TEMPERATURE CORRECTION OF A GAMMA DETECTOR
BACKGROUND
The present disclosure relates generally to drilling operations and, more
particularly, to temperature correction of a gamma detector.
Hydrocarbons, such as oil and gas, are commonly obtained from subterranean
formations that may be located onshore or offshore. The development of subterranean
operations and the processes involved in removing hydrocarbons from a subterranean
formation are complex. Typically, subterranean operations involve a number of
different steps such as, for example, drilling a wellbore at a desired well site, treating
the wellbore to optimize production of hydrocarbons, and performing the necessary
steps to produce and process the hydrocarbons from the subterranean formation.
When performing subterranean operations, it is often desirable to obtain
information about the formation.
The basic techniques for density logging for earth formations are well known.
Generally, a density logging tool consists of a logging source that emits gamma rays
and one or more detectors that detect gamma rays. Gamma rays from the logging
source pass into the earth formations. Some of the gamma rays are scattered back into
the tool and detected by one of the detectors. The detected gamma rays are processed
to obtain a measure of the formation density. In some cases a measure of the lithology
is also obtained. The measured formation properties may be recorded as a function
of the tool's depth or position in the borehole, yielding a formation log that can be
used to analyze the formation.
FIGURES
Some specific exemplary embodiments of the disclosure may be understood
by referring, in part, to the following description and the accompanying drawings.
FIGURE 1 is a diagram showing an illustrative logging while drilling
environment.
FIGURE 2 is a diagram showing an illustrative wireline logging environment.
FIGURE 3 is an illustration of an example embodiment of a logging tool.
FIGURE 4 is a graph with example effects on gamma ray detection by
resolution changes due to temperature changes.
FIGURE 5 is a graph of example response measured by a detector during
evaluation of a formation
FIGURE 6 is a more detailed graph of example response measured by a
detector during evaluation of a formation.
FIGURE 7 is a graph of example relationships between full width at half
maximum and resolution ratios
FIGURE 8 is an illustration of an example method for temperature correction
of a gamma detector.
While embodiments of this disclosure have been depicted and described and
are defined by reference to exemplary embodiments of the disclosure, such references
do not imply a limitation on the disclosure, and no such limitation is to be inferred.
The subject matter disclosed is capable of considerable modification, alteration, and
equivalents in form and function, as will occur to those skilled in the pertinent art and
having the benefit of this disclosure. The depicted and described embodiments of this
disclosure are examples only, and not exhaustive of the scope of the disclosure.
DETAILED DESCRIPTION
For purposes of this disclosure, an information handling system may include
any instrumentality or aggregate of instrumentalities operable to compute, classify,
process, transmit, receive, retrieve, originate, switch, store, display, manifest, detect,
record, reproduce, handle, or utilize any form of information, intelligence, or data for
business, scientific, control, or other purposes. For example, an information handling
system may be a personal computer, a network storage device, or any other suitable
device and may vary in size, shape, performance, functionality, and price. The
information handling system may include random access memory (RAM), one or
more processing resources such as a central processing unit (CPU) or hardware or
software control logic, ROM, and/or other types of nonvolatile memory. Additional
components of the information handling system may include one or more disk drives,
one or more network ports for communication with external devices as well as various
input and output (I/O) devices, such as a keyboard, a mouse, and a video display. The
information handling system may also include one or more buses operable to transmit
communications between the various hardware components. It may also include one
or more interface units capable of transmitting one or more signals to a controller,
actuator, or like device.
For the purposes of this disclosure, computer-readable media may include any
instrumentality or aggregation of instrumentalities that may retain data and/or
instructions for a period of time. Computer-readable media may include, for example,
without limitation, storage media such as a direct access storage device (e.g., a hard
disk drive or floppy disk drive), a sequential access storage device (e.g., a tape disk
drive), compact disk, CD-ROM, DVD, RAM, ROM, electrically erasable
programmable read-only memory (EEPROM), and/or flash memory; as well as
communications media such wires, optical fibers, microwaves, radio waves, and other
electromagnetic and/or optical carriers; and/or any combination of the foregoing.
Illustrative embodiments of the present disclosure are described in detail
herein. In the interest of clarity, not all features of an actual implementation may be
described in this specification. It will of course be appreciated that in the
development of any such actual embodiment, numerous implementation-specific
decisions are made to achieve the specific implementation goals, which will vary
from one implementation to another. Moreover, it will be appreciated that such a
development effort might be complex and time-consuming, but would, nevertheless,
be a routine undertaking for those of ordinary skill in the art having the benefit of the
present disclosure.
To facilitate a better understanding of the present disclosure, the following
examples of certain embodiments are given. In no way should the following
examples be read to limit, or define, the scope of the invention. Embodiments of the
present disclosure may be applicable to horizontal, vertical, deviated, or otherwise
nonlinear wellbores in any type of subterranean formation. Embodiments may be
applicable to injection wells as well as production wells, including hydrocarbon wells.
