Patent Application
20110313668
The present disclosure relates generally to neutron well logging and, more particularly, to techniques for reducing lithology error in neutron well logging.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Neutron well logging tools have been used to measure the porosity of oilfield subterranean formations for many years. Such devices generally include a neutron source and one or more neutron detectors. The neutron source may emit neutrons into the surrounding formation, which may be detected by the one or more neutron detectors in numbers that depend on the contents of the formation. In particular, the count of detected neutrons may be dominated by elastic scattering of the neutrons on hydrogen nuclei in the formation. Thus, all things being equal, when the formation includes more hydrogen, fewer neutrons may arrive at the one or more detectors.
As noted above, the porosity of the subterranean formation generally correlates with the quantity of hydrogen indicated by the neutron count, since the porosity of the formation may be typically filled with water or hydrocarbons. However, since some common downhole minerals contain bound water or hydroxyls, the neutron count may be more directly a measure of the hydrogen index. The hydrogen index represents a measure of the hydrogen content of the subterranean formation normalized to 100 for the amount of hydrogen in water at standard temperature and pressure. In addition to hydrogen index, however, the neutron count rate may also vary depending on, among other things, the concentration of elements in the formation besides hydrogen. Various techniques have been developed to attempt to minimize the lithology error due to these variations in formation composition by using epithermal detectors to avoid thermal neutron capture effects and by choosing a particular neutron detector spacing. However, these techniques may be inadequate for many formation compositions. In particular there may be no single optimal neutron detector spacing for minimizing the lithology effect on neutron porosity measurements.
A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below.
Embodiments of the present disclosure relate to systems, methods, and devices for determining the porosity of a subterranean formation with reduced lithology error. In one example, a downhole tool for such purposes may include a neutron source, a plurality of neutron detectors, and data processing circuitry. The neutron source may be configured to emit neutrons into a subterranean formation, and the plurality of neutron detectors may be configured to detect neutrons scattered from the subterranean formation. At least two of the plurality of neutron detectors may be disposed at different respective distances from the neutron source. The data processing circuitry may be configured to determine a porosity of the subterranean formation based at least in part on a weighted combination of the detector responses from each of the at least two of the plurality of neutron detectors.
Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which:
One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
Present embodiments relate to downhole neutron well logging tools. Such tools may include a neutron source and two or more neutron detectors at different respective distances from the neutron source. After lowering the downhole tool into a subterranean formation, neutrons emitted into the formation by the neutron source may interact with the formation in various ways. Among other things, the neutrons may elastically scatter off hydrogen nuclei in the formation. Accordingly, each of the neutron detectors of the downhole tool may obtain a count of neutrons that varies based on the number of hydrogen nuclei in the formation, which may be measured as the hydrogen index of the formation. As noted above, the hydrogen index represents a measure of the hydrogen content of the subterranean formation normalized to 100 for the amount of hydrogen in water at standard temperature and pressure.
The neutron counts of the neutron detectors mentioned above may be determined with respect to the total number of neutrons emitted by the neutron source. If a radioisotopic neutron source like 241AmBe is used for the neutron generation, its output can be determined through calibration in a known environment and the obtained calibration factor can be used to normalize the neutron counts. If the neutron source is an electronic neutron generator, such as a d-T generator, the neutron output may vary with time and/or operating conditions and a calibration may not be sufficient to obtain a normalization factor. In this case, the neutron output of the generator may be measured and the measurement used to derive the correct, time-varying normalization factor. In the following disclosure, neutron counts are assumed to be properly normalized through calibration or through the use of a monitor, measuring the instantaneous output of the neutron generator.
From these neutron counts, data processing circuitry may determine an apparent hydrogen index associated with each neutron detector. The apparent hydrogen index assumes “standard” well conditions (e.g., a calcite formation, fresh-water-filled porosity, 8 inch fresh-water-filled borehole, 20° C., 1 atm, and so forth). When the subterranean environment differs from these standard well conditions, the neutron counts may differ, and the apparent hydrogen index based on the neutron counts may differ from the actual hydrogen index. This difference between the actual hydrogen index and the apparent hydrogen index in the case where the formation lithology differs from the assumed standard lithology may be referred to as “lithology error.” Depending on the composition of the subterranean formation and the actual hydrogen index, the lithology error may be higher or lower when the neutron count is obtained at certain distances from the neutron source. That is, when the downhole tool includes neutron detectors at various distances from the neutron source, the apparent hydrogen indices derived from the counts of some neutron detectors may more accurately reflect the actual hydrogen index than those of other neutron detectors.
