDltage pulse having an amplitude functionally
related to the energy of the particular gamma ray is
delivered to detector board 22. It will be recalled that
detector board 22 amplifies each pulse and compares them to
an adjustable discriminator level, typically set at a value
corresponding to approximately 100 KeV. If such pulse has
an amplitude corresponding to an energy of at least
approximately 100 KeV, the voltage pulse is transformed into
a digital signal and passed to channel generator 26 of MCS
section 24.
The purpose of MCS section 24 may be more clearly
understood with the reference to the illustrative gamma ray
spectrum indicated in Figure 2. From the foregoing, it will
be recalled that each element in the formation when excited
my high energy neutrons, will emit gamma radiation having
energies characteristic of the particular element. In like
manner, the tine distribution of the occurrence of each of
these gamma ray pulses also yields extremely valuable
information. As but one example, in the system being
1;~36934
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described, a certain amount of "dead time" is experienced
due to finite time required for the processing of gamma ray
-I pulses and the like which adversely affect the spectra being
derived. Derivation of a gamma ray time distribult on-
spectrum would enable corrections for the dead time in thespectra. As another example of the use of gamma ray time
distribution spectrum, such a spectrum permits inference of
the neutron decay spectrum or flux in the formation of the
gamma rays thus received due to the source burst. This
information, in turn, will permit inferring the macroscopic
neutron absorption coefficient or capture coefficient.
From the foregoing, it will thus be noted that it
is desirable to have the capability of generating time
distribution spectra for the gamma rays being detected by LS
and SS detectors~l4 and 16, and thus MCS section 24 is
provided for this purpose. Figure 2 indicates a typical
time distribution spectrum generated by MCS section 24, in
rd;n~t~
which the e~rn~b~ te represents the number of gamma rays
being detected by a detector at time intervals corresponding
to discrete channel numbers indicated along the abscissa.
As source 18 is triggered and beco~Æs increasingly active,
the number of neutrons irradiating into the surrounding
formation per unit of time increases until the maximum burst
of source 18 is reached, after which the count rate of such
neutrons decreases. It will be expected that the gamma ray
count detected by a detector would observe a similar peak as
in fact evidenced by the peak in Figure 2. The abscissa
will conventionally be divided into a preselected number of
channels which, in the data depicted in Figure 2, is
arbitrarily selected as 100, with each increasing channel
number corresponding to a discrete window of time occurring
increasingly farther from a time reference point.
As aforemlentioned, the purpose of MCS section 24
is thus, in part, to form these time distribution spectra
for the detected gamma rays being delivered to MCS section
24 from LS and SS detectors 14 and 16. Prior to discussing
MCS section 24 in greater detail, it will be helpful to
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consider in general the manner in which these spectra are
thus created. It will be recalled that as each individual
gamma ray is detected by either LS or SS detector 14 or 16,
a corresponding voltage pulse is sent to detector board 22
where the pulse is amplified and compared to a discriminator
level. If the amplitude of such pulse is greater than such
discriminator level, the pulse is transformed into a digital
signal and sent to channel generator 26. In channel
generator 26, circuitry is provided for detecting the time
of occurrence of these voltage pulses relative or referenced
to a start time functionally related to the time of firing
or energizing of source 18. Thus, for each such voltage
pulse and corresponding detected gamma ray, a digital
representation of the time of occurrence of the pulse is
accordingly generated in channel generator 26.
Further discussing the time spectrum generating
function of MCS section 24, such as that illustrated in
Fi F e 2, it should be remembered that useful information is
provided by knowing the count of the total gamma rays
detected by kS and SS detectors 14 and 16 at preselected
discrete times or time windows relative to a start or
reference time. Thus, MCS section 24 is further provided
with appropriate time references for generating digital
words corresponding to the time occurrence of each detected
gamma ray pulse relative to a start time. This time period
over which a desired time spectrum is to be generated may be
divided into a plurality of discrete time windows in
corresponding channel numbers. Fbr example, if it is
desired to derive a time spectrum of detected gamma rays
over a 1000 microsecond interval (corresponding to a 1 KHz
repetition rate for source 18), this 1000 microsecond
interval may be divided into 100 consecutive chanrlels 1-100,
each of which is 10 microseconds in~width. Appropriate
memory locations may be provided in a suitable n~n~ry in the
circuitry of spectrum accumulator 28, each location
corresponding to a different one of these time channels.
