Neutron gamma density correction using elemental spectroscopy

A method for determining a corrected neutron gamma density of a formation includes emitting neutrons into a formation using a neutron source to generate gamma-rays. Additionally, the method includes detecting a first count rate of gamma-rays and a gamma-ray spectrum using at least a gamma-ray detector of the downhole tool. The method also includes detecting a second count rate of neutrons using a neutron detector. The method includes using a processor to perform a gamma-ray spectroscopy analysis on the formation based on the gamma-ray spectrum and determining a correction based on results of the gamma-ray spectroscopy analysis. The method includes applying the correction to the first count rate or the second count rate and determining a neutron gamma density of the formation based on a first corrected count rate of gamma-rays or a second corrected count rate of neutrons. The method also includes outputting the determined density of the formation.

BACKGROUND

This disclosure relates generally to neutron-gamma density (NGD) well logging and, more particularly, to techniques for obtaining an accurate NGD measurement in certain formations using a correction factor based on elemental spectroscopy.

Techniques have been developed to generate gamma-rays for a formation density measurement without radioisotopic gamma-ray sources. One such technique is referred to as a neutron-gamma density (NGD) measurement. An NGD measurement involves emitting neutrons into the formation using a neutron source, such as a neutron generator. Some of these neutrons may inelastically scatter off certain elements in the formation, generating inelastic gamma-rays that are detected by a gamma-ray detector in the tool that may enable a formation density determination. Although an NGD measurement based on these gamma-rays may be accurate in some formations, the NGD measurement may be less accurate in other formations, depending on the formation composition.

SUMMARY

In one example, a method includes emitting neutrons into a formation using a neutron source of a downhole tool, such that at least a portion of the neutrons scatter off the formation to generate gamma-rays. The method also includes detecting a first count rate of gamma-rays using a gamma-ray detector of the downhole tool. Additionally, the method includes detecting a gamma-ray spectrum using the gamma-ray detector. Further, the method includes detecting a second count rate of neutrons that return to the downhole tool using a neutron detector of the downhole tool. Furthermore, the method includes using a processor to perform a gamma-ray spectroscopy analysis on the formation based on the detected gamma-ray spectrum and determining a correction based at least in part on results of the gamma-ray spectroscopy analysis. Moreover, the method includes applying the correction to the first count rate, the second count rate, or both and determining a neutron gamma density of the formation based at least in part on a first corrected count rate of gamma-rays, a second corrected count rate of neutrons, or both. The method also includes outputting the determined density of the formation.

In another example, a system includes a downhole tool. The downhole tool includes a neutron source that emits neutrons into a formation at an energy sufficient to cause at least a portion of the neutrons to inelastically scatter off elements of the formation, generating inelastic gamma-rays. The downhole tool also includes a gamma-ray detection assembly including a gamma-ray detector that detects a first count rate of inelastic gamma-rays that scatter through the formation to reach the downhole tool and a gamma-ray spectrum of the formation, and a neutron detection assembly including a neutron detector that detects a second count rate of neutrons that return to the downhole tool. Additionally, the system includes data processing circuitry that performs a gamma-ray spectroscopy analysis on the gamma-ray spectrum of the formation. The data processing circuitry also corrects the first count rate, the second count rate, or both based on results of the gamma-ray spectroscopy analysis, and determines a density of the formation based at least in part on a first corrected count rate of inelastic gamma-rays, a second corrected count rate of neutrons, or both.

In another example, a non-transitory computer readable medium comprising executable instructions which, when executed by a processor, cause the processor to instruct a neutron source of a downhole tool to emit neutrons into a formation at an energy sufficient to cause at least a portion of the neutrons to inelastically scatter off elements of the formation, generating inelastic gamma-rays. The instructions also cause the processor to instruct at least a gamma-ray detector to detect a first count rate of inelastic gamma-rays that scatter through the formation to reach the downhole tool and a gamma-ray spectrum of the formation and instruct at least a neutron detector to detect a second count rate of neutrons that return to the downhole tool. Additionally, the instructions cause the processor to perform a gamma-ray spectroscopy analysis on the formation based on the gamma-ray spectrum and determine a neutron gamma density of the formation based at least in part of the first count rate of inelastic gamma rays and the second count rate of neutrons. Further, the instructions cause the processor to determine a correction of the neutron gamma density based at least in part on results of the gamma-ray spectroscopy analysis. Furthermore, the instructions cause the processor to apply the correction to the neutron gamma density of the formation to determine a corrected neutron gamma density and output the determined density of the formation.

Technical effects of the present disclosure include the accurate determination of a neutron-gamma density (NGD) measurement for a broad range of formations, including formations with a high concentration of iron and/or aluminum. These NGD measurements may remain accurate even when the configuration of a downhole tool used to obtain the neutron count rates and gamma-ray count rates used in the NGD measurement does not have an optimal configuration. Thus, despite an inability to directly measure fast neutrons (e.g., when a fast neutron detector is not present), an accurate NGD measurement still may be obtained using the disclosed systems and techniques.