Embodiments of the present disclosure account for the lithology error present in the neutron detectors of the downhole tool in several manners. The embodiments may determine the actual hydrogen index based on the apparent hydrogen indices obtained by multiple neutron detectors at various distances. In certain cases, the apparent hydrogen index of one neutron detector may have a negative lithology error, the apparent hydrogen index of another neutron detector may have a positive lithology error, and the actual hydrogen index may lie somewhere between the two apparent hydrogen indices. In certain other cases, the apparent hydrogen index of two neutron detectors may both have positive lithology errors, one lithology error exceeding the other, and the actual hydrogen index may be extrapolated. The mathematical formulations given below account for both of the above cases and others.
To reduce lithology error, the actual hydrogen index may be computed using a suitably weighted combination of the apparent hydrogen indices obtained by each neutron detector. The weighting of each of the apparent hydrogen indices may be positive or negative and, in some embodiments, may be chosen to favor the apparent hydrogen index of the detector or detectors closer to the optimal spacing. In some embodiments, rather than simply depending on an “optimal” detector positioning, since the difference in apparent hydrogen index at the various spacings is itself a measure of the lithology error, the weighting of each of the apparent hydrogen indices may be considered a function of this difference. Among other things, such a function may include a polynomial. By way of example, such a polynomial may be chosen to be quadratic or cubic. Thus, as used herein, the terms “weight,” “weighted,” “weighting,” and so forth, refer to the application of coefficients to apparent neutron detector values (e.g., apparent hydrogen index, count rates, ratios of count rates, etc.) to correct for lithology errors. These coefficients may be any numerical value, including any positive and/or negative value.
With the foregoing in mind,
The downhole tool 12 may include a housing 16 to house the various components of the downhole tool 12. Among other things, such a component of the downhole tool 12 may include a neutron source 18. By way of example, the neutron source 18 may be an electronic neutron source, such as a Minitron™ by Schlumberger Technology Corporation, which may produce pulses of neutrons through d-T reactions. Additionally or alternatively, the neutron source 18 may be a radioisotopic source such as AmBe or 252Cf. Neutron shields 20 may prevent neutrons from the neutron source 18 from contaminating the responses of various neutron detectors 22 of the downhole tool 12. In some embodiments, similar neutrons shields may also be placed between the neutron detectors 22 and the borehole-facing side of the downhole tool 12. This may reduce the number of neutrons that may reach the neutron detectors 22 via the borehole, versus those reaching the detector via the formation, thus increasing the sensitivity of the downhole tool 12 to formation properties versus those of the borehole.
As illustrated in
Each of the neutron detectors 22 may be separated from the neutron source 18 by a particular spacing measured from the neutron source 18 to the face nearest to the neutron source of the active region of the neutron detector 22, from a spacing 1, to a spacing i, to a spacing n. By way of example, suitable spacings may include 7 inches, 11 inches, 15 inches, 19 inches, and/or 23 inches from the neutron source 18. As should be appreciated, these spacings are intended to be exemplary and not exhaustive. As described in greater detail below, for a given lithology of a surrounding formation, a detector response from one of the neutron detectors 22 at a particular spacing may provide an apparent hydrogen index that more closely resembles the actual hydrogen index than the other neutron detectors 22 at other spacings. When the neutron source 18 includes an electronic neutron source, the downhole tool 12 may also include a neutron monitor 23. The neutron monitor 23 may measure the output of the neutron source 18 to provide a basis for normalizing the neutron counts detected by the neutron detectors 22. The neutron monitor 23 may be a plastic scintillation detector, sensitive only to high energy neutrons of energy levels emitted by the electronic neutron source 18, and may be located very close to the neutron source 18. In some embodiments, the neutron monitor 25 is not included, and the response of the downhole tool 12 may be based solely on ratios of count rates between the neutron detectors 22.
