U.S. patent application number 14/635430 was filed with the patent office on 2015-06-18 for downhole neutron activation measurement.
The applicant listed for this patent is Schlumberger Technology Corporation. Invention is credited to Bradley A. Roscoe, Christian Stoller, Peter Wraight.
Application Number | 20150168592 14/635430 |
Document ID | / |
Family ID | 41466538 |
Filed Date | 2015-06-18 |
United States Patent
Application |
20150168592 |
Kind Code |
A1 |
Stoller; Christian ; et
al. |
June 18, 2015 |
Downhole Neutron Activation Measurement
Abstract
Systems and methods for measuring neutron-induced activation
gamma-rays in a subterranean formation are provided. In one
example, a downhole tool for measuring neutron-induced activation
gamma-rays may include a neutron source and a gamma-ray detector.
The neutron source may emit neutrons according to a pulsing scheme
that includes a delay between two pulses. The delay may be
sufficient to allow substantially all neutron capture events due to
the emitted neutrons to cease. The gamma-ray detector may be
configured to detect activation gamma-rays produced when elements
activated by the emitted neutrons decay to a non-radioactive
state.
Inventors: |
Stoller; Christian;
(Princeton Junction, NJ) ; Wraight; Peter;
(Skillman, NJ) ; Roscoe; Bradley A.; (West
Chesterfield, NH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Schlumberger Technology Corporation |
Sugar Land |
TX |
US |
|
|
Family ID: |
41466538 |
Appl. No.: |
14/635430 |
Filed: |
March 2, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12996541 |
Mar 30, 2011 |
8969793 |
|
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PCT/US09/48810 |
Jun 26, 2009 |
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14635430 |
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61077524 |
Jul 2, 2008 |
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Current U.S.
Class: |
250/269.6 |
Current CPC
Class: |
G01V 5/102 20130101 |
International
Class: |
G01V 5/10 20060101
G01V005/10 |
Claims
1. A downhole tool comprising: a neutron source configured to emit
neutrons according to a pulsing scheme, wherein the pulsing scheme
includes a delay between two pulses, wherein the delay is
sufficient to allow substantially all neutron capture events due to
the emitted neutrons to cease; and a gamma-ray detector configured
to detect activation gamma-rays produced when elements activated by
the emitted neutrons decay to a non-radioactive state.
2. The downhole tool of claim 1, wherein the delay is greater than
or equal to approximately 2 ms.
3. The downhole tool of claim 1, wherein the delay is greater than
or equal to approximately 1 s.
4. The downhole tool of claim 1, wherein the pulsing scheme is
configured to vary depending on a logging speed of the downhole
tool.
5. The downhole tool of claim 1, wherein the pulsing scheme is
configured to vary depending on whether the downhole tool is moving
or is approximately stationary.
6. The downhole tool of claim 1, wherein the pulsing scheme is
configured to be independent of a logging speed of the downhole
tool and configured to comprise a plurality of predetermined burst
patterns for a plurality of logging speeds.
7. The downhole tool of claim 1, wherein the pulsing scheme is
configured such that one of the pulses of the pulsing scheme is
subdivided into a plurality of microbursts.
8. The downhole tool of claim 7, wherein the plurality of
microbursts comprises a carbon/oxygen (C/O) microbursting scheme, a
gamma-ray spectroscopy microbursting scheme, a dual-burst
microbursting scheme, or a Sigma measurement microbursting scheme,
or any combination thereof.
9. The downhole tool of claim 7, wherein the plurality of
microbursts comprises less than or equal to approximately 50% of
the one of the pulses of the pulsing scheme and wherein a plurality
of delays between the plurality of microbursts comprises greater
than or equal to approximately 50% of the one of the pulses of the
pulsing scheme.
10. A downhole tool for use in a subterranean formation,
comprising: a neutron source configured to emit neutrons into the
subterranean formation in pulses separated by a delay of at least
approximately 2 ms; and a gamma-ray detector configured to detect
activation gamma-rays and either or both inelastic gamma-rays or
neutron capture gamma-rays resulting from interactions between the
emitted neutrons and the subterranean formation.
11. The downhole tool of claim 10, wherein the neutron source
comprises a d-D neutron generator or a d-T neutron generator, or a
combination thereof.
12. The downhole tool of claim 10, wherein the gamma-ray detector
is configured for detecting counts of the activation gamma-rays or
detecting spectra of the activation gamma-rays, or any combination
thereof.
13. The downhole tool of claim 10, comprising a second gamma-ray
detector, wherein the neutron source is disposed in the downhole
tool between the gamma-ray detector and the second gamma-ray
detector.
14. The downhole tool of claim 10, comprising a second gamma-ray
detector, wherein the second gamma-ray detector is disposed in the
downhole tool between the gamma-ray detector and the neutron
source.
15. A method comprising: injecting fracture fluid containing an
inert tracer material into a subterranean formation; emitting
neutrons into the subterranean formation to activate the tracer
material using a neutron generator configured to emit neutrons
according to a pulsing scheme that includes a delay between pulses
of at least approximately 2 ms; and detecting activation gamma-rays
from the activated tracer material using a gamma-ray detector.
