Temperature correction of a gamma detector

A logging tool may include 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. The analysis module may be configured to determine a photopeak of the stabilization source in a gamma ray spectrum including counts of the gamma rays collected by the gamma ray detector and perform resolution calculations using the photopeak to determine a resolution of the gamma ray detector.

RELATED APPLICATION

This application is a U.S. National Stage Application of International Application No. PCT/US2013/073088 filed Dec. 4, 2013, which designates the United States, and which is incorporated herein by reference in its entirety.

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.

DETAILED DESCRIPTION

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.FIG. 1illustrates oil well drilling equipment used in an illustrative LWD environment. A drilling platform2supports a derrick4having a traveling block6for raising and lowering a drill string8. A kelly10supports the drill string8as it is lowered through a rotary table12. A drill bit14is driven by a downhole motor and/or rotation of the drill string8. As bit14rotates, it creates a borehole16that passes through one or more formations18. A pump20may circulate drilling fluid through a feed pipe22to kelly10, downhole through the interior of drill string8, through orifices in drill bit14, back to the surface via the annulus around drill string8, and into a retention pit24. The drilling fluid transports cuttings from borehole16into pit24and aids in maintaining integrity or borehole16.

A logging tool26may be integrated into the bottom-hole assembly near bit14(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 tool26may be integrated at any point along drill string8. Logging tool26may include gamma-ray receivers and/or gamma-ray sources. In one embodiment, logging tool26may 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 tool26may store received signals. Logging tool26may be configured to analyze these signals.

As the bit extends borehole16through formations18, logging tool26may 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 sub28may be included to transfer tool measurements to a surface receiver30and/or to receive commands from surface receiver30.

At various times during the drilling process, drill string8may be removed from borehole16as shown inFIG. 2. In one embodiment, once the drill string8has been removed, logging operations may be conducted using a wireline tool34. Wireline tool34may be implemented by an instrument suspended into borehole16by a cable15having conductors for transporting power to the tool and telemetry from the tool body to the surface. The wireline tool34may include one or more logging tools36according to the present disclosure. Logging tool36may be communicatively coupled to the cable15. A logging facility44(shown inFIG. 4as a truck, although it may be any other structure) may collect measurements from the logging tool36, and may include computing facilities (including, e.g., an information handling system) for controlling, processing, and/or storing the measurements gathered by the logging tool36. The computing facilities may be communicatively coupled to logging tool36by way of cable15.

Logging tool26and logging tool36may be implemented in any suitable manner.FIG. 3illustrates an example embodiment of a logging tool300. Logging tool300may fully or partially implement logging tool26or logging tool36. In one embodiment, logging tool300may include a gamma-gamma density tool. Logging tool300may include a logging module302communicatively coupled to one or more detecting modules304. Although logging module302and detecting modules304are illustrated as separate modules, the functionality of logging tool300may be implemented by any suitable kind, number, or combination of components.

Logging module302may be configured to control the operation of logging tool300. Logging module302may include a control module310and an analysis module312. Control module310may 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 module312may be configured to analyze the information collected by control module310. Such analysis may include temperature correction of a gamma ray detector. Control module310and analysis module312may 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 module310and analysis module312may be implemented by instructions, code, or logic on a memory308for execution by a processor306.

Processor306may 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, processor306may interpret and/or execute program instructions and/or process data stored in memory308. Memory308may be configured in part or whole as application memory, system memory, or both. Memory308may 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 tool300, such as configurations of components of control module310and analysis module312, may reside in memory116for execution by processor114.

Logging tool300may include any suitable number of detecting modules304. Although two detecting modules304are illustrated inFIG. 3, logging tool300may include, for example, one, two, three, or four detecting modules304. Detecting modules304may be arranged in any suitable manner, such as at different locations along the axis of the tool.

Although logging module302and detecting modules304are both illustrated as resident within a single logging tool300, logging module302and detecting modules304may be in the same or different locations. For example, inFIG. 1both logging module302and detecting modules304may be implemented in logging tool26. In the example ofFIG. 2, logging module302may be implemented in logging facility44while detecting modules304may be implemented in logging tool36. Furthermore, control module310and analysis module312may be implemented in different locations. In the example ofFIG. 1, analysis module312and control module310may be implemented in logging tool26. In the example ofFIG. 2, analysis module312may be implemented in logging facility44while control module310may be implemented in logging tool36. In another example regardingFIG. 2, portions of control module310may be implemented in both logging facility44and logging tool36.

Logging tool300may include a high voltage source326communicatively coupled to detecting modules304and logging module302. High voltage source326may be configured to provide voltage sufficient to power photodetection in detecting modules304. Logging module302may be configured to control the operation of high voltage source326by, for example, determining the power, voltage, phase, or current to be supplied by high voltage source326to detecting modules304. In one embodiment, a single high voltage source326may be sufficient to provide power to all detecting modules304. In such an embodiment, high voltage source326may be implemented separately than detecting modules304. In another embodiment, a single detecting module304may be used in logging tool300. In such an embodiment, detecting module304may include high voltage source326. In yet another embodiment, each detecting module304may include an instance of high voltage source326.