The responses of the neutron detectors 22 and/or the neutron monitor 23 may be provided to the data processing system 14 as data 24. The data processing system 14 may include a general-purpose computer, such as a personal computer, configured to run a variety of software, including software implementing all or part of the present techniques. Alternatively, the data processing system 14 may include, among other things, a mainframe computer, a distributed computing system, or an application-specific computer or workstation configured to implement all or part of the present technique based on specialized software and/or hardware provided as part of the system. Further, the data processing system 14 may include either a single processor or a plurality of processors to facilitate implementation of the presently disclosed functionality. For example, processing may take place at least in part by an embedded processor in the downhole tool 12.
In general, the data processing system 14 may include data acquisition circuitry 26 and data processing circuitry 28. The data processing circuitry 28 may be a microcontroller or microprocessor, such as a central processing unit (CPU), which may execute various routines and processing functions. For example, the data processing circuitry 28 may execute various operating system instructions as well as software routines configured to effect certain processes. These instructions and/or routines may be stored in or provided by a manufacture, which may include a computer readable-medium, such as a memory device (e.g., a random access memory (RAM) of a personal computer) or one or more mass storage devices (e.g., an internal or external hard drive, a solid-state storage device, CD-ROM, DVD, or other storage device). In addition, the data processing circuitry 28 may process data provided as inputs for various routines or software programs, including the data 24.
Such data associated with the present techniques may be stored in, or provided by, a memory or mass storage device of the data processing system 14. Alternatively, such data may be provided to the data processing circuitry 28 of the data processing system 14 via one or more input devices. In one embodiment, data acquisition circuitry 26 may represent one such input device; however, the input devices may also include manual input devices, such as a keyboard, a mouse, or the like. In addition, the input devices may include a network device, such as a wired or wireless Ethernet card, a wireless network adapter, or any of various ports or devices configured to facilitate communication with other devices via any suitable communications network, such as a local area network or the Internet. Through such a network device, the data processing system 14 may exchange data and communicate with other networked electronic systems, whether proximate to or remote from the system. The network may include various components that facilitate communication, including switches, routers, servers or other computers, network adapters, communications cables, and so forth.
The downhole tool 12 may transmit the data 24 to the data acquisition circuitry 26 of the data processing system 14 via, for example, internal connections within the downhole tool 12, a telemetry system communication downlink or a communication cable. In some embodiments, the data acquisition circuitry 26 may be located within the downhole tool, and the data processing circuitry 28 may be downhole, uphole, or in an office. After receiving the data 24, the data acquisition circuitry 26 may transmit the data 24 to the data processing circuitry 28 via, for example, a telemetry system communication downlink or a communication cable. In accordance with one or more stored routines, the data processing circuitry 28 may process the data 24 to ascertain one or more properties of a subterranean formation surrounding the downhole tool 12, such as hydrogen index or porosity. Such processing may involve, for example, normalizing and/or calibrating the neutron counts, determining an apparent hydrogen index from the counts of each neutron detector 22, and weighting and summing the apparent hydrogen indices. The data processing circuitry 28 may thereafter output a report 30 indicating the one or more ascertained properties of the formation. The report 30 may be stored in memory or may be provided to an operator via one or more output devices, such as an electronic display and/or a printer.
The interactions 42 of the neutron emission 40 with elements of the subterranean formation 34 may include, for example, inelastic scattering, elastic scattering, and neutron capture. The interactions 42 may result in neutrons 44 from the neutron emission 40 traveling through the subterranean formation 34 in varying numbers. Depending on the composition of the subterranean formation 34, the interactions 42 may vary, and the numbers of the neutrons 44 that reach the neutron detectors 22 at different distances from the neutron source 18 may also vary. As such, the different neutron detectors 22 may obtain correspondingly differing counts of the neutrons 44.
From the neutron counts obtained in the neutron well logging operation 32 of
As mentioned above, different compositions of the subterranean formation 34 may result in different interactions 42, which may cause the neutron counts obtained by the neutron detectors 22 to differ from those obtained under standard well conditions. As a result, the apparent hydrogen indices obtained by the neutron detectors 22 at different spacings may vary from the actual hydrogen index with the composition of the subterranean formation. As illustrated in
Turning first to
In
Finally,
As illustrated in
The neutron well logging system 10 may employ the relationships illustrated by
φ=Σwi(φ)φi (1),
where wi represents weighting factors that may be chosen to favor the apparent hydrogen index φi of the neutron detector 22 most optimal for the current (as yet unknown) actual hydrogen index φ.