Embodiments may be implemented using a tool that is made suitable for testing,
retrieval and sampling along sections of the formation. Embodiments may be
implemented with tools that, for example, may be conveyed through a flow passage in
tubular string or using a wireline, slickline, coiled tubing, downhole robot or the like.
"Measurement-while-drilling" ("MWD") is the term generally used for measuring
conditions downhole concerning the movement and location of the drilling assembly
while the drilling continues. "Logging-while-drilling" ("LWD") is the term generally
used for similar techniques that concentrate more on formation parameter
measurement. Devices and methods in accordance with certain embodiments may be
used in one or more of wireline (including wireline, slickline, and coiled tubing),
downhole robot, MWD, and LWD operations.
The terms "couple" or "couples" as used herein are intended to mean either an
indirect or a direct connection. Thus, if a first device couples to a second device, that
connection may be through a direct connection or through an indirect mechanical or
electrical connection via other devices and connections. Similarly, the term
"communicatively coupled" as used herein is intended to mean either a direct or an
indirect communication connection. Such connection may be a wired or wireless
connection such as, for example, Ethernet or LAN. Such wired and wireless
connections are well known to those of ordinary skill in the art and will therefore not
be discussed in detail herein. Thus, if a first device communicatively couples to a
second device, that connection may be through a direct connection, or through an
indirect communication connection via other devices and connections.
The present disclosure relates generally to subterranean drilling operations
and, more particularly, the present disclosure relates to formation sensing systems,
apparatus, and methods.
The present disclosure in some embodiments provides methods and systems
for analyzing characteristics of a subterranean formation (e.g., lithology, density,
resistivity, dielectric constant, or permittivity). The methods and systems of some
embodiments may include one or more logging tools. In some embodiments, a
logging tool may include a tool body and one or more antennas, emitters, and
detectors, each of which may act as a transmitter and/or a receiver of an
electromagnetic signal or signals.
Such electromagnetic signal(s) may be used to determine any suitable
characteristic, such as the lithology, density, resistivity, dielectric constant, or
permittivity of the formation. For example, the logging tools of some embodiments
may measure the count of gamma rays received relative to the number of gamma rays
sent. These measurements may be made at each of one or more receiving antennas or
detectors in response to signals transmitted by one or more transmitting sources. The
count of gamma rays may be used to determine, for example, whether a given portion
of a formation includes shale, sandstone, gypsum, coal, limestone, halite, dolomite, or
combinations of such substances.
The logging tools discussed above and herein may be implemented in any
suitable mechanism such as a drilling collar, mandrel, wireline tool, or other suitable
device. In some embodiments, such logging tools may be included and/or used in a
logging-while-drilling (LWD) environment. FIGURE 1 illustrates oil well drilling
equipment used in an illustrative LWD environment. A drilling platform 2 supports a
derrick 4 having a traveling block 6 for raising and lowering a drill string 8. A kelly
10 supports the drill string 8 as it is lowered through a rotary table 12. A drill bit 14 is
driven by a downhole motor and/or rotation of the drill string 8. As bit 14 rotates, it
creates a borehole 16 that passes through one or more formations 18. A pump 20 may
circulate drilling fluid through a feed pipe 22 to kelly 10, downhole through the
interior of drill string 8, through orifices in drill bit 14, back to the surface via the
annulus around drill string 8, and into a retention pit 24. The drilling fluid transports
cuttings from borehole 16 into pit 24 and aids in maintaining integrity or borehole 16.
A logging tool 26 may be integrated into the bottom-hole assembly near bit 14
(e.g., within a drilling collar, i.e., a thick-walled tubular that provides weight and
rigidity to aid in the drilling process, or a mandrel). In some embodiments, logging
tool 26 may be integrated at any point along drill string 8. Logging tool 26 may
include gamma-ray receivers and/or gamma-ray sources. In one embodiment, logging
tool 26 may communicate received signals to another portion of the illustrative LWD
environment. Such signals may be analyzed in the portion of the illustrative LWD
environment to which the signals are sent. In another embodiment, logging tool 26
may store received signals. Logging tool 26 may be configured to analyze these
signals.
As the bit extends borehole 16 through formations 18, logging tool 26 may
collect measurements relating to various formation properties as well as the tool
orientation and position and various other drilling conditions. The orientation
measurements may be performed using an azimuthal orientation indicator, which may
include magnetometers, inclinometers, and/or accelerometers, though other sensor types
such as gyroscopes may be used in some embodiments. In embodiments including an
azimuthal orientation indicator, resistivity and/or dielectric constant measurements may
be associated with a particular azimuthal orientation (e.g., by azimuthal binning). A
telemetry sub 28 may be included to transfer tool measurements to a surface receiver 30
and/or to receive commands from surface receiver 30.