DETAILED DESCRIPTION

Embodiments of this disclosure relate to systems and techniques for obtaining a neutron-gamma density (NGD) measurement that is accurate for various formations including formations with high concentrations of iron and/or aluminum. In general, a downhole tool for obtaining such an NGD measurement may include a neutron source, at least one neutron detector, and at least one gamma-ray detector. While the downhole tool is within a borehole of a formation, the neutron source may comprise a pulsed neutron generator emitting fast neutrons of at least 2 MeV into the formation for a brief period of time, referred to herein as a “burst gate,” during which the neutrons may inelastically scatter off certain elements in the formation (e.g., oxygen) to generate gamma-rays. The gamma-ray detectors of the downhole tool may detect these inelastic gamma-rays. The NGD measurement of the formation may be a function of a count rate of these inelastic gamma-rays, corrected by a neutron transport correction function based on a neutron count rate from the neutron detector(s).

It may be appreciated that the term “neutron transport” refers to the way the elements of the formation allow the neutrons to move through the formation. For example, in one formation, it may be statistically more likely that more of the neutrons will traverse deeper into the formation before inelastically scattering and generating inelastic gamma-rays. By contrast, in another formation, it may be statistically more likely that fewer of the neutrons will traverse deeper into the formation before inelastically scattering and generating inelastic gamma-rays. Since neutron transport affects the distribution of the generation of inelastic gamma-rays, the NGD measurement is, to a certain degree, a function of the neutron transport of the formation. Such a neutron transport correction function generally may accurately account for the neutron transport of most formations commonly encountered in an oil and/or gas well, resulting in an accurate NGD measurement. As used herein, an “accurate” NGD measurement may refer to an NGD measurement that is within about 0.03 g/cc the true density of a formation.

It is believed that neutron counts from some downhole tool configurations may not accurately account for fast neutron transport in certain formations. For instance, when the downhole tool does not include a fast neutron detector, thermal or epithermal neutron detectors may be used to estimate the fast neutron distribution, but count rates from thermal or epithermal neutron detectors may not always accurately reflect the fast neutron transport of some formations in the same way a fast neutron detector would. Moreover, the placement of such thermal, epithermal, and/or fast neutron detectors in the downhole tool may involve a variety of considerations for NGD, as well as many other well logging measurements. As such, some of these thermal or epithermal detectors may not be at a location within the downhole tool that is best suited to detect count rates of neutrons so as to accurately reflect the neutron transport of some formations, when applied in a neutron transport correction function. These situations may arise when an NGD measurement is obtained in certain formations including shale formations with heavy elements. As used herein, the term “formation with heavy elements” refers to a formation with a concentration of elements of atomic mass greater than the atomic mass of hydrogen beyond a concentration limit. Shales containing high concentrations of iron or aluminum may typically be environments that require a correction.

According to embodiments of the present disclosure, when an NGD measurement is obtained in a formation, having characteristics that detectably affect the fast neutron transport in a way that differs from other formations, the gamma-ray count rate(s) used for the NGD measurement and/or a neutron transport correction function may be modified to more accurately account for the fast neutron transport of the formation. These or any other suitable corrections may be applied when the formation has one or more characteristics that are expected to cause the count rate of thermal and/or epithermal neutrons not to accurately determine a fast neutron transport of the formation, when the count rate of neutrons and/or gamma-rays is applied in a neutron transport correction function.

With the foregoing in mind,FIG. 1illustrates a wellsite system in which the disclosed NGD system can be employed. The wellsite system ofFIG. 1may be onshore or offshore. In the wellsite system ofFIG. 1, a borehole11may be formed in subsurface formations by rotary drilling using any suitable technique. A drill string12may be suspended within the borehole11and may have a bottom hole assembly100that includes a drill bit105at its lower end. A surface system of the wellsite system ofFIG. 1may include a platform and derrick assembly10positioned over the borehole11, the platform and derrick assembly10including a rotary table16, kelly17, hook18and rotary swivel19. The drill string12may be rotated by the rotary table16, energized by any suitable means, which engages the kelly17at the upper end of the drill string12. The drill string12may be suspended from the hook18, attached to a traveling block (not shown), through the kelly17and the rotary swivel19, which permits rotation of the drill string12relative to the hook18. A top drive system could also be used, which may be a top drive system well known to those of ordinary skill in the art.

In the wellsite system ofFIG. 1, the surface system may also include drilling fluid or mud26stored in a pit27formed at the well site. A pump29may deliver the drilling fluid26to the interior of the drill string12via a port in the swivel19, causing the drilling fluid to flow downwardly through the drill string12as indicated by the directional arrow8. The drilling fluid26may exit the drill string12via ports in the drill bit105, and circulate upwardly through the annulus region between the outside of the drill string12and the wall of the borehole11, as indicated by the directional arrows9. In this way, the drilling fluid26lubricates the drill bit105and carries formation cuttings up to the surface, as the fluid26is returned to the pit27for recirculation.

