Note: Descriptions are shown in the official language in which they were submitted.
260710
1
METHOD FOR INSPECTING A
SUBTERRANEAN TUBULAR
BACKGROUND
1. Field of Invention
[0002] The invention relates generally to inspecting tubulars in a
subterranean wellbore.
More specifically, the present invention relates to a device and method that
uses a radiation
source for inspecting a subterranean tubular and a radiation detector for
detecting or
identifying the presence of an unwelcome or egregious substance or substances
deposited
in and/or adjacent the tubular.
2. Description of Prior Art
[0003] Subterranean wellbores used for producing hydrocarbons typically are
lined with
a casing string that is cemented to the formation intersected by the wellbore.
An inner casing
string may also be inserted within the first casing string and cemented in
place. Fluid
produced from the well flows to the surface within production tubing that is
inserted inside
the inner casing string. Over the life of a typical well the production tubing
may be removed
so that remediation, repair, or flow enhancement operations may be conducted
in the well.
There may also be a need at some time for removal of a portion or for all of
the casing.
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[0004] Generally drilling fluids fill the annular space between the concentric
tubulars.
Particulates, such as barite, within the drilling fluids may settle out or
precipitate over time and
form a cement like substance that binds together the concentric tubulars and
prevents the
removal of the inner tubular from the wellbore. While cutting tools can sever
the tubulars to
enable removing the unstuck portion, the tubular cannot be removed if the cut
is made at a depth
below where the tubulars are adhered together. Alternatively, too shallow a
cut can leave an
undesirably long portion of free pipe extending above the point of adhesion.
Asphaltene or sand
mixed with heavy hydrocarbons can form blockages inside the production
tubulars which will
limit well production capability. Junk-baskets, gauge rings and dummy tools
run through the
production tubing have been used to look for blockages.
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SUMMARY OF THE INVENTION
[0005] A method of inspecting a tubular in a subterranean wellbore is
disclosed. In one example
the method includes directing radiation from a radioactive source positioned
in a logging tool
into the adjacent sidewall of the tubular, detecting radiation scattered from
a material in the
annulus adjacent the tubular, estimating a rate and energy of the detected
radiation, and
identifying the substance based on the rate and energy of the detected
radiation. In one example,
the radiation is a gamma ray and the source is a 137Cs gamma ray source having
energy of about
662 keV. In this example, the Compton scattered radiation when detected has
energy of from
about 250 keV to about 650 keV. In one example, the step of detection is
performed using a
detector axially offset from the source. The emitted radiation can be directed
in a substantially
conical pattern from the source and wherein the energy of the detected
radiation is dependent
upon an angle of scatter of the radiation. The substance may be asphaltene in
the annulus and
adhered to the tubular, scale deposited in the annulus adjacent the tubular,
sand on the tubular, as
well as combinations thereof. The method can further include estimating a
location of the
substance, and help in removing the substance from the tubular based on the
steps of identifying
the substance and estimating the location of the substance. Optionally, the
substance can be a
production fluid inside or other deposits in the tubular, and where the
tubular is production
tubing.
[0006] Also provided herein is a method of imaging a subterranean wellbore. In
this example
method a logging instrument is provided that has a radiation source and a
scattered radiation
detector. In this example the method further includes introducing the logging
instrument in a
tubular that is inserted into the wellbore, directing radiation from the
source so that some of the
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radiation scatters from materials adjacent the tubular to define scattered
radiation, detecting the
scattered radiation with a scattered radiation detector, and identifying the
substance based on a
rate and energy of the scattered radiation detected. Optionally, a conically
shaped guide is
provided close to the radiation source and positioned in the logging
instrument so that an apex of
the guide is directed towards the source and the guide has an axis that is
substantially parallel
with an axis of the tubular. In an alternate embodiment, the energy of the
scattered radiation
corresponds to an angle of scatter of the detected radiation. In one example,
the material is one
or more of asphaltene, a paraffin, scale, sand, or combinations thereof.
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BRIEF DESCRIPTION OF DRAWINGS
[0007] Some of the features and benefits of the present invention having been
stated, others will
become apparent as the description proceeds when taken in conjunction with the
accompanying
drawings, in which:
[0008] FIG. I a schematic of an example embodiment of a downhole imaging
tool having a
low energy radiation source and detectors disposed in a wellbore in accordance
with the present
invention.