16. The method of claim 15, wherein injecting the fracture fluid
comprises injecting fracture fluid containing the inert tracer
material, wherein the inert tracer material is configured to be
activated through thermal neutron capture and wherein the emitted
neutrons have energies sufficient to cause neutron capture events
but not to cause substantially any inelastic scattering events.
17. The method of claim 15, wherein the activation gamma-rays are
detected at least approximately 2 ms after an emitted neutron pulse
has ended.
18. The method of claim 15, comprising detecting either or both
inelastic gamma-rays or neutron capture gamma-rays resulting from
interactions between the emitted neutrons and the subterranean
formation or the tracer material.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of co-pending U.S. patent
application Ser. No. 12/996,541, filed Jun. 26, 2009, which is a
371 of International Application No. PCT/US09/48810, filed Jun. 26,
2009, which claims benefit of U.S. Provisional Patent Application
Ser. No. 61/077,524, filed Jul. 2, 2008. Each of the aforementioned
related patent applications is herein incorporated by
reference.
BACKGROUND
[0002] The present disclosure relates generally to well logging
with neutron-induced gamma-rays and, more particularly, to well
logging with neutron-induced activation gamma-rays.
[0003] Using nuclear downhole tools, the elemental composition of a
subterranean formation may be determined in a variety of ways. An
indirect determination of formation lithology may be obtained using
information from density and photoelectric effect (PEF)
measurements from gamma-ray scattering in the formation. A direct
detection of formation elements may be obtained by detecting
neutron-induced gamma-rays. Neutron-induced gamma-rays may be
created when a neutron source emits neutrons into a formation,
which may interact with formation elements through inelastic
scattering, high-energy nuclear reactions, or neutron capture.
[0004] As a result of inelastic or capture reactions, certain
formation nuclei may become radioactive. Each radioactive isotope
in the formation may have a characteristic half-life and a
characteristic decay path to a non-radioactive element. The decay
of most radioactive elements may be accompanied by the emission of
one or more characteristic gamma-rays. These characteristic
gamma-rays may be used to identify the element of the formation
that is decaying, and thus may indicate a unique formation element
that has been activated by inelastic scattering or neutron
capture.
[0005] Various formation measurements may be obtained based on the
above-described nuclear reactions. For example, fracture height
determination in a formation may be undertaken by injecting
radioactive tracer elements into a formation with fracture fluid
and proppant, subsequently measuring characteristic gamma-rays
emitted by the tracer. However, the use of a radioactive tracer may
introduce a number of regulatory, environmental, and other
challenges, as the radioactive tracer may be in liquid form and
thus easily dispersible. As such, certain techniques have been
developed to avoid the use of radioactive tracer in fracture height
determination. These techniques may involve the injection of an
inert liquid tracer into the formation, which may be subsequently
bombarded with neutron radiation to activate the tracer in the
liquid. In carrying out these techniques, however, the source of
the activating neutron radiation may be moved away from the point
of measurement, and the activation radiation may be measured at a
later time when a gamma-ray detector or other detector passes by
this point. In certain cases, the intervening time between
activation and measurement may allow materials in the
tracer-containing fracture fluid to move, which may result in an
incorrect interpretation of a formation fracture or other formation
properties.
SUMMARY
[0006] Certain aspects commensurate in scope with the originally
claimed embodiments are set forth below. It should be understood
that these aspects are presented merely to provide the reader with
a brief summary of certain forms the embodiments might take and
that these aspects are not intended to limit the scope of the
embodiments. Indeed, the embodiments may encompass a variety of
aspects that may not be set forth below.
[0007] Embodiments of the presently disclosed subject matter relate
generally to systems and methods for measuring neutron-induced
activation gamma-rays. For example, a downhole tool for measuring
neutron-induced activation gamma-rays may include a neutron source
and a gamma-ray detector. The neutron source may emit neutrons
according to a pulsing scheme that includes a delay between two
pulses. The delay may be sufficient to allow substantially all
neutron capture events due to the emitted neutrons to cease. The
gamma-ray detector may be configured to detect activation
gamma-rays produced when elements activated by the emitted neutrons
decay to a non-radioactive state.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Advantages of the present disclosure may become apparent
upon reading the following detailed description and upon reference
to the drawings in which:
[0009] FIG. 1 is a schematic block diagram of a system including a
downhole tool and data processing circuitry for measuring
neutron-induced gamma-rays, in accordance with an embodiment;
[0010] FIG. 2 is a schematic block diagram of a well logging
operation using the downhole tool of FIG. 1, in accordance with an
embodiment;
[0011] FIG. 3 is a neutron pulse diagram illustrating a neutron
pulsing scheme for the downhole tool of FIG. 1, in accordance with
an embodiment;
[0012] FIG. 4 is a neutron pulse diagram illustrating a
microbursting scheme for use in the neutron pulsing scheme of FIG.