The bottom hole assembly100of the wellsite system ofFIG. 1may include a logging-while-drilling (LWD) module120and/or a measuring-while-drilling (MWD) module130, a roto-steerable system and motor150, and the drill bit105. The LWD module120can be housed in a special type of drill collar, as is known in the art, and can contain one or more types of logging tools. It will also be understood that more than one LWD module can be employed, as generally represented at numeral120A. As such, references to the LWD module120can also mean a module at the position of120A as well. The LWD module120may include capabilities for measuring, processing, and storing information, as well as for communicating with surface equipment. The LWD module120may be employed to obtain a neutron-gamma density (NGD) measurement, as will be discussed further below.

The MWD module130can also be housed in a special type of drill collar, as is known in the art, and can contain one or more devices for measuring characteristics of the drill string and drill bit. It will also be understood that more than one MWD can be employed, as generally represented at numeral130A. As such, references to the MWD module130can also mean a module at the position of130A as well. The MWD module130may also include an apparatus for generating electrical power to the downhole system. Such an electrical generator may include, for example, a mud turbine generator powered by the flow of the drilling fluid, but other power and/or battery systems may be employed additionally or alternatively. In the wellsite system ofFIG. 1, the MWD module130may include one or more of the following types of measuring devices: a weight-on-bit measuring device, a torque measuring device, a vibration measuring device, a shock measuring device, a stick slip measuring device, a direction measuring device, and/or an inclination measuring device.

The LWD module120may be used in a neutron-gamma density (NGD) system, as shown inFIG. 2, which can accurately measure a density in various types of formations including formations with heavy elements. It may be understood that the LWD module120is intended to represent one example of a general configuration of an NGD tool, and that other suitable NGD tools may include more or fewer components and may be configured for other means of conveyance. Indeed, embodiments of NGD tools employing the general configuration of the LWD module120are envisaged for use with any suitable means of conveyance, such as wireline, coiled tubing, slickline, and so forth. By way of example, the LWD module120may represent a model of the EcoScope™ tool by Schlumberger.

The LWD module120may be contained within a drill collar202that encircles a chassis204and a mud channel205. The chassis204may include a variety of components used for emitting and detecting radiation to obtain an NGD measurement. For example, a neutron generator206may serve as a neutron source that emits neutrons of at least 2 MeV, which is believed to be approximately the minimum energy to create gamma-rays through inelastic scattering with formation elements. By way of example, the neutron generator206may be an electronic neutron source, such as a Minitron™ by Schlumberger Technology Corporation, which may produce pulses of neutrons through deuteron-deuteron (d-D) and/or deuteron-triton (d-T) reactions. Thus, the neutron generator206may emit neutrons around 2 MeV or 14 MeV, for example. A neutron monitor208may monitor the neutron emissions from the neutron generator206. By way of example, the neutron monitor208may be a plastic scintillator and photomultiplier that primarily detects unscattered neutrons directly emitted from the neutron generator206, and thus may provide a count rate signal proportional to the neutron output rate from the rate of neutron output of the neutron generator206. Neutron shielding210, which may include lead or tungsten, for example, may largely prevent neutrons from the neutron generator206from passing internally through the LWD module120toward various radiation-detecting components on the other side of the shielding210.

As illustrated inFIG. 2, the LWD module120may include two near neutron detectors, namely, a near thermal neutron detector212and a near epithermal neutron detector214. Two far thermal neutron detectors216A and216B may be located at a spacing farther from the neutron generator206than the neutron detectors212and214. For example, the near neutron detectors212and214may be spaced approximately 10-14 in. from the neutron generator206, and the far neutron detectors216A and216B may be spaced 18-28 in. from the neutron generator206. A short spacing (SS) gamma-ray detector218may be located between the near neutron detectors212and214and the far neutron detectors216A and216B. A long spacing (LS) gamma-ray detector220may be located beyond the far neutron detectors216A and216B, at a spacing farther from the neutron generator206than the gamma-ray detector218. For example, the SS gamma-ray detectors218may be spaced approximately 16-22 in. from the neutron generator206, and the LS gamma-ray detector220may be spaced approximately 30-38 in. from the neutron generator206. Embodiments of the LWD module120may include more or fewer of such radiation detectors, but generally may include at least one gamma-ray detector and at least one neutron detector. For instance, the neutron detector may be a long spacing (LS) detector. The tool may also comprise one or more SS or LS neutron detectors, such as an additional thermal neutron detector. Configurations in which the tool comprises fewer detectors than in the embodiment ofFIG. 2are also included in the scope of the present disclosure.