[0009] FIG. 2 is a graph of an example of energy dependence of a single
Compton-scattered
0.662 MeV Cesium-137 gamma ray versus the scattering angle in accordance with
the present
invention.
100101 FIG. 3 is a perspective view of one embodiment of the tool of FIG.
1.
[0011] FIGS. 4A and 4B are sectional views of an example embodiment of the
tool of FIG. 3.
[0012] FIG. 5 is a graph of a single detector rate/logging response to an
anomaly in a gravel-
pack sand completion versus depth measured by an example embodiment of an
imaging tool in
accordance with the present invention.
100131 FIG. 6 is a partial side sectional view of an example of an imaging
tool in a tubular in
accordance with an embodiment of the present invention.
[0014] FIG. 7 is an example of a spectrum of count intensity versus count
energy in
accordance with an embodiment of the present invention.
260710
6
[0015] FIG. 8 is an example of a graph representing a high energy window (W6)
count rate
response to asphaltene deposits inside a production tubular in accordance with
an
embodiment of the present invention.
[0016] FIG. 9 is a schematic example of the imaging tool of FIG. 6 and a
single scatter region
of high energy radiation in accordance with an embodiment of the present
invention.
[0017] While the invention will be described in connection with the preferred
embodiments,
it will be understood that it is not intended to limit the invention to that
embodiment. On the
contrary, it is intended to cover all alternatives, modifications, and
equivalents, as may be
included within the scope of the invention as defined by the appended claims.
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DETAILED DESCRIPTION OF INVENTION
[0018] The method of the present disclosure will now be described more fully
hereinafter with
reference to the accompanying drawings in which embodiments are shown. The
method of the
present disclosure may be in many different forms and should not be construed
as limited to the
illustrated embodiments set forth herein; rather, these embodiments are
provided so that this
disclosure will be thorough and complete, and will fully convey its scope to
those skilled in the
art. Like numbers refer to like elements throughout.
[0019] It is to be further understood that the scope of the present disclosure
is not limited to the
exact details of construction, operation, exact materials, or embodiments
shown and described, as
modifications and equivalents will be apparent to one skilled in the art. In
the drawings and
specification, there have been disclosed illustrative embodiments and,
although specific terms
are employed, they are used in a generic and descriptive sense only and not
for the purpose of
limitation. Accordingly, the improvements herein described are therefore to be
limited only by
the scope of the appended claims.
[0020] Referring now to FIG. 1 a downhole imaging tool 100 is shown
positioned in a "base
pipe" or inner steel housing 110 of a gravel pack. It is recognized that a
tool housing 130 may be
constructed of any light metal wherein the term, "light metal," as used
herein, refers to any metal
having an atomic number less than 23. Downhole imaging tool 100 includes at a
minimum a
housing or pipe 130 carrying a radiation source 120 and plurality of detectors
140. In one
example embodiment, gamma radiation source 120 is centrally located in housing
130.
Optionally, detectors 140 are symmetrically spaced apart azimuthally at a
constant radius, but
also positioned within housing 130. In other words, in one example, the radius
on which
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detectors 140 are spaced apart is less than the radius of the housing 130.
Radiation source 120
emits radiation, in this case, gamma rays 124 into gravel pack 150.
[0021] The textured pattern of gravel pack 150 indicates possible regions of
gravel pack that
could be gravel-filled or not. For example, center region 151 may constitute a
void in gravel
pack 150 that has been filled with completion fluids or production fluids
whereas other regions
153 may constitute portions of the gravel pack that are properly completed or
fully sand filled.
Of course, those skilled in the art, with the benefit of this disclosure, will
appreciate that these
are for illustrative purposes only and that a void or vug could take any shape
and any position
relative to tool 100.
[0022] In the example of FIG. 1, gamma rays 124 propagating into gravel
pack 150 are
Compton scattered (as at point 155), with a loss of some energy, back towards
detectors 140
located within downhole imaging tool 100. The lower-energy gamma rays 126 are
detected by
detectors 140. The count-rate intensity of Compton scattered gamma rays 126
depends on,
among other factors, the density of the gravel pack material. Hence, higher
count rates represent
higher density in the gravel pack, whereas lower count-rates represent lower
density as a result of
fewer gamma rays being back-scattered towards the detectors.