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The purpose of each such location is to accumulate a running
count or total of the occurrence of the particular digital
wDrd assigned to that memory location and corresponding
uniquely to one of the time windbws or channels. Each time
a gamma ray is detected by a detector, the time of
occurrence relative to a start time is formed into a digital
wDrd corresponding to one of the time channels or nY~n~ry
locations. m at memory location is thence incremented by
one. Acaordingly, as additional gamma rays are detected and
their corresponding arrival times digitized and stored in
their appropriate memory locations or channel numbers, a
total count will be generated in each such memDry location
corresponding to gamma rays occurring within that time
windcw interval and within the time allotted for generation
of the time spectrum. By interrogating each of the memory
locations or channels consecutively in the memory of MCS
section 24, a visual indication such as that of Figure 2 may
be derive indicating the time distribution of occurrence of
gamma rays.
Again referring to Figure 1, the general purpose
of MCS section 24 may now be summarized. In addition to
deriving unique discrete digital representations of the time
of arrival of each gamma ray pulse detected by the detectors
and referenced to appropriate time references, MCS section
24 will accumulate a total of digital counts in memory
locations within spectrum accumulator 28, each location
oorresponding to the number of gamma rays detected as of the
interrogation occurring within the time window oorresponding
to the locations relative to a reference time. Thus, time
spectra may be accumulated and derived downhole as desired,
thereby avoiding the approach kncwn to the prior art wherein
the actual detected gamma ray pulses either in analog or
digitized form are transmitted up cable 20 for analysis at
the surface. Still referring to Figure 1, CPU 30 may at
appropriate times internally generate or in response to
comnands on the cable 20, interrogate spectrum accumulator
~236934
28 to retrieve spectral data as desired for transmission to
the surface. m is data will be delivered by CPU 30 to modem
32 or delivery to the surface on cable 20. In the .
alternative, however, it is contemplated that once the
spectra are thus derived, this spectral data may be stored
in the n~nory of CPU 30 for additional processing prior to
delivery to the surface.
It will be appreciated that due to the presence of
downhole computing and control capability afforded by MKS
section 24, an extremely powerful and flexible logging
system is thus provided which is not limited to particular
configurations. m us, the details of the spectra being
generated by MCS section 24 may be adaptively changed by CPU
30 through parameters delivered to CPU 30 in response to
either software program control resident in the memory of
CPU 30 or as a function of oontrol information delivered on
the cable 20 from the surface. thus, as but one simple
illustration, the discriminator widths of voltage pulse
signal threshold levels and the like employed by MCS section
24 may be varied at will in response to control from the CPU
30. m e flexibility afforded bY this feature will become
Gore apparent hereinafter wherein a more detailed
description of MC5 section 24 is provided.
Yet an additional extremely important feature of
the present invention relates to the ability of MCS section
24 to derive such spectra automatically wherein by means of
a direct memory access, CPU 30 may interrogate the nem~ry
of spectrum accumulator 28 to retrieve spectral data for
analysis or transmission without interrupting the spectral
acquisition process itself. Without such feature,
conventionally while CPU 30 is retrieving spectral data from
the n~Ynory, ability of the analyzer to derive further
portions of the spectra would thus be inhibited inasmuch as
the n~nory, while accessed by CPU 30 is thus not available
for storage of just-derived additional spectral data by
spectrum accumulator 28. High speed buffering in
lZ36934
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conjunction with the direct memory access enables this
independent spectral generation simultaneously along with
retrieval of such spectral information and additional,
,~ pFocessing thereof as desired, such buffering to be also
S dcsri later in greater detail.