The neutron detectors212,214,216A, and/or216B may be any suitable neutron detectors, such as3He neutron detectors. To detect primarily epithermal neutrons, the epithermal neutron detector214may be surrounded by thermal neutron shielding, while the thermal neutron detectors212,216A, and/or216B may not. In general, the detection of substantially only epithermal neutrons may allow the epithermal neutron detector214to measure the extent of a fast neutron distribution through most formations, and thus such a neutron count rate may be used to account for fast neutron transport through the formations in an NGD measurement.

Moreover, in formations with heavy elements, such as shales with high concentrations of iron or aluminum, the neutron detectors212,214,216A, and/or216B generally may not provide a neutron count rate that accurately reflects the fast neutron transport of such formations. Although it is believed that this deficiency could be addressed by using a fast neutron detector in the LWD module120, it may be difficult to implement such a fast neutron detector in a downhole tool. For example, it may be difficult to find a suitable sensor capable of working downhole that has high sensitivity and that is compact enough to fit within the LWD module120. As will be discussed below, for such formations with heavy elements, an NGD measurement obtained using the LWD module120may be corrected to approximate that which would be expected if the neutron detectors212,214,216A, and/or216B were fast neutron detectors.

The gamma-ray detectors218and/or220may be scintillator detectors surrounded by neutron shielding. The neutron shielding may include, for example,6Li, such as lithium carbonate (Li2CO3), which may substantially shield the gamma-ray detectors218and/or220from thermal neutrons without producing thermal neutron capture gamma-rays. The gamma-ray detectors218and220may detect inelastic gamma-rays generated when fast neutrons from the neutron generator206inelastically scatter off certain elements of a surrounding formation. As will be discussed below, a neutron-gamma density (NGD) measurement may be a function of the inelastic gamma-ray counts obtained from the gamma-ray detectors218and220, corrected for the fast neutron transport of the formation by the indirect measurement of neutron flux obtained from the neutron detectors212,214,216A, and/or216B. Using the systems and techniques disclosed herein, such an NGD measurement may provide enhanced accuracy to the system regardless of whether the formation is a formation with a high concentration of heavy elements or a formation that has one or more characteristics that may cause the count rate of neutrons not to accurately correspond to a fast neutron transport of the formation.

The count rates of gamma-rays from the gamma-ray detectors218and220and count rates of neutrons from the neutron detectors212,214,216A, and/or216B may be received by data processing circuitry200as data222. The data processing circuitry200may receive the data222and process the data222to determine one or more properties of the surrounding formation, such as formation density. The data processing circuitry200may include a processor224, memory226, and/or storage228. The processor224may be operably coupled to the memory226and/or the storage228to carry out the presently disclosed techniques. These techniques may be carried out by the processor224and/or other data processing circuitry based on certain instructions executable by the processor224. Such instructions may be stored using any suitable article of manufacture, which may include one or more tangible, computer-readable media to at least collectively store these instructions. The article of manufacture may include, for example, the memory226and/or the nonvolatile storage228. The memory226and the nonvolatile storage228may include any suitable articles of manufacture for storing data and executable instructions, such as random-access memory, read-only memory, rewriteable flash memory, hard drives, and optical disks.

The LWD module120may transmit the data222to the data processing circuitry200via, for example, internal connections within the tool, a telemetry system communication uplink, and/or a communication cable. The data processing circuitry200may be situated in the tool and/or at the surface. Accordingly, operations performed by the data processing circuitry200may be performed down-hole when the data processing circuitry200is situated in the tool or up-hole when the data processing circuitry200is situated at the surface. The data processing circuitry200may determine one or more properties of the surrounding formation. By way of example, such properties may include a neutron-gamma density (NGD) measurement of the formation. Thereafter, the data processing circuitry200may output a report indicating the NGD measurement of the formation. The report may be stored in memory or may be provided to an operator via one or more output devices, such as an electronic display.

As shown in a neutron-gamma density (NGD) well-logging operation240ofFIG. 3, the LWD module120may be used to obtain a neutron-gamma density (NGD) measurement that remains accurate in a variety of formations242. As seen inFIG. 3, the NGD well-logging operation240may involve lowering the LWD module120into the formation242through a borehole244. In the example ofFIG. 3, the LWD module120can be lowered into the borehole244while drilling, and thus no casing may be present in the borehole244. However, in other embodiments, a casing may be present. Although such casing could attenuate a gamma-gamma density tool that utilized a gamma-ray source instead of a neutron generator206, the presence of casing on the borehole244will not prevent the determination of an NGD measurement because neutrons246emitted by the neutron generator206may pass through casing without significant attenuation.

The neutron generator206may emit a burst of neutrons246for a relatively short period of time (e.g., 10 μs or 20 μs, or such) sufficient to substantially only allow for inelastic scattering to take place, referred to herein as a “burst gate.” The burst of neutrons246during the burst gate may be distributed through the formation242, the extent of which may vary depending upon the fast neutron transport of the formation242. For some formations242, counts of neutrons246obtained by the neutron detectors212,214,216A, and/or216B generally may accurately reflect the neutron transport of such formations242. However, for other formations242, such as formations with heavy elements, an additional correction based on an indirect measure of neutron flux may be used to more accurately account for the fast neutron transport of the formations242. This correction may be based on count rates of other detectors, ratios of the count rates of the other detectors, measured hydrogen index or ratios of the count rate of a principal neutron detectors212,214,216A, and/or216B with a count rate of another neutron detector212,214,216A, and/or216B, etc.