[0023] In an example, radiation source 120 includes barium, cesium, some other
radiation
source, or combinations thereof. By utilizing a source such as this, and
because the detectors are
located close to the source, detected energy originates only from a short
distance into the gravel
pack immediately adjacent a screen. For these same reasons, in one example
detectors 140 are
positioned in housing 130 proximate to radiation source 120. In one example
embodiment,
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radiation source 120 and detectors 140 are within about 3 to about 3.5 inches
apart along the
length of tool 100.
100241 Shielding (not shown in FIG. 1) may be applied around radiation
source 120 to
collimate or otherwise limit the emission of radiation from radiation source
120 to a restricted
longitudinal segment of gravel pack 150. In an embodiment, such shielding is a
heavy metal
shield, such as sintered-tungsten, which collimates the pathway for the
emitted gamma rays into
the gravel pack. Likewise, as described in more detail below, similar
shielding may be used
around each detector to limit the detector viewing aperture to only those
gamma rays that are
primarily singularly-scattered back to the detector from a specific azimuthal
section of the gravel
pack.
100251 Further, the energy levels of the emitted gamma rays 124 may be
selected to assess
gravel pack density at varying depths or distances from downhole imaging tool
100. As one
example, the radiation from a gamma ray source, such as a I33Ba source, may be
used to emit
various energy levels. Alternatively, a gamma ray radiation source with energy
close to that of
l37Cs may be used.
100261 In addition, the energy of Compton-scattered gamma rays depends on the
scattering
angle, as shown in FIG. 2. In an example, a careful choice of collimation
angles and energy
detection range can be used to discriminate scattering from different regions
in the well-bore
tubulars. Higher-energy single-scattered Compton gamma-rays, such as that
shown in FIG. 2,
come from shallow-angle scattering, and can be used to sense scattering
materials close to the
tool, such as asphaltene deposits inside the production tubing. In an optional
embodiment, the
support volume for the Compton-scattered gamma rays is defined by the
scattering angle and the
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detected gamma-ray energy so that different energy windows in the logging tool
can sense
different regions in the well bore.
100271 Techniques exist for converting radiation count rates from multiple
detectors positioned
axially around the logging tool into a complete 2D profile map of the gravel
pack integrity
include the SYSTAT's Table Curve 3D method. Other techniques include, but are
not limited to,
MATLAB, IMAGE, and advanced registration and techniques for making mosaic
representations from data points can be used to map the base-pipe and gravel-
pack environment.
Also, 3D geostatistical-based software can be adapted to convert the basic
gamma-ray count
rates to generate a map of the gravel-pack environment. In this way, the
integrity of a gravel
pack or formation may be determined.
100281 To produce accurately oriented maps, the azimuthal angle of the
logging tool relative
to the high side of the borehole is determined. This orientation can be
determined using any
orientation device known in the art. Orientation devices may contain one or
more attitude
sensors used to determine orientation of the logging tool with respect to a
reference vector.
Examples of suitable orientation devices include, but are not limited to,
those orientation devices
produced by MicroTesla of Houston, Tex. Each set of gamma ray measurements may
be
associated with such an orientation so that a 2D profile map of the gravel
pack can be accurately
generated in terms of the actual azimuthal location of the material in the
gravel pack.
[00291 FIG. 3 illustrates a perspective view of one embodiment of a gravel
pack imaging tool.
As shown, downhole imaging tool 200 includes a housing 230 which carries
radiation source
220, source collimator 225, and a plurality of radiation detectors 240 in an
array. The array of
detectors 240 may be positioned at a fixed distance from radiation source 220.
In certain
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embodiments, detector arrays may be positioned at differing distances from
radiation source 220.
Additionally, detector arrays on either side of radiation source 220 are also
envisioned in certain
embodiments. Electronics 260 may also be located in housing 230 or wherever
convenient.
[0030] Radiation source 220 may be one or more radiation sources, which may
include any
suitable low-energy gamma ray source capable of emitting gammy ray radiation
from about 250
keV to about 700 keV. Gamma ray sources suitable for use with embodiments of
the present
invention may include any suitable radioactive isotope including, but not
limited to, radioactive
isotopes of barium, cesium, a LINAC, high energy X-rays (e.g. about 200+ keV),
or any
combination thereof. Radiation from radiation source 220 may be continuous,
intermittent, or
pulsed.
[0031] In one example embodiment shown in FIG. 3, a radiation source 220 is
centrally
located in housing 230. In the illustrated embodiment, source 220 is
positioned along the axis of
housing 230.