At various times during the drilling process, drill string 8 may be removed from
borehole 16 as shown in FIGURE 2. In one embodiment, once the drill string 8 has been
removed, logging operations may be conducted using a wireline tool 34. Wireline tool 34
may be implemented by an instrument suspended into borehole 16 by a cable 15 having
conductors for transporting power to the tool and telemetry from the tool body to the
surface. The wireline tool 34 may include one or more logging tools 36 according to the
present disclosure. Logging tool 36 may be communicatively coupled to the cable 15. A
logging facility 44 (shown in Figure 4 as a truck, although it may be any other structure)
may collect measurements from the logging tool 36, and may include computing facilities
(including, e.g., an information handling system) for controlling, processing, and/or
storing the measurements gathered by the logging tool 36. The computing facilities may
be communicatively coupled to logging tool 36 by way of cable 15.
Logging tool 26 and logging tool 36 may be implemented in any suitable
manner. FIGURE 3 illustrates an example embodiment of a logging tool 300.
Logging tool 300 may fully or partially implement logging tool 26 or logging tool 36.
In one embodiment, logging tool 300 may include a gamma-gamma density tool.
Logging tool 300 may include a logging module 302 communicatively coupled to one
or more detecting modules 304. Although logging module 302 and detecting modules
304 are illustrated as separate modules, the functionality of logging tool 300 may be
implemented by any suitable kind, number, or combination of components.
Logging module 302 may be configured to control the operation of logging
tool 300. Logging module 302 may include a control module 310 and an analysis
module 312. Control module 310 may be configured to adjust the high voltage and
electronic gains of detectors so as to keep measurement windows within the same
channel number of the spectrum. Analysis module 312 may be configured to analyze
the information collected by control module 310. Such analysis may include
temperature correction of a gamma ray detector. Control module 310 and analysis
module 312 may be implemented in any suitable manner, such as by a card, function,
library, shared library, script, executable, application, process, computer, information
handling system, server, analog hardware, digital hardware, logic, instructions, code,
or any suitable combination thereof. Control module 310 and analysis module 312
may be implemented by instructions, code, or logic on a memory 308 for execution by
a processor 306.
Processor 306 may be implemented by, for example, a microprocessor,
microcontroller, digital signal processor (DSP), application-specific integrated circuit
(ASIC), or any other digital or analog circuitry configured to interpret and/or execute
program instructions and/or process data. In some embodiments, processor 306 may
interpret and/or execute program instructions and/or process data stored in memory
308. Memory 308 may be configured in part or whole as application memory, system
memory, or both. Memory 308 may include any system, device, or apparatus
configured to hold and/or house one or more memory modules. Each memory
module may include any system, device or apparatus configured to retain program
instructions and/or data for a period of time (e.g., computer-readable media or
machine-readable storage media). Instructions, logic, or data for configuring the
operation of logging tool 300, such as configurations of components of control
module 310 and analysis module 312, may reside in memory 116 for execution by
processor 114.
Logging tool 300 may include any suitable number of detecting modules 304.
Although two detecting modules 304 are illustrated in FIGURE 3, logging tool 300
may include, for example, one, two, three, or four detecting modules 304. Detecting
modules 304 may be arranged in any suitable manner, such as at different locations
along the axis of the tool.
Although logging module 302 and detecting modules 304 are both illustrated
as resident within a single logging tool 300, logging module 302 and detecting
modules 304 may be in the same or different locations. For example, in FIGURE 1
both logging module 302 and detecting modules 304 may be implemented in logging
tool 26. In the example of FIGURE 2, logging module 302 may be implemented in
logging facility 44 while detecting modules 304 may be implemented in logging tool 36.
Furthermore, control module 310 and analysis module 312 may be implemented in
different locations. In the example of FIGURE 1, analysis module 312 and control
module 310 may be implemented in logging tool 26. In the example of FIGURE 2,
analysis module 312 may be implemented in logging facility 44 while control module
310 may be implemented in logging tool 36. In another example regarding FIGURE
2, portions of control module 310 may be implemented in both logging facility 44 and
logging tool 36.
Logging tool 300 may include a high voltage source 326 communicatively
coupled to detecting modules 304 and logging module 302. High voltage source 326
may be configured to provide voltage sufficient to power photodetection in detecting
modules 304. Logging module 302 may be configured to control the operation of
high voltage source 326 by, for example, determining the power, voltage, phase, or
current to be supplied by high voltage source 326 to detecting modules 304. In one
embodiment, a single high voltage source 326 may be sufficient to provide power to
all detecting modules 304. In such an embodiment, high voltage source 326 may be
implemented separately than detecting modules 304. In another embodiment, a single
detecting module 304 may be used in logging tool 300. In such an embodiment,
detecting module 304 may include high voltage source 326. In yet another
embodiment, each detecting module 304 may include an instance of high voltage
source 326.