Further, even if the LWD module120does not include a fast neutron detector, a fast neutron signal may be accounted for by existing responses from the gamma-ray detectors218and220and the neutron detectors212,214,216A, and/or216B. In particular, elemental spectroscopy information may be obtained from the short spacing gamma-ray detector218. For example, the short spacing gamma-ray detector218may perform a capture gamma-ray spectroscopy operation and/or an inelastic gamma-ray spectroscopy operation by detecting a spectra of inelastic gamma-rays, and the data processing circuitry200may use the spectra to yield an estimate of iron, aluminum, potassium, chlorine, titanium, or other heavy elements within the formation242. Based on the elemental spectroscopy information obtained from the gamma-ray detectors218and/or220, in addition to the count rates of the neutron detectors212,214,216A, and/or216B, the fast neutron transport may be corrected for in determining the NGD of the formation242.

Many of the fast neutrons246emitted by the neutron generator206may inelastically scatter248against some of the elements of the formation242. This inelastic scattering248may produce inelastic gamma rays250, which may be detected by the gamma-ray detectors218and/or220. By determining a formation density by taking a ratio of inelastic gamma rays250detected using the two gamma-ray detectors218and220at different spacings from the neutron generator206, lithology effects may be mostly eliminated.

From count rates of the inelastic gamma rays250, one or more count rates of neutrons246, and a determination of the neutron output of the neutron generator206via the neutron monitor208, the data processing circuitry200may determine an initial estimate of electron density ρelectronof the formation242. In general, the electron density ρelectronmay be calculated according to a relationship that involves a function of a net inelastic count rate CRγinel, corrected by a neutron transport correction based on an indirect measure of neutron flux and a downhole tool calibration correction, which may be functions of one or more neutron count rate(s) CRneutronand the neutron output NSof the neutron generator206, respectively. For example, the electron density ρelectroncalculation may take the following form:

For some formations242, Equation (1) may result in an accurate density measurement. However, for other formations including formations242with relatively high concentrations of heavy elements (e.g., formations242having concentrations of heavy elements that may cause an NGD measurement to be inaccurate without additional correction), the neutron count rate from one or more of the neutron detectors212,214,216A, and216B may not adequately account for the fast neutron transport of such formations242. Thus, when an NGD measurement is being determined for such formations242, the neutron count rate CRneutron, the count rate of inelastic gamma-rays CRγinel, and/or the neutron transport correction function ƒ(CRneutron) may be corrected, as described by a flowchart260ofFIG. 4.

The flowchart260ofFIG. 4represents one embodiment of a method for carrying out the well-logging operation240ofFIG. 3. While the LWD module120is in the borehole244, the neutron generator206may emit a burst of neutrons246into the formation242(block262). The neutrons246may inelastically scatter248off certain elements of the formation242, generating inelastic gamma rays250. Count rate(s) of neutrons246as well as count rate(s) of inelastic gamma rays250may be obtained (block264). As discussed above with reference to Equation (1), such count rate(s) of neutrons246generally may relate well to the fast neutron transport of the formation242for some formations242encountered in an oil and/or gas well.

In other formations242, however, it is believed that the count rate(s) of neutrons246and/or the count rate(s) of gamma rays250may not adequately account for the neutron transport of such formations242. Thus, at block265, an elemental spectroscopy analysis is performed (e.g., a gamma-ray spectroscopy analysis) on the formation242. In particular, using information obtained from the elemental spectroscopy operation by the gamma-ray detector218(i.e., gamma-ray spectra detected by the gamma-ray detector218), an estimate of a concentration of iron, aluminum, potassium, chlorine, titanium, or other heavy elements within the formation242may be determined.

If, after receiving the information from the elemental spectroscopy analysis, the data processing circuitry200determines that the concentration of certain heavy elements exceeds a concentration threshold (decision block266), which indicates that the formation242has characteristics that imply need for correction, the data processing circuitry200may undertake a suitable correction of the count rate(s) of inelastic gamma rays250, and/or the neutron transport correction function ƒ(CRneutron), or may provide a global correction that applies to some or all of these terms. That is, it may be understood that modifying any of the terms in the numerator of Equation (1) could change the resulting NGD determination.