[0032] Gamma-Ray collimator 225, which is optional in certain embodiments,
may be-
configured adjacent to the source 220 in order to directionally constrain
radiation from the
radiation source 220 to an azimuthal radiation segment of the gravel pack. For
example,
collimator 225 may include fins or walls 226 adjacent source 220 to direct
gamma ray
propagation. By directing, focusing, or otherwise orienting the radiation from
radiation source
220, radiation may be guided to a more specific region of the gravel pack. It
is appreciated that
in certain embodiments, a heavy-met shutter mechanism could be further
employed to direct
radiation from radiation source 220. Additionally, the radiation energy may be
selected, by
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choosing different isotopic sources, so as to provide some lithological or
spatial depth
discrimination.
100331 In the illustrated embodiment, collimator 225 constrains radiation
from source 220. In
this embodiment, collimator 225 is also conically shaped as at 228, in the
direction of detectors
240 to collimate the gamma rays from source 220. Of course, those skilled in
the art will
appreciate that collimator 225 may be configured in any geometry suitable for
directing,
focusing, guiding, or otherwise orienting radiation from radiation source 220
to a more specific
region of the gravel pack.
[0034] In one non-limiting example, the radiation transmitted from source
220 into a gravel
pack (such as gravel 150 of FIG. 1) is Compton scattered back from the gravel
pack to tool 200
where the back-scattered radiation may be measured by radiation detectors 240.
Radiation
detectors 240 can be any plurality of sensors suitable for detecting
radiation, including gamma
ray detectors. In the illustrated embodiment, four detectors are depicted,
although any number of
detectors can be utilized. In another example embodiment, three detectors or
six detectors are
utilized; where optionally, each detector is disposed to "view" a different
segment of the gravel
pack. Employing multiple detectors, the tool can image the entire
circumference of the gravel
pack in separately identifiable segments. The resolution of the image of the
overall
circumference can depend on the number of detectors, the energy of the gamma
rays and the
degree of shielding provided around each detector,
[0035] In certain embodiments, gamma ray detectors may include a
scintillator crystal that
emits light proportional to the energy deposited in the crystal by each gamma
ray. A
photomultiplier tube may be coupled to the crystal to convert the light from
the scintillation
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crystal to measurable electron current or voltage pulse, which is then used to
quantify the energy
of each detected gamma ray. In other words, the gamma rays' energies are
quantified, counted,
and used to estimate the density of the gravel pack adjacent a screen.
Photomultiplier tubes may
be replaced with high-temperature charge-coupled devices (CCD) or micro-
channel photo-
amplifiers. Examples of suitable scintillator crystals that may be used
include, but are not
limited to, NaI(TI) crystals, BGO, and Lanthanum-bromide, or any combination
thereof. In this
way, count-rates may be measured from returned radiation, in this case,
returned gamma rays.
The intensity of the Compton scattered gamma rays depends on, among other
factors, the density
of the gravel pack material. Hence, lower density represents gaps in the
gravel pack and lower
count-rates represent lower density as a result of fewer gamma rays being back-
scattered towards
the detectors.
[0036] Still referring to FIG. 3, in an example embodiment detectors 240
are mounted inside
a housing at a radius smaller than the radius of housing 230 inset from the
surface of housing
230. Likewise, while they need not be evenly spaced, in the illustrated
embodiment, detectors
240 are evenly spaced on the selected radius. Although the illustrated example
shows four
detectors 240 spaced apart 90 degrees from one another, those skilled in the
art will appreciate
that any number of multiple detectors can be utilized in the invention.
Further, while the
embodiment illustrates all of the detectors 240 positioned at the same
distance from source 220,
they need not be evenly spaced. Thus, for example, one detector (or a multi-
detector array)
might be spaced apart 12 centimeters from the source, while another detector
(or a detector
array) is spaced apart 20 centimeters from the source or any other distance
within the tool.
[0037] Similarly, in another embodiment, detectors 240 can be positioned
both above and
below source 220. In such a case, collimator 225 would be appropriately shaped
to guide gamma
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rays in the direction of the desired detectors. In such embodiments with
multiple detectors
disposed on both sides of the radiation source, additional shielding may be
provided between the
collimators to prevent radiation scattering (i.e. cross-contamination of the
radiation) from
different segments of the gravel pack.
100381 Each detector 240 may be mounted so as be shielded from the other
detectors 240.