3, in accordance with an embodiment;
[0013] FIG. 5 is a neutron pulse diagram illustrating another
neutron pulsing scheme, in accordance with an embodiment;
[0014] FIG. 6 is a flowchart describing an embodiment of a method
for obtaining gamma-ray measurements for determining a
characteristic of a subterranean formation using the downhole tool
of FIG. 1, in accordance with an embodiment;
[0015] FIG. 7 is flowchart of an embodiment of a method for
obtaining gamma-ray measurements for a fracture height
determination using the downhole tool of FIG. 1, in accordance with
an embodiment;
[0016] FIG. 8 is a plot illustrating a relative gamma-ray count
over time using the downhole tool of FIG. 1, in accordance with an
embodiment;
[0017] FIG. 9 is a flowchart of an embodiment of a method for
obtaining gamma-ray measurements at specific times, in accordance
with an embodiment; and
[0018] FIG. 10 is a flowchart of an embodiment of a method for
obtaining neutron-induced gamma-ray measurements with neutrons
supplied by d-D and d-T reactions, in accordance with an
embodiment.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0019] One or more specific embodiments are 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.
[0020] Embodiments of the presently disclosed subject matter relate
generally to systems and methods for neutron-induced gamma-ray well
logging. In particular, the presently disclosed subject matter
relates to activating nuclei of a subterranean formation by
bombarding the formation with neutrons, which may thereafter emit
gamma-rays ("activation gamma-rays") having characteristic spectra.
Unlike other techniques, the presently disclosed subject matter may
involve bombarding the subterranean formation with neutrons using a
neutron pulsing scheme that includes a specific delay between
pulses.
[0021] Using such a neutron pulsing scheme, neutrons may be emitted
into the formation for a specific amount of time, during which the
formation nuclei may become activated by inelastic scattering
events and/or neutron capture events. These events may produce
"inelastic gamma-rays" and "neutron capture gamma-rays,"
respectively, while the neutrons are being emitted into the
formation and for a short time afterward. The presence of the delay
in the neutron pulsing scheme may allow time for the inelastic
gamma-rays and neutron capture gamma-rays to die away, leaving
substantially only the activation gamma-rays from the activated
formation nuclei. The activation gamma-rays may then be detected
during the delay, rather than at a later time when the neutron
source has been moved away. Additionally or alternatively, the
presently disclosed subject matter may also enable the measurement
of inelastic gamma-rays and/or neutron capture gamma-rays in
conjunction with the activation gamma-rays.
[0022] With the foregoing in mind, FIG. 1 illustrates a system 10
for determining subterranean formation properties using activation
gamma-rays that includes a downhole tool 12 and a data processing
system 14. By way of example, the downhole tool 12 may be a
slickline or wireline tool for logging an existing well, or may be
installed in a borehole assembly for logging while drilling (LWD).
The data processing system 14 may be incorporated into the downhole
tool 12 or may be at a remote location. The downhole tool 12 may be
surrounded by a housing 16.
[0023] The downhole tool 12 may include a neutron source 18
configured to emit neutrons into a formation according to a neutron
pulsing scheme, as described below. By way of example, the neutron
source 18 may be an electronic neutron source, such as a
Minitron.TM. by Schlumberger Technology Corporation, which may
produce pulses of neutrons through d-D and/or d-T reactions. In
some embodiments, a neutron monitor 20 may monitor neutron
emissions from the neutron source 18 to more precisely observe the
quantity of neutrons emitted by the neutron source 18. The neutron
monitor 20 may be a plastic scintillator and photomultiplier that
may primarily detect unscattered neutrons directly from the neutron
source 18, and may provide a count rate signal proportional to the
neutron output rate from the neutron source 18. A neutron shield 22
may separate the neutron source 18 from various detectors in the
downhole tool 12. A similar shield 24, which may contain elements
such as lead, may prevent gamma-rays from traveling between the
various detectors of the downhole tool 12.
[0024] The downhole tool 12 may include one or more gamma-ray
detectors, and may include three or more gamma-ray detectors. The
downhole tool 12 illustrated in FIG. 1 includes two gamma-ray
detectors 26 and 28. The relative positions of the gamma-ray
detectors 26 and/or 28 in the downhole tool 12 may vary. In some
embodiments, the gamma-ray detectors 26 and 28 may be located on
opposite sides of the neutron source 18.
[0025] The gamma-ray detectors 26 and/or 28 may be contained in
respective housings 30. Scintillator crystals 32 in the gamma-ray
detectors 26 and/or 28 may enable detection counts or spectra of
gamma-rays by producing light when gamma-rays scatter or are
captured in the scintillator crystals 32. The scintillator crystals
32 may be inorganic scintillation detectors containing, for
example, NaI(Tl), LaCl.sub.3, LaBr.sub.3, BGO, GSO, YAP, and/or
other suitable materials. Housings 34 may surround the scintillator
crystals 32. Photodetectors 36 may detect light emitted by the
scintillator crystals 32 when a gamma-ray is absorbed and the light
has passed through an optical window 38. The gamma-ray detectors 26
and/or 28 may be configured to obtain a gamma-ray count and/or
gamma-ray spectra.