To assist in accounting for the effects of heavy elements in the formation242on the measured count rates, in block268, the data processing circuitry200may undertake any suitable correction of any of the terms of Equation (1), based on results of the elemental spectroscopy analysis, that may cause the NGD measurement to be generally accurate for the formation242. Using the spectroscopy element concentration estimates, the count rate correction, in block268, may take the form:
CRnet-inelasticLSn,Corrected=CRnet-inelasticLSn,UnCorrected−αYAl−βYFe−γYK− . . . −ζYZ(2),
where Z represents the remaining heavy elements that may have an effect on the NGD measurement, CRnet-inelasticLSn,Correctedis a corrected net inelastic count rate from the gamma-ray detector220, CRnet-inelasticLSn,UnCorrectedis an uncorrected net inelastic count rate from the gamma-ray detector220, Y is the elemental concentration of the indicated heavy element within the formation242determined by the elemental spectroscopy process, and α, β, γ, and ζ are sensitivity parameters that are constants relating to a number of detected gamma-rays per unit of elemental concentration. The constants α, β, γ, and ζ may be determined in a laboratory or using a simulation by measuring a number of gamma-rays detected by the gamma-ray detector220from a given element versus a known concentration of that element in the formation242. The concentrations Y are determined by analyzing the gamma-ray spectrum coming from the formation as a linear combination at each wavelength of standard spectra (determined in laboratory or by simulation) corresponding to each of the elements present in the formation. Applying the corrections for the heavy elements in the formation242may result in an NGD response that is corrected for the fast-neutron effects seen in the formations242containing high concentrations of heavy elements.

Moreover, at block268, the spectroscopy elemental concentrations Y of Equation (2) may be used to correct count rates of the neutron detectors212,214,216A, and/or216B in computing NGD spines (i.e., a neutron transport correction). Using the spectroscopy elemental concentration estimates Y, the neutron transport correction, in block268, may take the form:
CRNeutronCorrected=CRNeutronUncorrected−αYAl−βYFe−γYK− . . . −ζYZ(3),
where CRNeutronCorrectedrepresents the corrected neutron count rates from the near neutron detectors212and/or214or the far neutron detectors216A and/or216B, CRNeutronUncorrectedrepresents the uncorrected neutron count rates from the near neutron detectors212and/or214or the far neutron detectors216A and/or216B, and α, β, γ, and ζ are sensitivity parameters that are constants relating the number of detected neutrons per unit of elemental concentration. More generally, Equation (3) may be the following:

CRNeutronCorrected=CRNeutronUncorrected-∑i⁢αi⁢Yi
Similar to Equation (2), the constants α, β, γ, and ζ may be determined in a laboratory or using a simulation by measuring a number of neutrons detected by the near neutron detectors212and/or214or the far neutron detectors216A and/or216B from a given element versus a known concentration of that element in the formation242. Applying the corrections for the heavy elements in the formation242may result in an NGD response that is corrected for the fast-neutron effects seen in the formations242containing high concentrations of heavy elements. In Equation (1) the uncorrected first count of gamma-rays and/or the uncorrected second count of neutrons may be replaced respectively by the corrected first count of gamma-rays and/or the corrected second count of neutrons.

If the data processing circuitry200does not determine that the formation242has such characteristics (e.g., concentrations of heavy elements above a predetermined threshold), at decision block266, the data processing circuitry200may not apply such a correction. In some embodiments, the processing circuitry200may not calculate or apply the count rate corrections when a concentration of heavy elements is below a threshold amount. For example, there may be inherent noise that is greater than the count rate correction when the concentrations of some of the heavy elements are below the threshold. In such a situation, the count rate correction for the particular heavy elements may not provide much or any benefit in determining the NGD of the formation242.

In any case, the data processing circuitry200may subsequently determine an NGD measurement of the formation242using the determined count rate(s) of neutrons246, as well as the (corrected or uncorrected) count rate(s) of inelastic gamma rays250(block270), and/or the (corrected or uncorrected) neutron transport correction function ƒ(CRneutron), and output the corrected density (block272). By way of example, the data processing circuitry200may determine the NGD measurement based on the relationship represented by Equation (1).

It may be appreciated that while the NGD measurement inFIG. 4is determined at block270after the correction is applied to the count rate(s), an uncorrected NGD measurement may be calculated prior to application of the correction. When the uncorrected NGD measurement is calculated prior to application of the correction, the effect of the correction on the NGD measurement may be subtracted from the uncorrected NGD measurement to determine the corrected NGD measurement. That is, instead of applying corrections on a front-end of the NGD calculation process, the correction effects on the NGD measurement may be applied on the back-end, after the uncorrected NGD measurement is calculated.

Additionally, the corrections may be performed for the formations242that lack high concentrations of heavy elements (e.g., clean sandstone, limestone, dolomite, etc.), but the count rate corrections may be minor. Accordingly,FIG. 5is a flowchart273of an embodiment of a method for carrying out the well-logging operation240ofFIG. 3without a determination as to whether formation characteristics imply a need for correction. While the LWD module120is in the borehole244, the neutron generator206may emit a burst of neutrons246into the formation242(block274). The neutrons246may inelastically scatter248off certain elements of the formation242, generating inelastic gamma rays250. Count rate(s) of neutrons246as well as count rate(s) of inelastic gamma rays250may be obtained (block275). As discussed above with reference to Equation (1), such count rate(s) of neutrons246generally may relate well to the fast neutron transport of the formation242for some formations242encountered in an oil and/or gas well.