Furthermore, logging tool300may include a logging source328. Logging source328may be configured to emit gamma rays into the formation, which may then been emitted back to logging tool300and detected by detecting modules304. Such received gamma rays may be used to analyze the properties of the formation. Logging source328may 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 source328may be implemented by a quantity of cesium-137. In one embodiment, a single logging source328may be used for logging tool300. If logging tool300includes multiple detecting modules304, logging source328may be implemented separately from detecting modules304. If logging tool300includes a single detecting module304, logging source328may be implemented within detecting module304.

Detecting modules304may be implemented in any suitable manner. In one embodiment, detecting module304may include a detector316and a stabilization source314. Detector316may be communicatively coupled to high voltage source326and logging module302. Detector316may 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 source314, and gamma rays from logging source328reflected by the formation. While various materials within a formation may emit gamma rays, the majority of the detected gamma rays may originate with logging source328and are scattered by the formation. The formation may also scatter gamma rays originating from stabilization source314. 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 modules304may include a stabilization source314. Stabilization source314may be configured to compensate for various variations in measurements made by detector316. Stabilization source314may be positioned in any suitable portion of detecting module. Stabilization source314may 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 source314may be relatively small in comparison to logging source328. Furthermore, detector316may be much closer to the position of stabilization source314than to the position of logging source328. The stablization source314may be incorporated into detector316. Given the location of stabilization source314to detector316, most gamma rays of stabilization source314detected by detector316may include gamma rays directly emitted from stabilization source314without 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 detector316, 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 source314may be of a higher energy than the gamma ray counts associated with the scattered and reflected gamma rays of logging source328. The measured voltage of a resultant photopeak may be kept relatively at the same level over time and through different uses of logging tool300. Although logging source328and stabilization source314may emit gamma rays of the same energy, the position of the sources may thus result in different detection profiles for each by detector316.

Detector316may 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. Detector316may include a substance configured to absorb gamma rays to determine such counts. For example, detector316may include a crystal320communicatively coupled to a photodetector318. Crystal320may include, for example, sodium iodide or lanthanum bromide crystals that may absorb the gamma rays. The absorption of gamma rays in crystal320may be detected by photodetector318, as the absorption of gamma rays may give off light. Photodetector318may be communicatively coupled to high voltage source326. High voltage source326may provide sufficient power for photodetector318to determine light emissions from crystal320. Furthermore, photodetector318may be configured to output its signals to any suitable electronics for signal processing.

In one embodiment, detecting module304may include electronics for signal processing the results of photodetector318. In another embodiment, such electronics may be included outside of detecting module304. The electronics may include, for example an amplifier322communicatively coupled to an analog to digital (A/D) converter324. Amplifier322may be configured to amplify signals received from photodetector318and pass the results to A/D converter324. Such amplification may correct for amplitude variations within the detected data. The gains of amplifier322may be set by commands received from logging module302. A/D converter324may be configured to convert the analog signals received from photodetector318into digital data and send the results to logging module302.

The ability of detector316to sufficiently perform its operations may depend upon the resolution of detector316. In one embodiment, the resolution of detector316may 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 detector316. Variations may be caused by, for example, light output from crystals in detector316, gain amplification in detector316, or non-linearities in electronics used to operate detector316. Various mechanisms, such as stabilization source314, may be employed to compensate for changes in gain or response of detector316. In another example, non-linearities may be accounted for when by characterizing logging tool300in ambient conditions.

In one embodiment, temperatures may affect the resolution of detector316. 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.

FIG. 4illustrates a graph400with example effects of resolution changes on gamma ray detection due to temperature changes. An example of a gamma ray energy spectrum is shown in graph400, wherein gamma ray counts are mapped on the y-axis and energy or channel identifiers are mapped on the x-axis. The axes of graph400may be measured in keV. In the example ofFIG. 4, each channel may represent a span of 3 keV.

Graph400may 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 inFIG. 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 graph300may result from placing drilling tool300in a marble block, which may emulate the characteristics of a zero-porosity limestone formation.

An initial response402may illustrate received gamma rays given the particular formation and other controlled, standard conditions. The temperature associated with initial response402may be room temperature or another suitable ambient temperature. A degraded response404may illustrate received gamma rays given the same conditions except for a temperature change. Degraded response404may result from a lowered resolution of detector316. As illustrated inFIG. 4, degraded response404may redistribute counts among the windows, thereby changing the counts in the windows.

Accordingly, logging tool300may be configured to correct gamma ray measurements based upon an instant resolution of detector316. Such corrections may be made, for example, in real-time or during post-measurement processing. Consequently, logging tool300may be configured to directly measure resolution of detector316and utilize the measured resolution to correct for temperature changes.

Logging tool300may be characterized such that gamma ray counts in a given formation may be adjusted to account for variations in resolution. Furthermore, logging tool300may 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, memory308. Thus, given an instant measurement of the resolution of detector316, correction in gamma ray counts accommodating temperature changes may be made.

