Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR INTERROGATING A
SUBTERRANEAN ANNULUS
BACKGROUND
1. Field of Invention
[0002] The invention relates generally to investigating an annulus between
tubulars
disposed in a subterranean wellbore. More specifically, the present invention
relates to a
device and method that uses a radiation source for inspecting a subterranean
annulus and a
radiation detector for identifying a substance or substances in the annulus.
2. Description of Prior Art
[0003] Subterranean wellbores used for producing hydrocarbons typically are
lined
with a string of casing that is cemented to the formation intersected by the
wellbore.
Often an inner casing string is inserted within the casing string cemented in
place. Fluid
produced from the well flows to the surface within production tubing that is
inserted into
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
commenced
in the well. There may also be a need at some time for removal of a portion or
for all of
the casing.
[0004] Generally drilling fluids fill the annular space between the
concentric tubulars.
Particulates within the drilling fluids may settle out or precipitate over
time and form a
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cement like substance that couples 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.
SUMMARY OF THE INVENTION
[0005] Provided herein is a method of investigating a subterranean
wellbore. In one
example the method includes generating radiation from within a tubular that is
disposed
in the subterranean wellbore. The radiation is directed along a path that is
oblique to an
axis of the tubular allowing some of the radiation to pass through the tubular
into an
annulus circumscribing the tubular and scatter back into the tubular. Some of
the
radiation that scatters back into the tubular is detected and a count of the
detected
radiation is used to identify a material in the annulus. Alternatively, the
radiation is a
first set of radiation and the path is a first path. In this example the
method further
includes directing a second set of radiation along a second path that points
away from the
first path. Some of the second set of radiation scatter from fluid disposed in
the tubular
and are detected. Thus in an example embodiment identifying a material in the
annulus is
further based on a rate of detection of the second set of radiation. The
radiation can be a
gamma ray from a gamma ray source having an energy of from about 250 keV to
about
700keV. In this example, the scattered radiation when detected have an energy
of from
about 50 keV to about 350 keV. In an alternate embodiment the method can also
include
detecting a substantially solid material in the annulus when a ratio of the
rate of detection
of the first set of radiation over the rate of detection of the second set of
radiation remains
substantially the same with changes in the thickness. In one example, the
material in the
annulus is a light weight cement.
[0006] Also included herein is a method of interrogating an annulus between
an inner
and outer coaxially disposed tubulars that includes providing a gamma ray
source against
an azimuthal section of the inner tubular. The method also includes directing
gamma
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rays from the source so that some of the gamma rays travel into the annulus
and scatter
from a material in the annulus back into the inner tubular. In this example
some of the
gamma rays travel away from the azimuthal section and scatter from a fluid in
the inner
tubular. The gamma rays that scatter back can be detected, the method can also
include
classifying by energy range those gamma rays that scatter from the fluid in
the inner
tubular and that scatter from the material in the annulus and identifying the
material in the
annulus based on a rate of detection of the scattered gamma rays. In an
alternative, a
conically shaped guide is provided proximate the gamma ray source and
positioned so
that an aperture of the guide is directed towards the source and the guide has
an axis that
is substantially parallel with an axis of the inner tubular. The detector can
be disposed
from about 2 inches to about 4 inches from the gamma ray source and wherein
the
detector is used to detect the scattered gamma rays. In an example, a
collimator is used to
strategically direct the gamma rays away from the source at an angle oblique
to an axis of
the inner tubular and along discrete paths disposed azimuthally around the
gamma ray
source, so that strategically located detectors respectively detect scattering
from discrete
azimuthal areas spaced radially outward from the gamma ray source. Optionally,
a rate
of detection of gamma rays scattering from fluid in the wellbore is used as a
reference for
determining the material in the annulus. The steps of generating and detecting
can be
repeated at different depths in a section of the wellbore. A substantially
solid material in
the annulus can be identified when a ratio of a rate of gamma rays detected
that are
scattered from the annulus over a rate of gamma rays detected that are
scattered from the
fluid in the inner tubular remains substantially the same with changes in
thickness of the
annulus. Optionally, a fluid can be identified in the annulus when a gamma ray
rate
detection ratio reduces with a reduction in thickness of the annulus.