In other formations242, however, it is believed that the count rate(s) of neutrons246and/or the count rate(s) of gamma rays250may not adequately account for the neutron transport of such formations242. Thus, at block276, an elemental spectroscopy analysis is performed (e.g., a gamma-ray spectroscopy analysis) on the formation242. In particular, using information obtained from the elemental spectroscopy operation by the gamma-ray detector218(i.e., inelastic or capture gamma-ray spectra detected by the gamma-ray detector218), an estimate of a concentration of iron, aluminum, potassium, chlorine, titanium, or other heavy and non-heavy elements within the formation242may be determined.

To assist in accounting for the effects of heavy elements in the formation242, or other elements in the formation242that may have a smaller effect on a formation density determination, on the measured count rates, in block277, the data processing circuitry200may undertake any suitable correction of any of the terms of Equation (1), based on results of the elemental spectroscopy analysis, that may cause the NGD measurement to be generally accurate for the formation242. Using the spectroscopy element concentration estimates, the count rate correction, in block268, may take the form of Equation (2), above, where Z represents the remaining heavy elements that may have an effect on the NGD measurement, CRnet-inelasticLSn,Correctedis a corrected net inelastic count rate from the gamma-ray detector220, CRnet-inelasticLSn,UnCorrectedis an uncorrected net inelastic count rate from the gamma-ray detector220, Y is the elemental concentration of the indicated heavy element within the formation242or any other elemental concentration within the formation242determined by the elemental spectroscopy process, and α, β, γ, and ζ are sensitivity parameters that are constants relating to a number of detected gamma-rays per unit of elemental concentration. The constants α, β, γ, and ζ may be determined in a laboratory or using a simulation by measuring a number of gamma-rays detected by the gamma-ray detector220from a given element versus a known concentration of that element in the formation242. Applying the corrections for the elemental concentration makeup of the formation242may result in an NGD response that is corrected for the fast-neutron effects seen in the formations242containing various elemental concentrations.

Moreover, at block268, the spectroscopy elemental concentrations Y of Equation (2) may be used to correct count rates of the neutron detectors212,214,216A, and/or216B in computing NGD spines (i.e., a neutron transport correction). Using the spectroscopy elemental concentration estimates Y, the neutron transport correction, in block268, may take the form of Equation (3), above, where CRCorrectedNearNeutron,FarNeutronrepresents the corrected neutron count rates from the near neutron detectors212and/or214or the far neutron detectors216A and/or216B, CRUncorrectedNearNeutron,FarNeutronrepresents the uncorrected neutron count rates from the near neutron detectors212and/or214or the far neutron detectors216A and/or216B, and α, β, γ, and ζ are sensitivity parameters that are constants relating the number of detected neutrons per unit of elemental concentration. Similar to Equation (2), the constants α, β, γ, and ζ may be determined in a laboratory or using a simulation by measuring a number of neutrons detected by the near neutron detectors212and/or214or the far neutron detectors216A and/or216B from a given element versus a known concentration of that element in the formation242. Applying the corrections for the heavy elements and non-heavy elements in the formation242may result in an NGD response that is corrected for the fast-neutron effects seen in the formations242containing various elemental concentrations.

Further, in some embodiments, a weighting factor may be applied to the corrections when a concentration of an element is very low to minimize the effect of the correction on the count rate, and the full correction may be applied when the concentration is sufficiently high to enable the full effect of the correction on the count rate. It may also be appreciated that the fast neutron correction may be used as a shale indicator (i.e., to indicate the presence of aluminum or iron in the formation242). Accordingly, the fast neutron correction may be used along with other shale indicators (e.g., natural gamma-ray, natural gamma-ray spectroscopy, capture and inelastic spectroscopy, neutron activation, etc.) to refine analysis of the lithology of the formation242.

The data processing circuitry200may subsequently determine an NGD measurement of the formation242using the determined count rate(s) of neutrons246, as well as the (corrected or uncorrected) count rate(s) of inelastic gamma rays250(block278), and/or the (corrected or uncorrected) neutron transport correction function ƒ(CRneutron), and output the corrected density (block279). By way of example, the data processing circuitry200may determine the NGD measurement based on the relationship represented by Equation (1).

It may be appreciated that while the NGD measurement inFIG. 4is determined at block278after the correction is applied to the count rate(s), an uncorrected NGD measurement may be calculated prior to application of the correction. When the uncorrected NGD measurement is calculated prior to application of the correction, the effect of the correction on the NGD measurement may be subtracted from the uncorrected NGD measurement to determine the corrected NGD measurement. That is, instead of applying corrections on a front-end of the NGD calculation process, the correction effects on the NGD measurement may be applied on the back-end, after the uncorrected NGD measurement is calculated.