Furthermore, logging tool 300 may include a logging source 328. Logging
source 328 may be configured to emit gamma rays into the formation, which may
then been emitted back to logging tool 300 and detected by detecting modules 304.
Such received gamma rays may be used to analyze the properties of the formation.
Logging source 328 may be implemented by, for example, a source of radioactive
material sufficiently large to generate gamma rays that are emitted into the formation
and returned for detection. For example, logging source 328 may be implemented by
a quantity of cesium-137. In one embodiment, a single logging source 328 may be
used for logging tool 300. If logging tool 300 includes multiple detecting modules
304, logging source 328 may be implemented separately from detecting modules 304.
If logging tool 300 includes a single detecting module 304, logging source 328 may
be implemented within detecting module 304.
Detecting modules 304 may be implemented in any suitable manner. In one
embodiment, detecting module 304 may include a detector 316 and a stabilization
source 314. Detector 316 may be communicatively coupled to high voltage source
326 and logging module 302. Detector 316 may be configured to detect and count
gamma rays as they are received from the formation. The gamma rays received by
the detector may include, in increasing amounts, gamma rays naturally emitted from
the formation, gamma rays from a stabilization source 314, and gamma rays from
logging source 328 reflected by the formation. While various materials within a
formation may emit gamma rays, the majority of the detected gamma rays may
originate with logging source 328 and are scattered by the formation. The formation
may also scatter gamma rays originating from stabilization source 314. The spectrum
of gamma rays received may be analyzed to determine the nature of the formation,
such as the lithology of the formation.
As mentioned above, detecting modules 304 may include a stabilization
source 314. Stabilization source 314 may be configured to compensate for various
variations in measurements made by detector 316. Stabilization source 314 may be
positioned in any suitable portion of detecting module. Stabilization source 314 may
include a source capable of emitting gamma rays. Such a source may emit high
energy gamma rays within a very limited range, such as radioactive isotope cesium-
137. The amount of cesium- 137 in stabilization source 314 may be relatively small in
comparison to logging source 328. Furthermore, detector 316 may be much closer to
the position of stabilization source 314 than to the position of logging source 328.
The stablization source 314 may be incorporated into detector 316. Given the
location of stabilization source 314 to detector 316, most gamma rays of stabilization
source 314 detected by detector 316 may include gamma rays directly emitted from
stabilization source 314 without being scattered and reflected in the formation. Some
of these detected gamma rays emitted within a limited high-energy range may deposit
all their energy in detector 316, resulting in a discernible photopeak of detected
gamma rays within such a high-energy range. The resultant photopeak or gamma ray
counts associated with stabilization source 314 may be of a higher energy than the
gamma ray counts associated with the scattered and reflected gamma rays of logging
source 328. The measured voltage of a resultant photopeak may be kept relatively at
the same level over time and through different uses of logging tool 300. Although
logging source 328 and stabilization source 314 may emit gamma rays of the same
energy, the position of the sources may thus result in different detection profiles for
each by detector 316.
Detector 316 may be configured to detect counts of gamma rays that are
received as well as categorize the received gamma rays according to energy channel.
The result may be an energy spectrum. Detector 316 may include a substance
configured to absorb gamma rays to determine such counts. For example, detector
316 may include a crystal 320 communicatively coupled to a photodetector 318.
Crystal 320 may include, for example, sodium iodide or lanthanum bromide crystals
that may absorb the gamma rays. The absorption of gamma rays in crystal 320 may
be detected by photodetector 318, as the absorption of gamma rays may give off light.
Photodetector 318 may be communicatively coupled to high voltage source 326.
High voltage source 326 may provide sufficient power for photodetector 318 to
determine light emissions from crystal 320. Furthermore, photodetector 318 may be
configured to output its signals to any suitable electronics for signal processing.
In one embodiment, detecting module 304 may include electronics for signal
processing the results of photodetector 318. In another embodiment, such electronics
may be included outside of detecting module 304. The electronics may include, for
example an amplifier 322 communicatively coupled to an analog to digital (A/D)
converter 324. Amplifier 322 may be configured to amplify signals received from
photodetector 318 and pass the results to A/D converter 324. Such amplification may
correct for amplitude variations within the detected data. The gains of amplifier 322
may be set by commands received from logging module 302. A/D converter 324 may
be configured to convert the analog signals received from photodetector 318 into
digital data and send the results to logging module 302.
The ability of detector 316 to sufficiently perform its operations may depend
upon the resolution of detector 316. In one embodiment, the resolution of detector
316 may be measured in terms of full width at half maximum (FWHM). FWHM may
be determined by evaluating the width of a given gamma ray peak at half of the
highest point of the peak. Resolution may be expressed in, for example, relative
terms or in electron-Volts (eV).