Referring now to Figure 3, an overall discussion
of MCS section 24 and its operation will be given. It will
be recalled that the purpose of MCS section 24 is to
accumulate a total count of gamma ray pulses occurring in
each of a preselected number of discrete tLme windows so as
to generate the desired time spectra. To accompiish this, a
series of sequential 8-bit words corresponding to memory
address locations in spectrum accumulator 28 will be
generated, in order, within counter 46 in response to
frequency pulses from a reference clock. MKS section 24 is
provided with two such reference clocks, decay clock 50 and
background clock 52. Considering for the noment only decay
clocX S0, such clock being utilized in determining the
timing of the discrete gating during the accumulation of the
gamma ray counts resulting from the pulsing of source 18,
decay clock 50 generates a reference pulse signal of
preselected a frequency, the period between pulses being
the desired width of the discrete time window. In the
preferred enbodiment, decay clock S0 has a reference
frequency of 100 KHz, that is, a period of 10 microseconds.
ffl e frequency pulses from decay clock S0 are passed through
multiplexer 54, which allows only the passage of the desired
reference pulse, in this case the frequency pulse from decay
clock 50, to counter 46. Upon detection of such frequency
pulse by counter 46, the 8-bit memary address word generated
therein is incremented by one. For example, after detection
of the first decay clock frequency pulse, the 8-bit output
from counter 46 may look like 00000000. After detection of
a second pulse, the 8-bit output is incremented by one to
look like 00000001. This incrementing will oontinuS until a
preselected count limit is detected by comparator ~3, that
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123693~
limit being preferably a decoded valve of 100 for the decay
count. When such limit is detected, comparator 56 generates
a signal which is passed to sync circuitry 58. In response
to this signal sync circuitry 58 generates a sync signal
_which is further passed to counter 46 and power source 15.
In response to this sync signal the count in counter 46 is
reset to 00000000 and a burst sequence from source 18 is
initiated in power source 15.
To summarize the above timing of MCS section 24,
counter 46 increments an 8-bit memory address word in
response to reference pulses from decay clock 50, a
reference frequency generator. After detection by compar-
ator 56 of a preselected increment limit, comparator 56
generates a signal which is sent to sync circuitry 58.
In response to such signal, sync circuitry 58 generates a
.
sync signal which initiates the resetting of the count in
counter 46 and the burst sequence of source 18.
In the operation of the above timing sequence, a
background data gathering process, such as is provided in
applicants U.S. Patent No. 4,540,883, issued September 10,
1985, may be utilized in conjunction with background clock
52 in the following manner. After detection by CPU 30 of the
passage of a preselected number of decay cycles, this pre-
selected number being preferablv 28, CPU 30 may instruct
comparator 56 to add to the increment limit, the preferred
increment increase being 20. In other words, upon in-
struction from CPU 30, counter 46 will be allowed to
Pgt)~ - 17 -
.~
~236934
increment to a decoded valve of 120 instead of 100.
Upon detection of this instruction signal from CPU 30,
comparator 56 further instructs multiplexer 54 to pass
the frequency pulses from background clock 52 to counter
46. In the preferred embodiment, background clock 52
produces a reference frequency of 5 K~z, that is, a
period of 200 microseconds. Once this new increment limit
17A -
1236939~ -
-18-
is detected by comparator 56, the system is reset to the
original decay cycle.
Further explaining the preferred timing, it will
be remembered that decay clock 50 has a preferred reference
S frequency of 100 KHz, background clock 52 has a preferred
reference frequency of 5 KHz and comparator 56 is preferably
set to detect a decoded valve of 100 increments during the
decay measurements with an extra 20 increments preferably
added during the background measurements. Upon initiation
of the first burst sequence of source 18, counter 46 is
incremented in response to decay clock 50 until a decoded
valve of 100 increments is detected by ccmparator 56. This
100 increments corresponds to a 1000 microsecond decay cycle
having 100 ten microsecond discrete time windows. Upon
detection of this 100 increment limit by comparator 56,
counter 46 is reset and the burst sequence of source 18 is
reinitiated. Upon the passage of the twenty-eighth decay
cycle, that is, after the twenty-eighth 1000 microsecond
interval, the limit in ccmparator 56 is extended to a
decoded valve of 120 and multiplexer 54 passes the reference
frequency of background clock 52 to counter 46, which ncw
increments in response to the 5 KHz reference signal.