[0007] A method of
analyzing an annulus between an inner tubular and an outer
tubular that are coaxially disposed in a subterranean wellbore is provided
herein that
includes providing a gamma ray source against an azimuthal section of the
inner tubular
and directing gamma rays from the source so that some of the gamma rays travel
through
the sidewall, into the annulus and scatter from a material in the annulus back
into the
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inner tubular, and so that some of the gamma rays travel away from the
azimuthal section
and scatter from a fluid in the inner tubular. In this example the scattered
gamma rays are
detected that scatter from the fluid in the inner tubular and that scatter
from the material
in the annulus and are classified. The material in the annulus is identified
based on a rate
of detection of the scattered gamma rays. The steps of this method can be
repeated at
different depths in a section of the wellbore and a substantially solid
material identified in
the annulus when a ratio of a rate of gamma rays detected that are scattered
from the
annulus over a rate of gamma rays detected that are scattered from the fluid
in the inner
tubular remains substantially the same with changes in thickness of the
annulus. A
substantially liquid material is identified in the annulus when a ratio of a
rate of gamma
rays detected that are scattered from the annulus over a rate of gamma rays
detected that
are scattered from the fluid in the inner tubular is reduced with a reduction
in thickness of
the annulus. In one alternative the detection rate of gamma rays that are
scattered from
the fluid in the tubular is a reference value for use in identifying a liquid
in the annulus.
BRIEF DESCRIPTION OF DRAWINGS
[0008] 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:
[0009] FIG. 1 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.
[0010] FIG. 2 is a perspective view of one embodiment of the tool of FIG.
1.
[0011] FIGS. 3A and 3B are sectional views of an example embodiment of the
tool of
FIG. 2.
[0012] FIG. 4 is an example of a graph that represents a source response in
a gravel
pack for Barite-Sag detection.
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[0013] FIG. 5 is a graph of a count rate versus depth measured by an
example
embodiment of an imaging tool in accordance with the present invention.
[0014] FIG. 6 is a side sectional view of an alternate example embodiment
of the tool
of FIG. 1 disposed in a cased wellbore in accordance with the present
invention.
[0015] FIG. 7 is a side sectional view of a portion of FIG. 6 shown with
additional
detail.
[0016] FIG. 8A is a lateral sectional view of the tool and wellbore of FIG.
7.
[0017] FIG. 8B is a side sectional view of the tool and wellbore of FIG.
8A.
[0018] FIG. 9A is a lateral sectional view of the tool and wellbore of FIG.
7 with
production tubing asymmetrically disposed within a casing string.
[0019] FIG. 9B is a side sectional view of the tool and wellbore of FIG.
9A.
[0020] FIG. 10 is a graphical representation of changes in the thickness of
annulus
between concentric downhole tubulars and count rates of gamma rays reflected
from
materials in the annulus.
[0021] FIG. 11 is a graphical representation of changes in the thickness of
annulus
between concentric downhole tubulars and a measured response of gamma rays
deflected
from materials in the annulus.
[0022] FIG. 12A is a side partial sectional view of an example of an
imaging tool in a
wellbore obtaining a baseline image of a gravel pack in accordance with the
present
invention.
[0023] FIG. 12B is a side partial sectional view of the tool of FIG. 12A
imaging the
gravel pack at a time after that of FIG. 12A in accordance with the present
invention.
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[0024] FIG. 12C is a side partial sectional view of an example of an
imaging tool
imaging a gravel pack after a step of remediation of the gravel pack in
accordance with
the present invention.
[0025] FIG. 13 is a side partial sectional view of an example embodiment of
an
imaging tool imaging a casing bond in accordance with the present invention.
[0026] FIG. 14 is a side partial sectional view of an example embodiment of
an
imaging tool imaging asphaltenes in a gravel pack in accordance with the
present
invention.
[0027] FIG. 15 is a side partial sectional view of an alternate embodiment
of the
imaging tool of FIG. 6 shown substantially centralized in a wellbore.
[0028] 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 spirit and scope of the invention
as defined by
the appended claims.
DETAILED DESCRIPTION OF INVENTION
[0029] The method and system of the present disclosure will now be
described more
fully hereinafter with reference to the accompanying drawings in which
embodiments are
shown. The method and system 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.
100301 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
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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.
[0031] 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 low
energy
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 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.
[0032] The alternating hatching 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 filled in. Of course, those skilled in the art,
with the
benefit of this disclosure, will appreciate that the foregoing regions are for
illustrative
purposes only and that a void or vug could take any shape and any position
relative to
tool 100.
[0033] 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. Upon scattering the
gamma
rays, they become lower energy gamma rays 126, which are detected by detectors
140.
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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.
[0034] In an example, radiation source 120 includes barium, cesium, some
other low
energy radiation source, or combinations thereof By utilizing a low energy
source such
as this, energy is only propagated a short distance into the gravel pack
immediately
adjacent a screen. For this same reason, in one example detectors 140 are
positioned in
housing 130 proximate to radiation source 120. In one example embodiment,
radiation
source 120 and detectors 140 are within about 3 to about 3.5 inches apart
along the length
of tool 100.
[0035] 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
limited 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.