[0026] The signals from the neutron monitor 20 and gamma-ray
detectors 26 and/or 28 may be transmitted to the data processing
system 14 as data 40. 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. Processing may be done at least
in part by an embedded processor in the downhole tool.
[0027] In general, the data processing system 14 may include data
processing circuitry 44, which 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 44 may execute various operating system
instructions as well as software routines configured to effect
certain processes and stored in or provided by a manufacture
including 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 44 may process
data provided as inputs for various routines or software programs,
including the data 40.
[0028] Such data associated with the present techniques may be
stored in, or provided by, the memory or mass storage device of the
data processing system 14. Alternatively, such data may be provided
to the data processing circuitry 44 of the data processing system
14 via one or more input devices. In one embodiment, data
acquisition circuitry 42 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.
[0029] The downhole tool 12 may transmit the data 40 to the data
acquisition circuitry 42 of the data processing system 14 via, for
example, a telemetry system communication downlink or a
communication cable. After receiving the data 40, the data
acquisition circuitry 42 may transmit the data 40 to data
processing circuitry 44. In accordance with one or more stored
routines, the data processing circuitry 44 may process the data 40
to ascertain one or more properties of a subterranean formation
surrounding the downhole tool 12. Such processing may involve, for
example, one or more techniques for determining a formation
property based on activation gamma-rays and/or inelastic or neutron
capture gamma-rays. The data processing circuitry 44 may thereafter
output a report 46 indicating the one or more ascertained
properties of the formation. The report 46 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. By way of example,
the data processing circuitry 44 may determine a composition of a
subterranean formation based on activation gamma-rays, using such
techniques as discussed in U.S. Pat. No. 4,810,876, "LOGGING
APPARATUS AND METHOD FOR DETERMINING ABSOLUTE ELEMENTAL
CONCENTRATIONS OF SUBSURFACE FORMATIONS," and/or U.S. Pat. No.
5,237,594, "NUCLEAR ACTIVATION METHOD AND APPARATUS FOR DETECTING
AND QUANTIFYING EARTH ELEMENTS," both of which are assigned to
Schlumberger Technology Corporation and incorporated by reference
herein in their entirety.
[0030] FIG. 2 illustrates a neutron-induced gamma-ray well-logging
operation 48, which involves the placement of the downhole tool 12
into a surrounding subterranean formation 50. In the operation 48
depicted in FIG. 2, the downhole tool 12 has been lowered into an
existing well 52. The well-logging operation 48 may begin when the
neutron source 18 outputs a series of neutron bursts 54 according
to a neutron pulsing scheme incorporating an activation delay.
Suitable neutron pulsing schemes are discussed in greater detail
below. If the neutron source 18 emits neutrons produced via d-T
reactions, the neutron burst 54 may include neutrons of
approximately 14.1 MeV. These 14.1 MeV neutrons may collide with
nuclei in the surrounding formation 50 through inelastic scattering
events 56, which may produce inelastic gamma-rays 58 and may cause
the neutrons of the burst of neutrons 54 to lose energy. As the
neutrons of the burst of neutrons 54 lose energy, the neutrons may
be absorbed by formation 50 nuclei in neutron capture events 60,
which may produce neutron capture gamma-rays 62.
[0031] Both the inelastic scattering events 56 and the neutron
capture events 60 may cause the formation 50 elements involved in
the events 56 and/or 60 to activate 64, or become radioactive. Each
of the activated 64 radioactive isotopes may have a characteristic
half-life and a characteristic decay path to a non-radioactive
element. In particular, the decay of most radioactive elements of
the formation 50 may be accompanied by the emission of one or more
characteristic activation gamma-rays 66. Because the activation
gamma-rays 66 may correspond to the element that is decaying, by
detecting the activation gamma-rays 66, unique formation 50
elements may be identified.
[0032] If the neutron source 18 emits neutrons produced via d-D
reactions, the neutron burst 54 may include neutrons of
approximately 2.5 MeV. These 2.5 MeV neutrons may not cause
inelastic scattering events 56, which may require neutrons of
higher energy, but may cause neutron capture events 60. Thus, the
activation 64 that occurs, when the neutron burst 54 includes
substantially only neutrons produced via d-D reactions, may result
only from neutron capture events 60. Since certain formation 50
elements may be activated 64 only through neutron capture events
60, and certain other formation 50 elements may be activated only
through inelastic scattering events 56, certain techniques
described herein may involve multiple passes of the downhole tool
12 through the formation 50, during which neutron bursts 54 of
different energy levels are emitted.
[0033] The inelastic gamma-rays 58, neutron capture gamma-rays 62,
and/or activation gamma-rays 66 may be detected by the gamma-ray
detectors 26 and/or 28. As noted briefly above, the gamma-rays 58,
62, and 66 may be produced at different points in time after the
neutron burst 54. In particular, during and immediately after the
neutron burst 54, the gamma-ray detectors 26 and/or 28 may detect
mostly inelastic gamma-rays 58 and neutron capture gamma-rays 62.