As mentioned above, although an NGD measurement such as determined using Equation (1) may accurately represent a density measurement for some formations242, such an NGD measurement may not be accurate for other formations242such as formations having a relatively high concentration of heavy elements. This effect is apparent in a crossplot280ofFIG. 6, which represents a crossplot modeling the known density of a variety of types of formations242against an NGD measurement for the formations242obtained using Equation (1) for which, for example, the neutron transport correction function ƒ(CRneutron) has not been corrected in the presence of, for example, a high concentration of heavy elements. In the crossplot280, an ordinate282represents the logarithm of a neutron-transport-corrected gamma-ray count rate as detected by the LS gamma-ray detector218, and an abscissa284represents electron density of the formation242in units of g/cc. A legend indicates various types of formations242that have been modeled in the crossplot280, including limestone, sandstone, dolomite, alumina, and sandstone with hematite. A line286represents an accurate correlation between the neutron-transport-corrected gamma-ray count rate and the known formation density.

As seen in the crossplot280, for certain formations242, despite variations in the densities of the formations242, the calculated logarithm of neutron-transport-corrected gamma-ray count rates lies along the line286and accurately corresponds to the known density. These points represent the general accuracy of the NGD determination for these formations242. However, for formations242that have heavy elements290A, the calculated logarithm of neutron-transport-corrected gamma-ray count rates lies above the line286. Since the calculated logarithm of neutron-transport-corrected gamma-ray count rates of these formations242with heavy elements290A does not follow the same function of change with density as the other formations242(i.e., not falling along the line286), NGD measurements for the heavy element formations290A obtained using the same (uncorrected) calculations as the other formations242may be inaccurate.

It is believed that insufficient fast neutron transport correction may be responsible for the inaccurate calculations for these formations with the heavy elements290A. Neutron transport corrections may be obtained by modifying, for example, the count rate(s) of inelastic gamma rays250and/or the neutron transport correction function ƒ(CRneutron) in a suitable manner, such that the calculated logarithm of neutron-transport-corrected gamma-ray count rates of the formations242that have heavy elements290A are shifted to their proper placement along the line286. Equations (2) and (3), discussed above in the discussion ofFIG. 4, provide the shifting mechanism for the count rates of the inelastic gamma rays250and the transport correction function ƒ(CRneutron), respectively.

The correction to the count rate(s) of inelastic gamma rays250, and/or the neutron transport correction function ƒ(CRneutron) that is applied in block268ofFIG. 4may depend on the indirect measurement of the fast neutron signal. In the crossplot300ofFIG. 7, which represents a crossplot modeling the known density of a variety of types of formations242against an NGD measurement for the formations242obtained using Equation (1) for which, for example, the count rate(s) of inelastic gamma rays250, and/or the neutron transport correction function ƒ(CRneutron) have been corrected in the presence of, for example, a high concentration of heavy elements. In the crossplot300, the ordinate282represents the logarithm of a neutron-transport-corrected gamma-ray count rate as detected by the LS gamma-ray detector218, and the abscissa284represents electron density of the formation242in units of g/cc. The legend indicates various types of formations242that have been modeled in the crossplot300, including limestone, sandstone, dolomite, alumina, and sandstone with hematite. The line286represents an accurate correlation between the neutron-transport-corrected gamma-ray count rate and the known formation density.

As seen in the crossplot300, despite variations in the densities of the formations242, the calculated logarithm of neutron-transport-corrected gamma-ray count rates lies along the line286and accurately corresponds to the known density. These points represent the general accuracy of the NGD determination for these formations242. Additionally, for formations242that have heavy elements290B, the calculated logarithm of neutron-transport-corrected gamma-ray count rates lies along the line286after applying the correction at blocks268and/or277. Neutron transport corrections may be obtained by modifying, for example, the count rate(s) of inelastic gamma rays250and/or the neutron transport correction function ƒ(CRneutron), for example using Equations (2) and (3), such that the calculated logarithm of neutron-transport-corrected gamma-ray count rates of the formations242that have heavy elements290B are shifted to their proper placement along the line286.

It may be appreciated that the techniques described above may be used in combination with other techniques for determinations of NGD measurements. For example, elemental spectroscopy corrections described above may be used in conjunction with a weighted least squares technique, a windowing technique, or various techniques for correcting for high concentrations of individual elements. In combining various techniques, the element spectroscopy corrections may provide an additional level of accuracy to the other techniques for determinations of NGD measurements.

Technical effects of the present disclosure include the accurate determination of a neutron-gamma density (NGD) measurement for a broad range of formations, including formations with heavy elements. These NGD measurements may remain accurate even when the configurations of a downhole tool used to obtain the neutron count rates and gamma-ray count rates used in the NGD measurement do not have optimal configurations. Thus, an accurate NGD measurement may be obtained using the systems and techniques disclosed above.