100361 Further, the energy levels of the emitted gamma rays 124 may be
selected to
assess gravel pack density at varying depths or distances from dovvnhole
imaging tool
100. As one example, the radiation from a low-energy gamma ray source, such as
a l33Ba
source, may be used to emit various energy levels. Alternatively, a gamma ray
radiation
source with an energy close to that of I37Cs may be used.
[0037] Techniques for converting radiation count rates 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
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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.
10038] 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 orientation sensors used to determine orientation of the
logging tool
with respect to a reference plane. 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.
100391 FIG. 2 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 embodiments, detector arrays may be
positioned at
differing distances from radiation source 220. Additionally, detector arrays
on either side
of radiation source 220 is also envisioned in certain embodiments. Electronics
260 may
also be located in housing 230 or wherever convenient.
100401 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
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radiation source 220 may be continuous, intermittent, periodic, or in certain
embodiments, amplitude, frequency, phase modulated, or any combination
thereof.
[0041] In one example embodiment, radiation source 220 is centrally located
in
housing 230. In the illustrated embodiment. source 220 is positioned along the
axis of
housing 230.
[0042] 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 choosing different isotopic sources,
so as to
provide some Ethological or spatial depth discrimination.
[0043] 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.
[0044] 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
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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.
[0045] 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 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
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
device
(CCD) or micro-channel photo-amplifiers. Examples of suitable scintillator
crystals that
may be used include, but are not limited to, NaI crystals, NaI(T1), 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.
[0046] 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
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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.
100471 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 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.
[0048] 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.
[0049] 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
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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 is separated from one another by radiation absorbent
material. By
eliminating detector-to-detector radiation scattering, more precise azimuthal
readings are
achieved.
[0050] While source collimator 225 is shown as a single, integrally formed
body,
having fins 226, 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.
[0051] 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.
[0052] 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.
[0053] 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
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also control the gain of the 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.
[0054] 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.
[0055] 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, heavy-
met, lead, dense and very-high atomic number (Z) materials, or any combination
thereof.
[0056] Further, while a 111/16 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.
[0057] 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.
[0058] As a further illustration of an exemplary geometry of the embodiment
illustrated in FIG. 2, FIGS. 3A and 3B show cross-sectional views of another
embodiment of the tool disposed in base pipe or screen 330, which is further
disposed in
casing 310, which is further disposed in gravel pack 350. where FIG. 3A shows
a cross-
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section taken from the X-Y plane and where FIG. 3B 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. 3A in
openings or
slots 345, whereas radiation source 320 is shown depicted in FIG. 3B. As shown
in FIG.
3A, 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.
[0059] Methods of using the present invention may include the use of
different energy
windows to discriminate the gravel pack in low to high density completion
fluids. In
certain embodiments, at least three energy windows are used where each window
depends on the source energy. For example, for a Cs source (662 keV), the Low
Energy
(LE) window (typically from about 50 keV to about 200 keV) is sensitive to
multiple
scattered source gamma-rays, whereas the High Energy (HE) window (typically
from
about 200 keV to about 350 keV) is sensitive to single-scattered source gamma
rays. A
Broad Window (BW) typically may include gamma rays in the range of about 50
keV to
about 350 keV. The BW count rate has the highest statistical precision and is
used for the
base gravel pack imaging. The LE and HE windows may be used for specific
applications, such as deep-reading and maximum-dynamic-range imaging
capabilities.
Combinations of these different energy 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.
[0060] 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
CA 02799809 2012-12-20
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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.
[0061] Radioactive tracers may be used in conjunction with certain
embodiments to
produce enhanced images of the gravel pack. The introduction of radioactive
tracers
allows production of an image of the azimuthally distributed radioactive
tracer material.
Radioactive tracers may be attached to the gravel pack before building the
gravel pack or
as the gravel pack is being placed. Alternatively, radioactive tracers may be
injected or
otherwise introduced into the gravel pack after installation of the gravel
pack (e.g. as a
fluid or slurry). More generally, radioactive tracers may be introduced into
any portion
of the formation as well.
[0062] Where radioactive tracer material is attached to the gravel itself
before
placement, void areas show up on the images as low count-rate (or "dark")
regions,
whereas where the radioactive tracer material is injected as a fluid or
slurry, void areas
void areas show up on the images as high count-rate ("bright") regions within
the gravel
pack. Further image enhancement may be achieved by using a variety of tracers
to create
a multiple-isotope log. When used for this purpose, source 320 in FIG. 3, 220
in FIG. 2,
or 120 in FIG. 1 may be omitted from the tool. Alternatively, tracer
radioactivity may be
determined in the presence of the radiation source or multiple tracers can be
identified by
using the energy discrimination capability of electronics 260.
[0063] 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.