Cbunter 46 is then allcwed to increment to a decoded valve
of 120, at which time the system resets to the previously
described timing of the decay cycle. Stated another way,
counter 46 will increment to 100 twenty-eight times, thereby
prod w ing 28 decay spectra. Following the twenty-eighth
decay cycle, counter 46 will increment an extra 20 twc-
hundred microsecond intervals, thereby producing a single
background spectrum. Following this background cycle, the
system is reset to again detect the 28 decay cycles followed
again by the background cycle and so forth throughout the
measurement interval.
m ese 8-bit memory address words so generated by
the above described timing system are in turn communicated
through buffer 48 to spectrum accumulator 28 wherein such
8-bit words are altered and accumulated to reflect the
~236934
--19--
detection of gamma rays by either LS or SS detector 14 or 16
as will be described below. Also communicated through
buffer 48 to spectrum accumulator 28 are signals generated
by detection and processing (DIP) circuitry 64 in response
to the signals generated by the impingement of gamma
radiation upon LS and SS detectors 14 and 16. The purpose
for and generation of such nip signals will also be
described below.
Buffer 48 is preferably of a high speed first in -
first out (FIFO) type. It is a significant feature of thepresently described system that the memory in spectrum
accumulator 28 which accumulates the desired time spectra
may be accessed by both CPU 30 as well as by the remaining
portions of MCS section 24. If this memory was, at a given
time, being accessed by CPU 30 data on
`~ 24 wDuld thus be impeded reducing throughput of the system
inasmuch as the data acquisition capability of MCS section
24 would have to be held in abeyance until CPU 30
relinquishes control of the memory. Otherwise data vzlves
being generated by MCS section 24 would be lost during CPU
30 control of the m~m~ry inasmuch as this data would have
nowhere to be stored. Accordingly, one of the functions of
buffer 43 is to provide temporary storage for the data being
derived by the rest of the MCS section 24 until such time as
this data can be stored in the appropriate memory locations
in the nemDry of spectrum accumulator 28.
Referring back to the generation of DAP signals
from CAP circuitry 64, it will be recalled that detector
bcard 22 generates digital representations in response to
voltage pulses generated by LS and SS detectors 14 and 16
indicating the impingement upon such detectors of gamma
radiation. Those digital signals generated in response to
voltage pulses from LS detector 14 are latched into DAP
circuitry 64 through LS latch 60, while those digital
signals generated in response to voltage pulses from SS
detector 16 are latched into nAP circuitry 64 through SS
1Z36934
-20-
latch 62. LS and SS latches 60 and 62 are provided for
temporarily holding their respective signals until such tire
as DIP circuitry 64 is available for processing. In ,
response to these signals DAP circuitry 64 generates signals
indicative of the occurrence of the impingement of gamma
radiation upon a specific detector. These DAP signals, as
previously mentioned, are then passed through buffer 48 to
spectrum accumulator 28 for further processing.
Upon reaching spectrum accumulator 28, the nip
signals alter the eighth bit of the 8-bit ward currently
residing in spectrum accumulator 28 to indicate the specific
detector to which such pulse is attributable. Such altered
8-bit word is then latched into the memory of spectrum
accumulator 28, wherein the cccurrence of such altered 8,
bit word is accumulated within the corresponding memory
address location. For example, assume during a first time
interval a first signal is generated by DIP circuitry 64,
such signal indicating the impingemRnt of a gamma ray upon
one of the detectors. Assume further during a following
time interval a second signal is generated by nAP circuitry
64, such signal indicating the impingement of a gamma ray
upon the other detector. Assuming still further that the 8-
bit word residing in spectrum accumulator 28 during the
first time interval is 00000000, upon detection by spectrum
accumulator 28 of the first nAP signal such 8-bit word will
be altered to 10000000 and latched into the corresponding
memory location for accumulation. When such memory location
is read by CPU 30, it will show one count occurred in the
first detector during the first time interval. Likewise
assuming still further that the 8-bit word residing in
spectrum accumulator 28 during the seoond time interval is
00000001, upon detection of the second DIP signal such 8-bit
word will be altered, or rather remain unaltered, to
00000001 and latched into the corresponding memory location
for accumulation. When this nY~noory-~location is read by CPU
30, it will show one count occurred m the second detector
during the second time interval.