Various factors may affect the response of detector 316. Variations may be
caused by, for example, light output from crystals in detector 316, gain amplification
in detector 316, or non-linearities in electronics used to operate detector 316. Various
mechanisms, such as stabilization source 314, may be employed to compensate for
changes in gain or response of detector 316. In another example, non-linearities may
be accounted for when by characterizing logging tool 300 in ambient conditions.
In one embodiment, temperatures may affect the resolution of detector 316.
Such temperature effects may manifest themselves by affecting measured counts of
various channels. Changing the detector's ability to finely detect gamma rays may
change the actual, observed counts.
FIGURE 4 illustrates a graph 400 with example effects of resolution changes
on gamma ray detection due to temperature changes. An example of a gamma ray
energy spectrum is shown in graph 400, wherein gamma ray counts are mapped on
the y-axis and energy or channel identifiers are mapped on the x-axis. The axes of
graph 400 may be measured in keV. In the example of FIGURE 4, each channel may
represent a span of 3 keV.
Graph 400 may illustrate various characteristics of a formation being drilled.
The various characteristics may manifest themselves by variations in the energy
spectra. These variations may be quantified by grouping a range of channels into
windows and summing the gamma ray counts of the window. When the counts are
normalized by time, the results are referred to as count rates. Example window
groupings may be shown in FIGURE 4. Each window may characterize a different
aspect of the formation or drilling environment. Count rates from the windows of the
one or more detectors may be combined to obtain formation properties such as
formation density and formation lithology. For the purposes of example only, the
information shown in graph 300 may result from placing drilling tool 300 in a marble
block, which may emulate the characteristics of a zero-porosity limestone formation.
An initial response 402 may illustrate received gamma rays given the
particular formation and other controlled, standard conditions. The temperature
associated with initial response 402 may be room temperature or another suitable
ambient temperature. A degraded response 404 may illustrate received gamma rays
given the same conditions except for a temperature change. Degraded response 404
may result from a lowered resolution of detector 316. As illustrated in FIGURE 4,
degraded response 404 may redistribute counts among the windows, thereby changing
the counts in the windows.
Accordingly, logging tool 300 may be configured to correct gamma ray
measurements based upon an instant resolution of detector 316. Such corrections may
be made, for example, in real-time or during post-measurement processing.
Consequently, logging tool 300 may be configured to directly measure resolution of
detector 316 and utilize the measured resolution to correct for temperature changes.
Logging tool 300 may be characterized such that gamma ray counts in a given
formation may be adjusted to account for variations in resolution. Furthermore,
logging tool 300 may be repeatedly characterized in this manner for different types of
expected formations. The relationship between a given resolution and adjustment of
gamma ray counts may be established and defined according to equations,
experimental data, look-up tables, functions, or any other suitable mechanism. Such
mechanisms may be stored in, for example, memory 308. Thus, given an instant
measurement of the resolution of detector 316, correction in gamma ray counts
accommodating temperature changes may be made.
Measurement of the resolution of detector 316 may be made by analyzing the
gamma ray spectrum of detector 316. Such analysis may be performed by analysis
module 312. In one embodiment, analysis module 312 may analyze count rates from
the photopeak of stabilization source 314. The photopeak of stabilization source 314
may include a peak of gamma ray counts associated with stabilization source 314. In
a further embodiment, the count rates may be analyzed according to narrow energy
windows defined adjacent to the photopeak. In another, further embodiment, the
count rates may be analyzed according to two such windows. In yet another, further
embodiment, the two windows may be defined on the high-energy side of the
photopeak. The windows may be of equal width in terms of channel count or eV.
The width of the two windows combined may be sufficient to stretch between the
channel corresponding to the top count of the photopeak and the channel
corresponding to a count rate near zero.
FIGURE 5 is an illustration of a graph 500 of example response measured by
detector 316 during evaluation of a formation. Graph 500 may include information
compiled by, for example, analysis module 312. Within a viewable range suitable to
illustrate gamma counts for determining properties such as density and lithology of
the formation, a photopeak corresponding to stabilization source 314 may not be
visible.
A photopeak may include a distribution of gamma rays corresponding to the
energy levels of an original source of such gamma rays. For example, logging source
328 and stabilization source 314 may both emit gamma rays from cesium-137
sources. The energy level of such gamma rays may be, for example, 662 keV. This
may correspond to, for example, channel #232. However, logging source 328 may be
positioned such that its emitted gamma rays do not flow directly to detector 316. Any
gamma rays received at detector 316 as a result of emissions from logging source 328
may have first travelled into the formation and are subsequently scattered. The
scattering may lessen the energy of the gamma rays. Accordingly, the plot of gamma
ray counts illustrated in graph 500 between channel #0 and channel #200 may
correspond to such gamma rays that were originally emitted by logging source 328
that entered the formation, were scattered, and are now detected as having less energy
than 662 keV. Therefore, no photopeak might be available in graph 500 associated
with logging source 328. In contrast, the proximity of stabilization source 314 to
detector 316 may result in high-energy gamma rays of stabilization source 314
depositing all of their energy in detector 316, instead of such gamma rays first
dispersing into the formation, scattering, and then being detected. Accordingly, a
photopeak of stabilization source 314 should appear in graph 500 at the energy level
of the high-energy gamma rays emitted by stabilization source 314. For example, a
photopeak of stabilization source 314 may appear at channel #232.