While any type of shielding configuration may be utilized for the detectors
240, in the illustrated
embodiment, collimator 248 is provided with a plurality of openings or slots
245 spaced apart
around the perimeter of collimator 248. Although openings 245 could have any
shape, such as
round, oval, square or any other shape, in one example embodiment openings 245
are shaped as
elongated slots and will be referred to as such herein.
[0039] A detector 240 is mounted in each slot 245, so as to encase detector
240 in the shield.
The width and depth of the slot 245 can be adjusted as desired to achieve the
desired azimuthal
range. In certain embodiments the length of slots 245 can be as long as the
sensitive region of
the gamma-ray detector (e.g. the crystal height). It will be appreciated that
since a detector is
disposed within the slot, the detector is not on the surface of the collimator
where it might
otherwise detect gamma rays from a larger azimuthal range. In an example
embodiment, slot
245 is 360/(number of detectors) degrees wide and the detector face to inner
diameter of the
pressure housing is a few millimeters deep (e.g. from about 2 to about 5 mm).
However, tighter
collimation is possible. Optionally, the azimuthal range of each slot is
limited to 360/(number of
detectors) degrees. In this way, the view of each radiation detector 240 may
be more focused on
a particular region of the gravel pack. Additionally, such shielding
eliminates or at least
mitigates radiation scattered from one detector to another detector. As can be
seen, each detector
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is separated from one another by radiation absorbent material. By eliminating
detector-to-
detector radiation scattering, more precise azimuthal readings are achieved.
[0040] While source collimator 225 is shown as a single, integrally formed
body, having fins
226, and conical surface 228, it need not be and could be formed of separate
structural
components, such as a source collimator combined with a detector collimator
248, so long as the
shielding as described herein is achieved.
[0041] In the illustrated embodiment, the region of housing 230 around the
opening in source
collimator and detectors 240 may be fabricated of beryllium, aluminum,
titanium, or other low
atomic number metal or material, the purpose of which is to allow more of the
gamma rays to
enter detectors 240. This design is especially important for lower energy
gamma rays, which are
preferentially absorbed by any dense metal in the pressure housing.
[0042] Alternatively, or in addition to detector shielding or collimator
248, an anti-
coincidence algorithm may be implemented in electronics 260 to compensate for
detector-to-
detector radiation scattering. In this way, a processor can mitigate the
effects of multiply-
detected gamma rays via an anti-coincidence algorithm. In certain embodiments,
electronics
260, 262, and 264 are located above detectors 240 or below source 220.
[0043] Electronics 260 may include processor 262, memory 263, and power
supply 264 for
supplying power to gravel pack imaging tool 200. Power supply 264 may be a
battery or may
receive power from an external source such as a wireline (not shown).
Processor 262 is adapted
to receive measured data from radiation detectors 240. The measured data,
which in certain
embodiments includes count rates, may then be stored in memory 263 or further
processed
before being stored in memory 263. Processor 262 may also control the gain of
the
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photomultiplier or other device for converting scintillations into electrical
pulses. Electronics
260 may be located below source 220 and above detectors 240 or removed
therefrom.
[0044] In one embodiment, the tool further includes an accelerometer, a 3
axis inclinometer
or attitude sensor to unambiguously determine the position of an azimuthal
segment. In certain
embodiments, a compass device may be incorporated to further determine the
orientation of the
tool.
[0045] Gravel pack imaging tool 200 may be constructed out of any material
suitable for the
downhole environment to which it is expected to be exposed, taking into
account in particular,
the expected temperatures, pressures, forces, and chemicals to which the tool
will be exposed. In
certain embodiments, suitable materials of construction for source collimator
225 and detector
collimator 248 include, but are not limited to, sintered tungsten (known as
heavy-met), lead,
dense and very-high atomic number (Z) materials, or any combination thereof.
[0046] Further, while a 1 11/16 inch diameter configuration tool is
illustrated, the tool 100
can be sized as desired for a particular application. Those skilled in the art
will appreciate that a
larger diameter tool would allow more detectors and shielding to provide
further segmentation of
the view of the gravel pack.
[0047] This tool may be deployed to measure the integrity of the gravel
pack in new
installations and to diagnose damage to the gravel pack from continuing
production from the
well. A person of ordinary skill in the art with the benefit of this
disclosure will appreciate how
to relate the log results of count rates and inferred densities of gravel pack
material to the
structure of the pack and to reason from the results to the condition of the
pack.