By way of example, responses from two of the neutron detectors 22 of the downhole tool 12 may be used to determine the actual hydrogen index φ. The neutron detectors 22 may be respectively located 1 ft and 2 ft from the neutron source 18 (respectively referred to as the near neutron detector 22 and the far neutron detector 22). Under such conditions, the weighting factors should be chosen such that
wnear≈0 & wfar≈1 at low φ
As such, when the actual hydrogen index is low (e.g., near to 0), the apparent index of the far neutron detector 22 is weighted most heavily because, regardless of the composition of the subterranean formation 34, the far neutron detector 22 will have the least lithology error. When the actual hydrogen index is high (e.g., 40 or greater), the apparent index of the near neutron detector 22 is weighted most heavily because, regardless of the composition of the subterranean formation 34, the near neutron detector 22 will have the least lithology error. When the actual hydrogen index is an intermediate value (e.g., between approximately 10 and 40), the value of the weighting factors may be selected between 0 and 1. As may be appreciated, however, the weighting factors described above depend upon the actual hydrogen index φ, which is not known a priori. As such, in step 84, the actual hydrogen index may be solved for iteratively in the data processing system 14 using any numerical method. By way of example, the data processing system 14 may use Newton's method to obtain an approximate value of actual hydrogen index in step 84.
The techniques described above with reference to
φi−φj,
or, more simply, in the two detector case,
φnear−φfar,
which represents a measure of a lithology error and, therefore, an expected dependency.
φ=Σƒk(φ, φ1−φ2, . . . , φi−φj, . . . φn-1−φn)φk (3).
More simply, when responses from only two neutron detectors 22 are used, and approximating the true actual hydrogen index φ in the coefficients with the average of the apparent hydrogen indices, the following may be expected:
φ=ƒnear(φnear+φfar,φnear−φfar)φnear+ƒfar(φnear+φfar,φnear−φfar)φfar (4).
Although various functional forms may be chosen for the functions fi, the simplest choice may be to approximate them as polynomials in the sum and differences of the apparent hydrogen indices φi obtained by the neutron detectors 22 at different spacings. By way of example, in the case involving only two neutron detectors 22, the following relationship may be employed:
φ=(Σ aij(φnear+φfar)i(φnear−φfar)j)φnear+(Σ bij(φnear+φfar)i(φnear−φfar)j)φfar (5),
or, equivalently:
φ=Σ cij(φnear)i(φfar)j (6),
where the coefficients cij may be derived during the characterization of the downhole tool 12 in various experimental and/or modeled settings. In certain embodiments, the coefficient c00=0, though this is not a necessary condition. Also, in some embodiments, in the case where φnearφfar, a first set of coefficient cij may be used and a different set of coefficient dij may be used for the case where φnear
φfar. In some embodiments, the first set of coefficients cij may include terms up to cubic and the second set of coefficients dij may include terms only up to quadratic. In general, the coefficients cij and/or dij may be constrained to make the derived hydrogen index φ continuous at φnear=φfar.
It should be further noted that, since the apparent hydrogen indices φj are computed from the respective individual neutron detector 22 count rates, an alternative expression for improved multi-neutron-detector 22 hydrogen index could also be derived directly from these count rates. The optimal “weighting functions” or coefficients also may depend on the borehole 36 conditions (e.g., borehole size, borehole fluid composition, and so forth). These techniques may be extended to account for those other conditions, and may involve, for example, including terms in the polynomial expansion that depend on borehole size.
Following the step 98 of the flow chart 94, the data processing system 14 may solve for the actual hydrogen index φ in step 100. While solving for the actual hydrogen index φ, if only two neutron detectors 22 are employed, Equation (6) may be used in step 98. Thus, in step 100, the prederived coefficients cij may be used to compute the hydrogen index φ from the apparent hydrogen indices using Equation (6).
φfar and a different set of coefficients dij was used for the case where φnear
φfar. In particular, for the φnear
φfar case, terms up to cubic were used, as might be expected from Equation (6) above and the nonlinear hydrogen index dependence of the lithology error in some minerals as seen in
φfar case, terms up to quadratic were used.
Turning to
As can be seen from a comparison of
The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.
This application claims the benefit of U.S. Provisional Application No. 61/115,754, filed on Nov. 18, 2008.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US2009/064812 | 11/17/2009 | WO | 00 | 9/6/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61115754 | Nov 2008 | US |