FIGURE 6 is a more detailed illustration of graph 500 of example response
measured by detector 316 during evaluation of a formation. In FIGURE 6, gamma
counts corresponding to the lower-energy portion of the spectrum may be cut off (as
such measurements are too high) so that a photopeak corresponding to stabilization
source 314 may be illustrated. In the example of FIGURE 6, such a photopeak
corresponding to stabilization source 314 may be centered on approximately channel
#232. The photopeak may be generated by stabilization source 314. In various
embodiments, the photopeak may include a Gaussian distribution. However, the
photopeak may be distorted by a pile-up of lower-energy signals.
Logging tool 300 may measure the resolution of detector 316 by analyzing the
photopeak corresponding to stabilization source 314. In one embodiment, logging
tool 300 may analyze the photopeak by determining energy resolution of the
photopeak. For example, logging tool 300 may analyze the photopeak by measuring,
indirectly, the FWHM. In another example, logging tool 300 may analyze the
photopeak by measuring the photopeak' s peak standard deviation. In another
embodiment, logging tool 300 may analyze the photopeak by using consecutive
windows of channels or ranges of channels. The counts of gamma rays within a given
window may be counted. In yet another embodiment, logging tool 300 may analyze
the photopeak by using windows on the high-energy side of the photopeak. Such a
high-energy side may be indicated by channel numbers higher than the photopeak.
By using windows on the high-energy side of the photopeak, errors due to gamma
rays generated by the formation and collected on the low-energy side may be avoided.
Thus, logging tool 300 may calculate the gamma ray counts in windowl 604
and in window2 606. Logging tool 300 may divide the counts determined in
window2 606 by the counts determined in windowl 604 to find a resolution ratio.
The resolution ratio may be generally smaller when resolution of detector 316 is
better. The resolution ratio may thus be generally smaller when the photopeak is
sharper.
The correlation between resolution ratio and resolution of detector 316 may be
established in any suitable manner. For example, the relationship between resolution
and resolution ratios may be determined through characterizations using different
temperatures (and thus resolutions). The result may be expressed in, for example,
functions, look-up tables, or linear approximations of the data points. In another
example, assuming that the photopeak follows Gaussian distribution, the Gaussian
peak values and widths of windows may similarly yield functions.
FIGURE 7 illustrates a graph 700 of example relationships between resolution
(such as FWHM) and resolution ratios. Graph 700 may express a function to
determine resolution given a resolution ratio. In the example of FIGURE 7, assuming
that windowl 604 and window2 606 are 5 1 keV wide and that the photopeak is 662
keV, the relationship between resolution and resolution ratio may be largely linear
between approximately 8% resolution and 25% resolution. The function may be
expressed as resolution = 27.958(resolution ratio)+8.0666. Given other distributions,
other results may arise. The relationships between resolution and resolution ratios
may be stored in, for example, memory 308.
In operation, logging tool 300 may make measurements of gamma rays during
drilling of formation 18. Control module 310 may issue commands to detecting
module 304, to the extent that components of detecting module 304 may be enabled
or tunable. For example, control module 310 may enable and control high voltage
source 326 and each instance of amplifier 322. Stabilization source 314 and logging
source 328 may emit gamma rays. The powering of each instance of photodetector
318 may enable detector 316 to begin recording gamma ray counts from formation 18
and stabilization source 314.
Each instance of photodetector 318 may observe light given off by reactions of
the gamma rays in respective instances of crystal 320. Photodetector 318 may
generate signals indicating quantifications of the observed light and pass the signals to
amplifier 322. Amplifier 322 may apply gains specified by logging module 302 to the
signals and pass the result to A/D converter 324 for conversion to digital data. The
resultant digital data may be sent to analysis module 312.
The counts may be organized according to channel or energy level. Analysis
module 312 may determine the resultant photopeaks for instances of stabilization
source 314. Such a photopeak may be located at a higher channel than the counts
associated with formation characteristics and may be centered at the channel at which
stabilization source 314 emits gamma rays.
Once the photopeak of stabilization source 314 is determined, two consecutive
windows adjacent to the channel of the photopeak may be selected for use in
resolution determination. The two windows may be pre-determined or calculated. If
the high voltage and or electronic gains are being adjusted to keep the photopeak in
the same energy channel, then the two consecutive windows might always span the
same channels. Otherwise, the window channels may be determined based on the
peak location. The two windows may be on the high-energy side of the photopeak.
The windows may be of a predetermined width, or may be selected such that the two
windows reach a channel with a gamma ray count rate of near zero.