However, following a sufficient delay after the neutron burst 54,
the gamma-ray detectors 26 and/or 28 may detect substantially only
activation gamma-rays 66, since the inelastic scattering events 56
and neutron capture events 60 may largely cease.
[0034] For this reason, among others, the neutron pulsing scheme
used by the neutron source 18 for emitting neutrons into the
formation 50 may include a delay between neutron bursts 54. The
delay between neutron bursts 54 may be sufficient to permit
substantially all inelastic gamma-rays 58 and neutron capture
gamma-rays 62 to die away (e.g., greater than 2 ms), leaving among
the neutron-induced gamma-rays substantially only activation
gamma-rays 66. A neutron pulsing scheme that incorporates such a
delay between pulses may be described in a neutron pulse diagram
68, shown in FIG. 3. The neutron pulse diagram 68 includes an
ordinate 70 indicating whether the neutron source 18 is set to ON
or OFF, and an abscissa 72 indicating time in unit of seconds
(s).
[0035] In the neutron pulse diagram 68 of FIG. 3, the neutron
bursts 54 may be pulsed from the neutron source 18 in a relatively
slow sequence. By way of example, a neutron ON time for a pulse 74
of the neutron pulsing scheme may be approximately one second, and
the neutron OFF time between pulses 74 may also be on the order of
approximately one second. The neutron OFF time between the neutron
pulses 74 may be shorter or longer than the neutron ON time for the
neutron pulses 74. By way of example, if a slower logging speed is
chosen, the neutron ON and OFF times may be lengthened. Moreover,
the neutrons need not be emitted at a constant rate during each
neutron pulse 74. Rather, a microbursting scheme, such as those
used in C/O logging, Sigma, or spectroscopy logging may be
employed. As such, each of the neutron pulses 74 may be subdivided
into multiple neutron microbursts and pauses representing such a
microbursting scheme, as discussed below. Moreover, more than one
distinct microbursting scheme may be employed in the neutron
pulsing scheme described by the neutron pulse diagram 68. In
particular, one neutron pulse 74 may be subdivided into a
microbursts and pauses representing a first bursting scheme, and
another neutron pulse 74 may be subdivided into a microbursts and
pauses representing a second bursting scheme.
[0036] FIG. 4 is a neutron pulse diagram 76 illustrating a
microbursting scheme that may be employed within pulses 74 of a
neutron pulsing scheme, such as the neutron pulsing scheme
described by the neutron pulse diagram 68 of FIG. 3. The neutron
pulse diagram 76 includes an ordinate 78, which indicates whether
the neutron source 18 is set to ON or OFF, and an ordinate 80,
which indicates time in units of microseconds (.mu.s). As shown by
the neutron pulse diagram 76, each neutron microburst 82 of ON time
may have a length of approximately 20 .mu.s, which may be followed
by approximately 80 .mu.s of OFF time. This sequence may be
repeated rapidly throughout a pulse 74. Alternative microbursting
schemes may be employed. Such schemes may include the dual-burst
scheme described in U.S. Pat. No. 4,926,044, "THERMAL DECAY TIME
LOGGING METHOD AND APPARATUS", or the scheme used in the EcoScope
tool by Schlumberger and described in U.S. Pat. No. 6,703,606
"NEUTRON BURST TIMING METHOD AND SYSTEM FOR MULTIPLE MEASUREMENT
PULSED NEUTRON FORMATION EVALUATION." Both of the above-referenced
patents are assigned to Schlumberger Technology Corporation and are
incorporated herein by reference in their entirety.
[0037] It should be understood that such microbursting schemes are
intended to be exemplary and not exhaustive, and that any number of
microbursting schemes may be employed during the neutron pulse 74.
Moreover, the microbursting scheme employed during the neutron
pulse 74 may be used to obtain additional measurements, such as
inelastic gamma-ray spectroscopy, capture gamma-ray spectroscopy,
and/or Sigma measurements. The particular measurements that may be
obtained may vary depending on the particular microbursting scheme
employed during the neutron pulse 74.
[0038] FIG. 5 is a neutron pulse diagram 84 describing another
neutron pulsing scheme for use by the neutron source 18 in the
downhole tool 12. The neutron pulse diagram 84 includes an ordinate
86, which indicates whether the neutron source 18 is set to ON or
OFF, and an ordinate 88, which indicates time in units of
microseconds (.mu.s). The neutron pulse diagram 84 may be divided
into an ON segment 90 and an OFF segment 92. During the ON segment
90, microbursts of neutrons may be emitted as the neutron source 18
is switched ON and OFF approximately 600 times with a period of 100
.mu.s. Thus, the neutron source 18 may be ON for 20 .mu.s, followed
by a pause of approximately 80 .mu.s, for each microburst during
the ON segment 90. During the OFF segment 92, which may last
between approximately 2 ms to 100 ms, the thermal neutron
population may disappear completely, such that substantially no
neutron capture gamma-rays 62 may be observed by the end of the OFF
segment 92.