Measurement of the resolution of detector316may be made by analyzing the gamma ray spectrum of detector316. Such analysis may be performed by analysis module312. In one embodiment, analysis module312may analyze count rates from the photopeak of stabilization source314. The photopeak of stabilization source314may include a peak of gamma ray counts associated with stabilization source314. 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.

FIG. 5is an illustration of a graph500of example response measured by detector316during evaluation of a formation. Graph500may include information compiled by, for example, analysis module312. 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 source314may 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 source328and stabilization source314may 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 source328may be positioned such that its emitted gamma rays do not flow directly to detector316. Any gamma rays received at detector316as a result of emissions from logging source328may 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 graph500between channel #0and channel #200may correspond to such gamma rays that were originally emitted by logging source328that entered the formation, were scattered, and are now detected as having less energy than 662 keV. Therefore, no photopeak might be available in graph500associated with logging source328. In contrast, the proximity of stabilization source314to detector316may result in high-energy gamma rays of stabilization source314depositing all of their energy in detector316, instead of such gamma rays first dispersing into the formation, scattering, and then being detected. Accordingly, a photopeak of stabilization source314should appear in graph500at the energy level of the high-energy gamma rays emitted by stabilization source314. For example, a photopeak of stabilization source314may appear at channel #232.

FIG. 6is a more detailed illustration of graph500of example response measured by detector316during evaluation of a formation. InFIG. 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 source314may be illustrated. In the example ofFIG. 6, such a photopeak corresponding to stabilization source314may be centered on approximately channel #232. The photopeak may be generated by stabilization source314. 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 tool300may measure the resolution of detector316by analyzing the photopeak corresponding to stabilization source314. In one embodiment, logging tool300may analyze the photopeak by determining energy resolution of the photopeak. For example, logging tool300may analyze the photopeak by measuring, indirectly, the FWHM. In another example, logging tool300may analyze the photopeak by measuring the photopeak's peak standard deviation. In another embodiment, logging tool300may 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 tool300may 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 tool300may calculate the gamma ray counts in window1604and in window2606. Logging tool300may divide the counts determined in window2606by the counts determined in window1604to find a resolution ratio. The resolution ratio may be generally smaller when resolution of detector316is better. The resolution ratio may thus be generally smaller when the photopeak is sharper.

The correlation between resolution ratio and resolution of detector316may 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.

FIG. 7illustrates a graph700of example relationships between resolution (such as FWHM) and resolution ratios. Graph700may express a function to determine resolution given a resolution ratio. In the example ofFIG. 7, assuming that window1604and window2606are 51 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, memory308.

In operation, logging tool300may make measurements of gamma rays during drilling of formation18. Control module310may issue commands to detecting module304, to the extent that components of detecting module304may be enabled or tunable. For example, control module310may enable and control high voltage source326and each instance of amplifier322. Stabilization source314and logging source328may emit gamma rays. The powering of each instance of photodetector318may enable detector316to begin recording gamma ray counts from formation18and stabilization source314.

Each instance of photodetector318may observe light given off by reactions of the gamma rays in respective instances of crystal320. Photodetector318may generate signals indicating quantifications of the observed light and pass the signals to amplifier322. Amplifier322may apply gains specified by logging module302to the signals and pass the result to A/D converter324for conversion to digital data. The resultant digital data may be sent to analysis module312.

The counts may be organized according to channel or energy level. Analysis module312may determine the resultant photopeaks for instances of stabilization source314. 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 source314emits gamma rays.

Once the photopeak of stabilization source314is 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 module312may count the total gamma rays in the first window, adjacent to the photopeak. Furthermore, analysis module312may count the total gamma rays in the second window, adjacent to the first window. In addition, analysis module312may 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 module312may 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 module312may 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 module312may apply a gamma ray correction to count rates obtained for the various energy windows of the spectrum. Alternatively or in addition, analysis module312may 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 formation18may be accurately used for further analysis or drilling guidance.

Analysis module312may separately analyze the data for each instance of detecting module304. The results of each such analysis may all be used as appropriate for formation analysis.

FIG. 8is an illustration of an example method800for temperature correction of a gamma detector. AlthoughFIG. 8discloses a particular number of steps to be taken with respect to example method800, method800may be executed with more or fewer steps than those depicted inFIG. 8. In addition, althoughFIG. 8discloses a certain order of steps to be taken with respect to method800, the steps of these methods may be completed in any suitable order. Method800may be implemented using the system ofFIGS. 1-7or any other suitable mechanism. In certain embodiments, method800may be implemented partially or fully in software embodied in computer-readable storage media.

Program instructions may be used to cause a general-purpose or special-purpose 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. Method800may 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, method800may begin at805. Method800may be used in conjunction with a logging tool, such as logging tool300, present within a formation. At805, the gamma rays from a stabilization source of the logging tool may be emitted. At810, 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.

At815, 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.

At820, 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. At825, the counts within the first window on the high-energy side of the photopeak may be determined. At830, the counts within the second window on the high-energy side of the photopeak may be determined.

At835, a resolution ratio of the second window's count to the first window's count may be determined. Given the resolution ratio, at840a resolution measurement, such as FWHM expressed in percentage, may be determined with a function, look-up table, or other suitable relationship expression. At845, 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.