Analysis module 312 may count the total gamma rays in the first window,
adjacent to the photopeak. Furthermore, analysis module 312 may count the total
gamma rays in the second window, adjacent to the first window. In addition, analysis
module 312 may determine the ratio of the count of the second window to the count
of the first window. This ratio may be the resolution ratio.
Given the resolution ratio, analysis module 312 may determine a resolution
value based upon the resolution ratio. The determination may be made by, for
example, a function, look-up table, or other mechanism. Given the resolution value,
analysis module 312 may determine a gamma ray correction amount. The gamma ray
correction determination may be made by, for example, a function, look-up table, or
other mechanism. Analysis module 312 may apply a gamma ray correction to count
rates obtained for the various energy windows of the spectrum. Alternatively or in
addition, analysis module 312 may apply a gamma ray correction to formation
properties computed from uncorrected count rates. Characteristics such as density
and lithology may thus be corrected. This correction may be based upon the
resolution changes which in turn may have been caused by temperature changes.
Given corrected characteristics, information about formation 18 may be accurately
used for further analysis or drilling guidance.
Analysis module 312 may separately analyze the data for each instance of
detecting module 304. The results of each such analysis may all be used as
appropriate for formation analysis.
FIGURE 8 is an illustration of an example method 800 for temperature
correction of a gamma detector. Although FIGURE 8 discloses a particular number
of steps to be taken with respect to example method 800, method 800 may be
executed with more or fewer steps than those depicted in FIGURE 8. In addition,
although FIGURE 8 discloses a certain order of steps to be taken with respect to
method 800, the steps of these methods may be completed in any suitable order.
Method 800 may be implemented using the system of FIGURES 1-7 or any other
suitable mechanism. In certain embodiments, method 800 may be implemented
partially or fully in software embodied in computer-readable storage media.
Program instructions may be used to cause a general-purpose or specialpurpose
processing system that is programmed with the instructions to perform the
operations described below. The operations may be performed by specific hardware
components that contain hardwired logic for performing the operations, or by any
combination of programmed computer components and custom hardware
components. Method 800 may be provided as a computer program product that may
include one or more machine readable media having stored thereon instructions that
may be used to program a processing system or other electronic device to perform the
methods.
In some embodiments, method 800 may begin at 805. Method 800 may be
used in conjunction with a logging tool, such as logging tool 300, present within a
formation. At 805, the gamma rays from a stabilization source of the logging tool
may be emitted. At 810, gamma rays emitting from the formation and associated with
the emission by the stabilization source may be collected. Such collection may be
performed by a gamma ray detector of the logging tool.
At 815, gamma ray spectra may be acquired. As such, counts of gamma rays
for a range of channels or energy levels may be determined. The counts at each
channel may form a gamma ray spectrum. The counts of the various channels may be
used for determining characteristics of the formation such as density and lithology.
At 820, a photopeak of the stabilization source may be determined within the
gamma ray spectrum. In one embodiment, such a photopeak may be at a higher
channel count or energy level than channels associated with characteristics such as
density and lithology, or higher than channels associated with a logging source. The
magnitude of the gamma counts for the photopeak may be much smaller than the
gamma counts of the characteristics such as density and lithology. Once a channel
count of the photopeak is determined, two channel windows may be established.
Such windows may be of the same channel width. In addition, the windows may be
consecutive without a channel gap. Furthermore, such windows may be located on
the high-energy side of the photopeak. At 825, the counts within the first window on
the high-energy side of the photopeak may be determined. At 830, the counts within
the second window on the high-energy side of the photopeak may be determined.
At 835, a resolution ratio of the second window's count to the first window's
count may be determined. Given the resolution ratio, at 840 a resolution
measurement, such as FWHM expressed in percentage, may be determined with a
function, look-up table, or other suitable relationship expression. At 845, given the
resolution, a correction to the computation of formation characteristics may be made.
In one embodiment, a correction for gamma ray counts in channels associated with
characteristics such as density and lithology may be made. The correction may be
made given the resolution and a function, look-up table, or other suitable relationship
expression. Once corrected gamma ray counts have been determined, the
characteristics may be evaluated and, consequently, the formation. In another
embodiment, the formation properties may be computed with uncorrected count rates
and then corrected for resolution.
As would be appreciated by those of ordinary skill in the art, with the benefit
of this disclosure, in one exemplary embodiment, the methods, systems, and apparatus
disclosed herein may be implemented using an information handling system. In one
embodiment, each of the one or more detectors of a logging tool may be
communicatively coupled to an information handling system through a wired or
wireless network. Operations of such systems are well known to those of ordinary
skill in the art and will therefore not be discussed in detail herein. The information
handling system may control generation, transmission, and/or receipt of signals
received from each detector to analyze a subterranean formation. Specifically,
software including instructions in accordance with the methods disclosed herein may
be stored in computer-readable media of an information handling system. The
information handling system may then use those instructions to carry out the methods
disclosed herein. In one exemplary embodiment, the information handling system
may store the values of the measured signal in each of multiple iterations as it carries
out the methods disclosed herein. In one embodiment, the information handling
system may include a user interface that may provide information relating to
formation properties to a user in real time.