[0039] FIG. 6 is a flowchart 94 describing an embodiment of a
method for obtaining gamma-ray measurements due to activation,
neutron capture, and/or inelastic scattering using the downhole
tool 12. In a first step 96, the downhole tool 12 may be moved
through the formation 50 via the well 52. As described above, the
downhole tool 12 may move through the well 52 on a wireline, a
slickline, or as part of a borehole assembly (BHA). The downhole
tool 12 may be moved through the formation 50 at a predetermined
logging speed, or the logging speed may vary based on the
particular neutron pulsing scheme applied or based on the resulting
gamma-ray measurements.
[0040] In step 98, the neutron source 18 may emit neutrons
according to a particular neutron pulsing scheme. The neutron
pulsing scheme may include, for example, any of the neutron pulsing
schemes or neutron microburst schemes described above with
reference to FIGS. 3-5, any suitable variations thereof, or any
neutron pulsing scheme incorporating a sufficient delay to allow
the detection of activation gamma-rays 66. The neutron pulsing
scheme applied during step 98 may include neutron pulses 74 and
delays short enough such that, following activation 64 of formation
50 nuclei, the gamma-ray detectors 26 and/or 28 may not have moved
substantially.
[0041] The neutron pulsing scheme applied in step 98 may or may not
be adapted to the logging speed of step 96. In one example, if the
neutron pulsing scheme is adapted to the logging speed, neutron
pulses 74 and OFF times between the neutron pulses 74 may be
proportional to the movement rate of the downhole tool 12 through
the formation 50. In another example, if the downhole tool 12
becomes stationary in the formation 50, the neutron pulsing scheme
applied in step 98 may be different from a neutron pulsing scheme
applied when the downhole tool 12 is currently moving through the
formation. If the neutron pulsing scheme applied in step 98 is not
adapted to the logging speed, the neutron pulsing scheme may be a
single, predetermined neutron pulsing scheme configured to
effectively activate 64 the formation 50 while permitting
sufficient time for the detection of resulting activation
gamma-rays 66. By way of example, such a predetermined neutron
pulsing scheme may involve multiple neutron pulses 74 of various
duration and/or including various microbursting schemes. The single
predetermined neutron pulsing scheme may include sufficient
variation to effectively enable a range of logging speeds for
activation gamma-ray 66 logging. For example, the predetermined
neutron pulsing scheme may include certain pulses 74 with
corresponding delays tailored for measurements at certain logging
speeds, and may include other pulses 74 with corresponding delays
tailored for stationary measurements.
[0042] As described above with reference to FIG. 1, the neutron
source 18 may be capable of generating neutrons at one or more
energy levels. For example, the neutron source 18 may be a d-T
neutron generator, capable of emitting 14.1 MeV neutrons, or a d-D
neutron generator, capable of emitting 2.5 MeV neutrons. As such,
the neutron pulsing scheme applied in step 98 may involve neutron
pulses or microbursts of substantially only 14.1 MeV neutrons, of
substantially only 2.5 MeV neutrons, or of both 14.1 MeV neutrons
and 2.5 MeV neutrons. As described above with reference to FIG. 2,
when a neutron burst 54 that occurs during a neutron pulse 74
includes the 14.1 MeV neutrons, inelastic scattering events 56 may
occur in the formation 50, producing inelastic gamma-rays 58, and
providing certain formation 50 elements a path to activation
64.
[0043] If substantially only 2.5 MeV neutrons are emitted in a
neutron burst 54 during a neutron pulse 74 of the pulsing scheme
applied in step 98, certain specific elements of the formation 50
may be determined more easily. At 2.5 MeV, the neutron burst 54 may
produce almost no inelastic scattering events 56, and thus neutron
capture events 60 may dominate. Thus, the activated 64 isotopes may
be limited almost entirely to those activated 64 by thermal neutron
capture events 60. This may eliminate, for example, the production
of .sup.28Al through the high-energy reaction
.sup.28Si(n,p).sup.28Al. As a result, the activation 64 of
.sup.27Al through the .sup.27Al(n,.gamma.).sup.28Al thermal capture
reaction 60 may be unambiguously detected.
[0044] The neutron pulsing scheme applied in step 98 may also
involve the use of a neutron source 18 that can produce neutrons
through d-T and d-D reactions in a separate controlled manner. This
may allow the separation of activation 64 caused by fast neutrons
of approximately 14.1 MeV (via inelastic scattering events 56) and
thermal neutrons of approximately 2.5 MeV (via neutron capture
events 60). The activation 64 may be accomplished using alternate
pulses 74 or alternate bursts 54 of these low and high energy
neutrons. Such a neutron generator 18 may also emit fast and
thermal neutrons in parallel, as disclosed in U.S. Patent
Application Serial No. 2007/839757 "DOWNHOLE TOOLS HAVING COMBINED
D-D AND D-T NEUTRON GENERATORS" assigned to Schlumberger Technology
Corporation and incorporated by reference herein in its
entirety.