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[0048] As a further illustration of an exemplary geometry of the embodiment
illustrated in FIG.
3, FIGS. 4A and 4B show cross-sectional views of another embodiment of the
tool disposed in
base pipe or gravel-pack screen 330, which is further disposed in casing 310.
A gravel pack 350
is disposed between the casing 310 and the base-pipe 330, where FIG. 4A shows
a cross-section
taken from the X-Y plane and where FIG. 4B shows a cross-section taken from
the X-Z plane.
As shown in the illustrated embodiment, source collimator 328 is conical
shaped in the X-Z
plane or Y-Z plane. Detector 340 is shown in FIG. 4A in openings or slots 345,
whereas
radiation source 320 is shown depicted in FIG. 4B. As shown in FIG. 4A,
detector collimators
348 are fan-shaped in the X-Y plane and rectangular in the X-Z or Y-Z planes.
In certain
embodiments, a conical source collimator 328 is desirable as it reduces
multiple scattering events
in the gravel pack.
[0049] Methods of using the present invention may include the use of different
energy windows
to map voids or blockages in the gravel pack in low to high density completion
fluids. In certain
embodiments, at least four energy windows (Figure 7) are used. For example,
for a I37Cs source
(source energy is 662 keV), the Low Energy (LE or Wi) window (typically from
about 50 keV to
about 200 keV) is sensitive to multiple scattered source gamma-rays, whereas
the Medium
Energy (W2) window (typically from about 200 keV to about 250 keV) is
sensitive to single-
scattered source gamma rays. A Broad Window (BW or W3) typically may include
gamma rays
in the range of about 50 keV to about 250 keV. A high energy window W6 (also
referred to
herein as HE) typically may include gamma rays in the range between about 250
keV to about
650 keV nearly the source energy. The BW count rate has the highest
statistical precision. The
LE and Medium Energy windows may be used for specific applications, such as
deep-reading
and maximum-dynamic-range imaging capabilities. Combinations of these
different energy
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window logs can be combined using special methods (e.g. ad-hoc adaptive or
Kalman-type
processing algorithms) for enhanced precision and resolution. It is recognized
that multiple-
intensity energy sources may be utilized in the same tool, either
simultaneously or sequentially.
[0050] In addition to the energy levels of the radiation source, other
factors that may be
adjusted to discriminate segmented views of the gravel pack include, but are
not limited to the
angle of the collimators and the source to detector spacing. Examples of
suitable angles of the
source collimator include, but are not limited to, angles from about 15 degree
to about 85 degree
and from about 65 degree to about 85 degree in other embodiments. Examples of
suitable source
to detector spacing include, but are not limited to, from about 1 inch to
about 3.5 inches to about
8 inches, and in other embodiments, from about 6 inches to about 10 inches,
and in still other
embodiments to about 12 inches.
[0051] Moreover, it is recognized that the downhole tool is capable of
measuring count rates
while being lowered or raised in the wellbore. In certain embodiments, the
downhole tool may
perform measurements while the tool is stationary in the wellbore. Exemplary
raising and
lowering rates include displacement rates of up to about 1800 feet/hour.
[0052] To facilitate a better understanding of the present invention, the
following examples of
certain aspects of some embodiments are given. In no way should the following
examples be
read to limit, or define, the scope of the invention.
EXAMPLES
[0053] In one non-limiting example of use, FIG. 5 shows a graph of a count
rate versus depth in
centimeters as measured by a 2.5 inch gravel-pack imaging tool in a 7 inch
gravel pack. These
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logs were produced by processing individual detector gamma-ray count rates.
The plot in FIG. 5
is an MCNP-modeled example of the count-rate sensitivity to a 1-inch annulus
wash-out in a
gravel pack centered at a depth index of 4-centimeters. It shows significant
sensitivity to
changes in the gravel pack density. MCNP refers to Monte Carlo N-Particle,
developed by Los
Alamos Monte Carlo Group and specifically titled "MCNP-A General Monte Carlo N-
Particle
Transport Code Version 5," Los Alamos National Laboratory Vols. I-Ill, April,
2003; and is
available from Radiation Safety Information Computational Center at Oak Ridge
National
Laboratory as CCC-740. Qualitative image logs will be produced by displaying
the relative
count rates from each detector sector at each depth. Another means of
analyzing the counts can
be used to compute a more quantitative multi-sector density (i.e. in grams/cc)
profile. Such a
density log can be derived from the count rates by using a calibrated logging
count rate-to-
density algorithm.