:~236934
It will be recalled that, after all of the channels have had
their data accumulated, CPU 30 reads the spectrum, or colleotion of
data from all of the channels and either stores such data in the
internal memory of CPU 30 or sends such data to modem 32 for
transmission to the surface. In the preferred embodiment, CPU 30
reads suoh spectrum in response to the movement of instrument 12 of a
preseleoted length interval, preferably 1/4 foot (four samples per
foot or ln the alternative twenty sample per meter). As will be
recalled, depth controller 36 generates signals sent to master
oontroller 34 indicating the movement of instrument 12 within well
10. Upon detection of matter controller 34 of the movement of
instrument 12 of 1/4 foot, master controller 34 generates a signal
which instructs CPU 30 to read such data and either store such data
within its internal memory or transmits such data uphole. Such data
may further be processed, for example, to produce a time distribution
spectrum as shown in Figure 2.
CPU 30, as previously mentioned, also implements system
operation programs thereby controlling many parameters of the well
logging operation. Specifically, CPU 30 performs several functions
including communication of data and commands between the surface and
subsurface; setting Or such operating parameters as the discriminator
level of deteotor board 22, the filament ourrent supplied to souroe
18, and the ao¢elerator voltage supplied to source 18; and starting
and stopping of sync circuitry 58 including oontrol over the
baokground oyole. Suoh examples of the oontrolling funotion of CPU 30
are for illustrative purposes only and in no way limit the invention
herein desoribed.
The time diBtribution Bpeotra BO generated by the
aforede~oribed system allow greater flexibility in utilization of such
gathered data. For example, by utilizing the preferred timing, counts
of the impingement of gamma radiation are aocumulated over 10
microseoond intervals, thereby improving upon the resolution and
statistical accuracy of prior systems. Further, such data
accumulation over a plurality of discrete intervals covering the
entire
Pg/
- 21 -
f
~Z36934
-22-
interval during and between bursts of the neutron source
allows the use of both the inelastic and capture portions of
the spectrum, or any other portion thereof, thereby gFeatly
increasing the amount of information available for
processing and greatly expanding the ability to determine
borehole and formation conditions.
As but one example of the use of such accumulated
data to predict borehole conditions, the inelastic portion
of the spectrum may be utilized to generate parameters
highly sensitive to changes of the borehole. This result
stems from the shallcw depth of investigation available from
the inelastic gamma radiation. m e borehole conditions most
evidenced by such parameters, therefore, are those related
to changes in the geometry of the borehole.
One such borehole parameter is determined by
utilizing the counts of the impingement of primarily
inelastic gamma radiation upon the short-spaced detector.
Cne embodiment of this parameter, RIN, is the ratio of the
primarily inelastic counts upon the short-spaced detector to
the primarily inelastic counts upon the long-spaced
detector. RIN is highly sensitive to borehole geometry
changes, and may be used as an indicator of changes in the
bo~ehole, a diagnostic for system irregularities and an
indicator of borehole effects on other parameters.
In utilizing the preferred timing of the system as
described above, RIN is determined by first accumulating
counts of the Lmpingement of gamma radiation upon the short-
spaced detector during the 100 microsecond interval
following the initiation of each neutron burst, that is,
during the first 10 microseoond intervals. The short-spaced
count is then normalized to remove the effects of variations
in the neutron bursts by taking the ratio of the short-
spaced count to the long-spaced count over the same time
interval. It should be noted that such normalization may be
accomplished by any measure capable of reflecting such
variations in the neutron bursts. By utilizing another
~Z36939~
-23-
inelastic count to provide such normalization, however, RIN
is primarily sensitive to changes in borehole gecmetry.
Such inelastic data is also useful in predicting
formation conditions such as formation porosity. One such
formation parameter is also determined by utilizing the
counts of the impingement of primarily inelastic gamma
radiation upon the short-spaced detector. One embodiment of
this parameter, RICS, is the ratio of the primarily
inelastic counts uFon the short-spaced detector to the
primarily capture counts upon the short-spaced detector.
Once again utilizing the preferred timing, RICS is
determined in the same manner as RIN except the short-spaced
inelastic count is nonmalized by a short-spaced capture
count accumulated over the 900 microsecond interval
preceeding the initiation of each burst, that is, from the
eleventh to the one hundredth 10 microsecond interval. It
should again be noted that such normalization may be
accomplished by any measure capable of reflecting variations
in the neutron bursts.
The basis for the sensitivity of RlCS to formation
porosity arises from the fact that the inelastic gamma
radiation count depPnds upon the number of heavier elements
present in the formation. If a formation is less porous,
that isl has a greater density of heavier elements, more
inelastic gamma radiation will be produced due to the higher
number of heavier elements to collide with. Conversely, if
the formation is mDre porous, that is has a lower density of
heavier elements, fewer collisions will occur and less
inelastic gamma radiation will be produced.