[0045] In step 100, based on the neutron pulsing scheme applied in
step 98, resulting gamma-rays 58, 62, and/or 66 may be detected due
to inelastic scattering events 56, neutron capture events 60,
and/or activation events 64, respectively. For example, if the
neutron pulsing scheme applied in step 98 is similar to the neutron
pulsing scheme illustrated in FIG. 5 and the neutron source 18
emits neutrons of approximately 14.1 MeV, the gamma-ray detectors
26 and/or 28 may detect inelastic gamma-rays 58 and/or neutron
capture gamma-rays 62 throughout the ON segment 90. Meanwhile,
during the OFF segment 92, after the inelastic gamma-rays 58 and
neutron capture gamma-rays 62 have died away, the gamma-ray
detectors 26 and/or 28 may detect substantially only activation
gamma-rays 66. Due to the pulsing scheme applied in step 98, the
gamma-ray detectors 26 and/or 28 may detect the gamma-rays 58, 62,
and/or 66 before the downhole tool 12 has moved away from the
location in the formation 50 where the neutrons were emitted. Thus,
the downhole tool 12 may gain measurements of inelastic gamma-rays
58 and/or neutron capture gamma-rays 62 substantially
simultaneously with activation gamma-rays 66.
[0046] It should be understood that, in step 100, the gamma-ray
detectors 26 and/or 28 may obtain gamma-ray counts and/or measure
spectra of the gamma-rays 58, 62, and/or 66. In this way, the
neutron-induced inelastic gamma-ray 58 and/or neutron capture
gamma-ray 62 counts or spectra may be obtained in concert with
activation gamma-ray 66 counts or spectra. For example, the
obtained gamma-ray 58 and/or 62 spectra may be processed in the
data processing system 14 to enhance and/or complement the
information of the activation gamma-ray 66 spectra. Additionally,
if the neutron monitor 20 is present in the downhole tool 12, the
measured gamma-ray 58, 62, and 66 intensity may be related to the
total neutron output during the neutron pulsing scheme applied in
step 98.
[0047] Depending on the neutron pulsing scheme applied in step 98,
in step 100, the macroscopic formation capture cross section
(Sigma) may also be measured. Neutron pulsing schemes suitable for
a Sigma measurement are discussed briefly above with reference to
FIG. 4. The Sigma measurement may provide additional information
and may be important for various environmental corrections, and
particularly for the measurement of activation gamma-rays 66.
[0048] FIG. 7 is a flowchart 102 of an embodiment of a method for
obtaining activation gamma-ray 66 measurements for fracture height
determination in the formation 50. In a first step 104, a fracture
fluid containing an inert tracer material may be injected into
fractures inside a formation 50 proximate to a well, such as the
well 52. In step 106, the downhole tool 12 may be moved through the
formation via the well 52.
[0049] In step 108, a neutron pulsing scheme may be applied to
activate 64 the inert tracer materials in the fracture fluid. The
neutron pulsing scheme applied in step 108 may be any of the
neutron pulsing schemes described above with reference to FIGS.
3-5, as well as any variation of the neutron pulsing schemes
described above with reference to step 98 of FIG. 6. In particular,
in one embodiment, the neutron pulsing scheme 108 may involve
emitting the neutron burst 54 using only a d-D neutron generator,
which may emit substantially only 2.5 MeV neutrons. The use of
low-energy neutrons from the d-D reaction may be useful for the
detection of non radioactive tracers that may be activated 64 by
thermal neutron capture events 60, but not inelastic scattering
events 56.
[0050] In step 110, the gamma-ray detectors 26 and/or 28 may record
the gamma-ray response from the activated fracture fluid. It should
be appreciated that the activation gamma-rays 66 detected from the
activated fracture fluid may be used for determination of fracture
heights in the formation 50.
[0051] FIG. 8 is a plot 112 representing an exemplary gamma-ray
response that may result after bombarding the formation 50 with
neutrons emitted according to one of the neutron pulsing schemes
described herein. By way of example, the exemplary gamma-ray
response of the plot 112 may represent a count of gamma-rays
obtained during step 100 of the flowchart 94 or during step 110 of
the flowchart 102. In the plot 112, an ordinate 114 represents a
relative gamma-ray count including inelastic gamma-rays 58, neutron
capture gamma-rays 62, and activation gamma-rays 66. An abscissa
116 represents relative time, starting during a neutron pulse 74
and ending during a delay that follows the neutron pulse 74.
[0052] Time bins A, B, and C represent times during which only
certain gamma-rays may be observed. In particular, since time bin A
represents a time when the neutron source 18 is emitting neutrons
into the formation 50, during time bin A, the detected gamma-rays
may include mostly inelastic gamma-rays 58, but may also include
some neutron capture gamma-rays 62 and activation gamma-rays 66.
During time bin B, which may begin immediately after the final
neutron burst 54 of a neutron pulse 74, the detected gamma-rays may
include mostly neutron capture gamma-rays 62, but may also include
some activation gamma-rays 66. During time bin C, which may begin
following a delay sufficient to allow the neutron capture
gamma-rays 62 to die away, substantially only activation gamma-rays
66 may be detected. Time bin C may be further subdivided into time
bins based on the half-lives of various activated 64 isotopes of
the formation 50. Each of the subdivided time bins within time bin
C may correspond to certain isotopes that may be present and
activated 64 in the formation 50.