10054] Notably, traditional prior art density tools used to measure the gravel
pack generally have
a relatively large spacing between the source and the detector. The reason for
this is that the tool
is provided to evaluate the entire gravel pack at one azimuth without high
spatial resolution. The
source and detector are both typically located centrally in the tool along the
tool's axis. Shielding
may be provided along the axis between the source and the detector to prevent
energy coupling
between the two, i.e., energy passing directly from the source to the detector
without scattering
within the gravel pack. In the prior art, because of the relatively large
spacing between the
source and detector, the gamma ray radiation undergoes significant multiple
scattering and
absorption before it is detected and counted. The more dense the gravel pack,
the fewer counts
that are recorded. In other words, in the tools of the prior art, the count
rate decreases with
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gravel pack density because the multiple scattering and absorption attenuates
the total amount of
radiation measured by the detectors.
[0055] In one example embodiment of the device and method of the present
disclosure, the
source and the detectors are closely positioned to one another, such as about
3.5 inches apart.
Because of this close physical relationship, energy propagated into the gravel
pack and scattered
back to the detector undergoes much less scatter, i.e., typically only a
single scatter (back to the
detector) as opposed to multiple scattering. In fact, the count rates increase
with the density of
the gravel pack utilizing the tool of the invention. This is significant
because improved density
resolution is realized as compared to prior art.
[0056] Moreover, the prior art does not utilize a conically shaped
collimator to direct the
energy propagated into the gravel pack. Again, by utilizing such a collimator
in the prior art
tool, multiple scattering can be minimized and improve upon the imaging of the
prior art tools.
[0057] FIG. 6 illustrates in a partial side sectional view, an example of an
imaging tool 400
inserted within a tubular 402. Embodiments exist wherein the tool 400 can be
the same or
substantially the same as the tools 100, 200 respectively of FIGS. 1 and 3 and
described above.
In the example of FIG. 6, the tubular 402 is inserted into a wellbore 404 that
is shown
intersecting a subterranean formation 406. Casing 408 is optionally provided
in the wellbore 404
for lining the sidewalls of the wellbore 404. Thus in this example, the
tubular 402 is production
tubing. Alternate examples of use exist wherein the tool 400 is inserted
within casing 408
having no production tubing within. The tool 400 is deployed in the wellbore
404 on a line 410,
where the line 410 can be a wireline, slickline, cable, or coil tubing. The
line 410 is shown
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inserted through a wellhead assembly 412 that is mounted on surface above an
opening to the
wellbore 404.
[0058] Further illustrated in the embodiment of FIG. 6 are deposits 414 that
adhere to a sidewall
of the tubular 402 at various depths in the wellbore 404 and azimuthal
positions around an axis
Ax of the wellbore 404. The deposits 414 may include scale that forms on an
inside of the
tubular 402, as well as residue from fluid 416 shown disposed in the tubular
402. Other
examples of residue include asphaltenes, paraffins, heavy hydrocarbons, sand,
and combinations
thereof. In the example of FIG. 6, the fluid 416 substantially occupies an
annular space between
a body 418 of the imaging tool 400 and an inner surface of the tubular 402.
Further included
with the embodiment of the tool 400 of FIG. 6 is a radiation source 420, which
can be
substantially the same as sources 220, 320 respectively of Figures 2 and 4B
and described above.
Radiation emitted from the source 420 can travel along a path represented by
arrows A, which
initially diverges from the axis Ax. Some of the radiation undergoes
scattering and is redirected
to converge with the axis Ax at a location axially away from the source 420.
As shown, the
redirected radiation contacts sensor 422 where a count and associated energy
of the radiation is
detected. Examples exist wherein the sensor 422 includes detectors 140, 240
respectively of
FIGS. 1 and 3 and discussed above.
[0059] Still referring to the example embodiment of FIG. 6, the radiation is
directed in a conical
pattern away from the source 420 and generally about a line intersecting the
source 420 and
sensor 422. As such, the radiation is shown Compton scattering from the fluid
416 in the tubular
402, an area proximate the sidewall of the tubular 402, and the formation 406.
It should be
pointed out that paths the radiation follows from the source 420 to the sensor
422 can intersect
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any point in a plane bisected by the axis Ax, and is not limited to the select
number of arrows A
that are illustrated for clarity.