Therefore, the present disclosure is well adapted to attain the ends and
advantages mentioned as well as those that are inherent therein. The particular
embodiments disclosed above are illustrative only, as the present disclosure may be
modified and practiced in different but equivalent manners apparent to those skilled in
the art having the benefit of the teachings herein. Furthermore, no limitations are
intended to the details of construction or design herein shown, other than as described
in the claims below. It is therefore evident that the particular illustrative embodiments
disclosed above may be altered or modified and all such variations are considered
within the scope and spirit of the present disclosure. Also, the terms in the claims
have their plain, ordinary meaning unless otherwise explicitly and clearly defined by
the patentee. The indefinite articles "a" or "an," as used in the claims, are defined
herein to mean one or more than one of the element that it introduces.

What is claimed is:
1. A logging tool comprising:
a stabilization source configured to emit gamma rays;
a gamma ray detector configured to collect gamma rays from the stabilization
source and a formation; and
an analysis module configured to:
determine a photopeak of the stabilization source in an energy
spectrum of gamma rays collected by the gamma ray detector; and
perform resolution calculations using the photopeak to determine a
resolution of the gamma ray detector.
2. The logging tool of Claim 1, wherein performing resolution
calculations comprises:
determining a sum of the gamma ray counts of a first window in the spectrum;
determining a sum of the gamma ray counts of a second window in the
spectrum;
determining a ratio of the sum of the second window to the sum of the first
window;
wherein the first window is adjacent to the photopeak.
3. The logging tool of Claim 2, wherein the first window and the second
window are on a high-energy side of the photopeak.
4. The logging tool of Claim 2, wherein performing resolution
calculations further comprises determining resolution from the determined ratio.
5. The logging tool of Claim 1, wherein the analysis module is further
configured to correct formation properties based upon the determined resolution.
6. The logging tool of Claim 1, wherein the analysis module is further
configured to correct gamma ray counts based upon the determined resolution.
7. The logging tool of Claim 1, wherein the analysis module is further
configured to make corrections based upon the determined resolution, wherein the
corrections include variations due to temperature changes experienced by the logging
tool.
8. A method of analyzing a formation, comprising:
collecting, at a gamma ray detector, gamma rays from a stabilization source
and a formation;
determining, at a logging tool, a photopeak of the stabilization source in a
gamma ray spectrum of gamma rays collected by the gamma ray detector; and
performing, at the logging tool, resolution calculations using the photopeak to
determine a resolution of the gamma ray detector.
9. The method of Claim 8, wherein performing resolution calculations
comprises:
determining a sum of the gamma ray counts of a first window of the spectrum;
determining a sum of the gamma ray counts of a second window of the
spectrum;
determining a ratio of the sum of the second window to the sum of the first
window;
wherein the first window is adjacent to the photopeak.
10. The method of Claim 9, wherein the first window and the second
window are on a high-energy side of the photopeak.
11. The method of Claim 9, wherein performing resolution calculations
further comprises determining resolution from the determined ratio.
12. The method of Claim 8, further comprising correcting formation
properties based upon the determined resolution.
13. The method of Claim 8, further comprising correcting gamma ray
counts based upon the determined resolution.
14. The method of Claim 8 further comprising making corrections based
upon the determined resolution, wherein the corrections include variations due to
temperature changes experienced by the logging tool.
15. An article of manufacture, comprising a machine-readable storage
medium, the computer-readable storage medium including machine-executable
instructions, the instructions readable by a processor and, when read and executed,
configured to cause the processor to:
determine a photopeak of a stabilization source in a gamma ray spectrum of
gamma rays collected by a gamma ray detector, the gamma rays from the stabilization
source and a formation; and
perform resolution calculations using the photopeak to determine a resolution
of the gamma ray detector.
16. The article of Claim 15, wherein performing resolution calculations
comprises:
determining a sum of the gamma ray counts of a first window of the spectrum;
determining a sum of the gamma ray counts of a second window of the
spectrum;
determining a ratio of the sum of the second window to the sum of the first
window;
wherein the first window is adjacent to the photopeak.
17. The article of Claim 16, wherein the first window and the second
window are on a high-energy side of the photopeak.
18. The article of Claim 16, wherein performing resolution calculations
further comprises determining resolution from the determined ratio.
19. The article of Claim 15, further comprising instructions configured to
cause the processor to correct formation properties based upon the determined
resolution.
20. The article of Claim 15, further comprising instructions configured to
cause the processor to correct gamma ray counts based upon the determined
resolution.