[0053] FIG. 9 is a flowchart 120 representing an embodiment of a
method for obtaining activation gamma-ray 66 measurements and
storing the gamma-ray measurements into specific time bins. In this
way, the temporal characteristics of the gamma-ray response
described above with reference to FIG. 8 may be employed to
identify the origination of certain detected gamma-rays. The method
of the flowchart 120 may or may not involve the injection of an
inert tracer in fracture fluid into the formation 50. In a first
step 122, the downhole tool 12 may be moved through the formation
50 via the well 52 in the same manner as described in steps 96 or
106. Similarly, in step 124, one of the neutron pulsing schemes may
be applied in the same manner as in steps 98 or 108.
[0054] Following the application of the neutron pulsing scheme in
step 124, in step 126, the gamma-ray detectors 26 and/or 28 may
detect the inelastic gamma-rays 58, neutron capture gamma-rays 62,
and/or activation gamma-rays 66 that result. In particular, when
the gamma-rays 58, 62, and/or 66 recorded by the gamma-ray
detectors 26 and/or 28, they may be stored in particular time bins.
The time bins may have equal lengths or may have lengths that vary.
For example, the length of the time bins may depend on the amount
of time that has passed since the last neutron burst 54 of a
neutron pulse 74. Additionally or alternatively, the length of the
time bins may vary depending on the logging speed of the downhole
tool 12 or depending on the pulsing scheme applied during step 124.
By way of example, the time bins may have relative lengths
comparable to the time bins A, B, and/or C shown in FIG. 8. As
another example, the lengths of the time bins corresponding to
measured activation gamma-rays 66 may be shorter or longer if the
pulsing scheme applied in step 124 involves primarily 2.5 MeV
neutrons or 14.1 MeV, as may be suitable.
[0055] FIG. 10 is a flowchart 128 representing an embodiment of a
method for obtaining activation gamma-ray 66 measurements using two
distinct neutron energy levels. Specifically, steps 130-134
represent a first pass through the formation 50, during which
substantially only 2.5 MeV neutrons may be emitted. Steps 136-140
represent a second pass through the formation 50, during which
substantially only 14.1 MeV neutrons may be emitted. The method of
the flowchart 120 may or may not involve the injection of an inert
tracer in fracture fluid into the formation 50.
[0056] In the first step 130 of the first pass through the
formation 50, the downhole tool 12 may be moved through the
formation 50 via the well 52 in the same manner as described in
steps 96 or 106. Similarly, in step 132, one of the neutron pulsing
schemes may be applied in the same manner as in steps 98 or 108. In
particular, in step 132, the neutron pulsing scheme applied in step
130 may involve emitting neutron bursts 54 using 2.5 MeV neutrons
from d-D reactions. The use of low-energy neutrons from d-D
reactions may be particularly useful for the detection of
non-radioactive tracers that may be activated 64 by thermal neutron
capture events 60, but not inelastic scattering events 56.
Additionally, however, the low-energy neutrons may activate 64 only
certain elements in the formation 50. For example, at 2.5 MeV, the
neutron burst 54 may produce almost no inelastic scattering events
56, and thus neutron capture events 60 may dominate. As such, the
activated 64 isotopes may be limited almost entirely to those
activated 64 by thermal neutron capture events 60. This may
eliminate, for example, the production of .sup.28Al through the
high-energy reaction .sup.28Si(n,p).sup.28Al. As a result, the
activation 64 of .sup.27Al through the
.sup.27Al(n,.gamma.).sup.28Al thermal capture reaction 60 may be
unambiguously detected. In step 134, measurements of resulting
neutron capture gamma-rays 62 and/or activation gamma-rays 66 may
be obtained.
[0057] In the first step 136 of the second pass through the
formation 50, the downhole tool 12 may be moved through the
formation 50 via the well 52 in the same manner as described in
step 130. Similarly, in step 138, one of the neutron pulsing
schemes may be applied in approximately the same manner as in step
132, except that the neutron pulsing scheme applied in step 138 may
involve emitting neutron bursts 54 using 14.1 MeV neutrons from d-T
reactions. The 14.1 MeV neutrons may cause both inelastic
scattering events 56 and neutron capture events 60. Thus, in the
second pass of steps 136-140, certain formation 50 elements and/or
tracer elements that may only become activated 64 via inelastic
scattering events 56, which were not activated during the first
pass of steps 130-134, may become activated 64. In step 140,
measurements of resulting inelastic gamma-rays 58, neutron capture
gamma-rays 62, and/or activation gamma-rays 66 may be obtained.
[0058] While only certain features have been illustrated and
described herein, many modifications and changes will occur to
those skilled in the art. It is, therefore, to be understood that
the appended claims are intended to cover all such modifications
and changes as fall within the true spirit of the present
disclosure.
* * * * *