[0060] As is known, the energy of the radiation detected by the sensor 422 is
affected by the
Compton scatter angle of the radiation (i.e. the angle of the directional
change of the radiation).
Generally, the energy decreases with increasing angles of scatter, as shown in
FIG. 2; thus the
radiation flowing directly from the source 420 to the sensor 422 which
undergoes only minimal
scattering will have a greater detected energy than the radiation scattered
from adjacent the
tubular 402 and the formation 406, and the radiation scattered from adjacent
the tubular 402 will
have a greater detected energy than the radiation scattered from the formation
406. Radiation
counts detected by sensor 422 are binned based on an energy level of each
count. As shown in
the example of FIG. 7, the counts versus their corresponding energy are
plotted to create a
spectrum 424 to illustrate a distribution of detected energy of the radiation.
Energy windows W1
- W6 are shown superimposed on the spectrum 424 that extend along the energy
axis. Counts of
scattered radiation coming from the materials inside gravel pack, or materials
between tubulars,
or inside tubulars adjacent to the logging tool fall into windows Wi, W2, W3,
or W6. Counts of
radiation that flow un-scattered and directly from the source 420 to the
sensor 422 (FIG. 6) fall
into windows W4 or W5 and can be used for tool gain stabilization. The counts
of radiation that
scatter from material deposited on or adjacent the tubular 402 are illustrated
as being in window
W6. It is within the capabilities of those skilled in the art to create a
spectrum as found in FIG. 7
and to identify the substances from which the radiation scatters based on the
counts and
corresponding energy of the created spectrum. Moreover, those skilled in the
art are capable of
identifying a spatial location of the identified substances based on the
relative arrangement of the
energy windows W1-W6.
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[0061] FIG. 8 provides a plot 426 illustrating example MCNP modeled results
for the High
Energy window W6 count rate response dependence on the density-thickness
product of an
asphaltene deposit inside a 5 Y2-inch tubular 402 of Figure 6. In the example
plot of FIG. 8, the
W6 count rate increases with increasing asphaltene-thickness, as expected from
single shallow-
angle Compton scattering adjacent to the logging tool. In an exemplar
embodiment, count rates
in the high energy window W6 can depend on very shallow-angle Compton
scattering, and are
sensitive to density changes in the materials close to the logging tool. In
one example, the high
energy count rates W6 can be used to detect blockages caused by an
accumulation or build up in
the production tubing of asphaltene, sand, scale, or combinations thereof. As
such, by analyzing
the portion of the spectrum 424 falling in window W6 (FIG. 7), the matter
adjacent the tubular
402 can be identified. The matter adjacent the tubular 402 includes the
deposit 414 as illustrated
in FIG. 6 on the inner surface of tubular 402, the fluid 416 in the tubular,
matter in the fluid 416,
the tubular 402 itself, matter on the outer radial surface of the tubular 402,
and matter in the
annulus between the tubular 402 and casing 408.
[0062] Referring now to FIG. 9, illustrated is a side partial sectional view
of a schematic
example of the imaging tool 400 inserted within the tubular 402. Further shown
is a region 428
that represents a zone where shallow-angle single scattered gamma rays are
scattered from
materials near the logging tool. Because shallow-angle Compton-scattered gamma
rays lose
very little of their initial energy, they fall into high energy window W6 of
the plot 424 of FIG. 7.
In the example of FIG. 9, the region 428 has an outer periphery with inner and
outer lateral edges
I, 0 that angle away from the axis Ax of the tool 400 and are joined at their
distal ends by a distal
edge D and a proximate edge P. In the example, the region 428 extends from
adjacent the tool
400 past an outer surface of the tubular 402. More specifically, an
intersection of the inner and
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proximate edges I, P is adjacent an outer surface of the tool 400 and an
intersection of the distal
and outer edges D, 0 is outside of the tubular 402. A Compton scattering
equation can be used
for generating the spatial locations that make up the region 428. Radiation
scatter occurring in
the region 428 has a relatively low angle compared with scatter that occurs
radially past the
region 428, as such, an energy level of radiation detected by the sensors 422
that is scattered
from within the region 428, is greater than that of the energy level of
shallow-angle radiation that
scatters from areas radially past the region 428. In an example, the energy
level of radiation
scattered from within the region 428 and detected in W6 by sensors 422 can
range up to the
source energy of about 662 keV.