RlCS may further be utilized in conjunction with
another formation sensitive parameter to indicate the
presence of gas in a fluid-filled zone. m e other parameter
is preferably dependent upon capture gamma radiation. ox
such other parameter is RATO, the ratio of primarily capture
gamma radiation upon the short-spaced detector to primarily
capture gamma radiation upon the long spaced detector.
Again utilizing the preferred tLming, RATO is determmined by
1236934
-24-
first accumulating counts of the impingement of gamma
radiation upon the short-spaced detector during the interval
from 200 microseconds to 400 microseconds following the
initiation of each source burst, that is from the twenty-
first to the fortieth 10 microsecond interval. This firstaccumulation is then normalized by taking the ratio of this
first accumulation to the long-spaced count over the
interval from 200 microseconds to 1000 microseconds
following the initiation of each source burst, that is from
the twenty-first to the one hundreth 10 microsecond
interval.
R~TO is sensitive to the hydrogen density in the
formation since the production of capture gamma radiation is
dependent upon such hydrogen density. In a gas-filled zone,
which has a lower hydrogen density than a liquid-filled
zone, RATO will be lower due to this lower hydrogen
density. RICS, hx~ever, is not sensitive to hydrogen
density, and by overlaying the values of RICS and RATO in
liquid-filled zone, gas-filled zones can be located when the
RICS and R~TD values separate. The use of RICS and RATO in
conjunction, therfore, provides a good indicator of the
presenoe of gas in a fluid filled zone.
As previously mentioned, the data SQ generated by
the system herein disclosed may further be utilized to
measure the macroscY~pic thermal neutron capture cross-
section, , of a formation. The of a reservoir rock is
indicative of the porosity, formation water salinity, and
quantity and type of hydrocarbons contained in the pores of
such rock. The is further one of the more important
parameters measured by pulsed neutron capture systems such
as herein described, and a quantitative measure of the
repeatability of such is highly desirable in determining the
statistical validity of such measure.
The basis for such quantitative measure of the
repeatability arises from the fact that the system herein
disclosed accumulates data in depth increments smaller than
1~36934
the actual depth resolution of such system. m e neutron
cloud generated by the pulsing of the neutron source extends
over tens of inches, while data i6 accumulated by the, system
preferably over 1/4 foot (3 inch) intervals. Thus, for a
constant earth formation character, the measured from
successive 1/4 foot intervals should be equal.
Any differences between the ~w~ values,
therefore, must be oonsidered deviations. By taking a
standard root-mean-square of the two values, a quantitative
value of the deviation may be obtained. This calculated
deviation may then be filtered to produced a measured mean
standard deviation, MSD, over a specified interval. In the
preferred embodiment, these calculated deviations are
filtered with an ll-point binomial filter, which in this
case has a 90% response in a two foot (8 value) interval,
to produce the MSD value. mis MSD value may then be
further processed or compared to a purely statistical
standard deviation as an indication of the quality of the
values so obtained.
ffl is MSD value may further be utilized to control
the weight of a filter used for the raw values. Such
filter is again a ll-point binomial filter with variable
weighting. In response to chkLnges of the MSD value outside
of preselected statistical limit, the weight of such filter
can be made faster thereby decreasing the response time of
the filtered values to indicate more quickly such changes
in the values. Large changes in the MSD values are
generally caused by zone changes, and the statistical limits
of the MSD curve should be adjusted appropriately. By
m king the weight of such filter faster, the filtered
values will more accurately disclose such a zone charge.
It should here be noted that MSD may be calculated
for any parameter generated by a counting log in which data
acquisition is made at depth intervals smaller than the
depth resolution of the system. Accordingly, it should bs
- Z6 - ~2~9~
clearly understood that the application of the MSD to the
measured is exemplary only, and is not intended as a
limitation on the scope of the present invention.
m us, there has been described and illustrated new
and improved methcds for induced gamma ray logging of
subsurface formations. However, those skilled in the art
will recogni7e that obvious edifications can be made to the
em}~-L~m=nts without departing from the spirit of the present
invention.