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[0064] 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
[0065] In one non-limiting example of use, FIG. 4 depicts a graph of a
spectrum of
gamma rays incident on one of the detectors as a response to being scattered
in a gravel
pack. Here, typical gamma ray intensity is shown plotted versus gamma ray
energy
(MeV). This graph shows an MCNP-modeled detector energy spectrum simulation of
an
actual tool resulting from the I33Ba 356 keV gamma ray Compton back scattered
in
various gravel pack scenarios. This graph signifies an advantage of choosing a
low-
energy gamma source. By using an energy source that is low enough, one can
ensure that
the gravel-pack tool is sensitive primarily to the near-region variations of
the gravel pack
and not significantly affected by scattering in deeper regions of the cement
around the
casing or the formation and subsequent formation density variations. However,
in cases
of thick base pipes and metal screens between the gravel pack and the gravel-
pack tool
detectors, the source energy must be sufficiently high to penetrate into the
gravel-pack
screen. In this way, gravel pack imaging tools may be designed to "focus" on
particular
depths or portions of a gravel pack.
[0066] In one non-limiting example of use, FIG. 5 shows a graph of a count
rate
versus depth in centimeters as measured by a 3.5 inch gravel-pack imaging tool
in a 7
inch gravel pack. These 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.
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
17
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density log can be derived from the count rates by using a calibrated logging
count rate-
to-density algorithm.
[0067] 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. 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 gravel pack density because the multiple
scattering and
absorption attenuates the total amount of radiation measured by the detectors.
[0068] 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 this means that the radiation does not
undergo the
attenuation associated with tools of the prior art.
[0069] 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.
18
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[0070] Referring now to Figure 6, a partial side sectional view is provided
illustrating
an example embodiment of the tool 400 disposed in a wellbore 402 that
intersects a
formation 404. The tool 400 is suspended on wireline 406, that may be threaded
through
a wellhead assembly 408 shown set on the surface and at the opening to the
wellbore 402.
Optionally, the tool 400 could be deployed on tubing, slick line, cable, or
any other
known or later developed deployment means. An outer casing 410 that can be
cemented
to the formation 404, lines the wellbore 402 and an inner casing 412 is shown
coaxially
inserted within outer casing 410. The tool 400 is suspended within production
tubing 414
that inserts within the inner casing 412; a decentralizer 416 shown mounted on
a lateral
side of the tool 400 for urging the tool 400 up against the inner surface of
the production
tubing 414. In one example, and as described in more detail below,
decentralizing the
tool 400 enhances imaging of the annular spaces between concentric tubulars.
[0071] Referring now to Figure 7, shown in greater detail is a side
sectional view of a
portion of the embodiment of Figure 6. Fluid 418 is shown in the example of
Figure 7 in
the production tubing 414. Fluid 418 also occupies the higher length of an
annulus 420
between the inner and outer strings of casing 410, 412 and a portion of an
annulus 422
between the inner casing 412 and production tubing 414. Below the fluid 418
and
annulus 422 is a substantially solid precipitate 424 that extends between the
tubing 414
and inner casing 412. In the example of Figure 7, the precipitate 424 adheres
the tubing
414 with inner casing 412. In one example, the precipitate 424, which may fall
out of or
precipitate from the fluid 418, may be substantially made up of barite.
[0072] Figure 8A is a sectional view of the tool 400 in the wellbore 402
taken along
lines 8A-8A of Figure 7. In the example of Figure 8A, the production tubing
414 is
substantially concentric within the inner casing 412. As such, in the example
of Figure
8A the thickness Ar is substantially the same around the entire circumference
of the
annulus 422. Also illustrated in Figure 8A are detectors 4261_6 that are
provided within
the body of the tool 400. In the example of Figure 8A, the tool 400 is
positioned against
the inner surface of the tubing 414 so that detector 4261 is the closest
detector to the side
19
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wall of the tubing 414. In contrast, detector 4264, which is illustrated as
being about 1800
from detector 4261, is the farthest detector from the sidewall of the tubing
414 against
which the tool 400 is urged against.
[0073] Referring now to Figure 813, a side partial sectional view of the
embodiment of
Figure 8A is shown taken along lines 8B-8B that depicts a spatial relationship
of the
detectors 4261, 4264 and a radiation source 428. In the example of Figure 8B
and as
described above, radiation source 428 emits radiation that is directed along
dedicated
paths to enhance detection of those scattered gamma rays. Still referring to
Figure 8B,
paths P1, P4 are shown that illustrate one example direction of radiation
directed for
detection respectively by detectors 4261, 4264. By asymmetrically disposing
the tool 400
within the tubing 414, radiation directed along path P1 exit the tool 400 and
pass through
the tubing 414 and into the annulus 422. At least some of the radiation along
P1 that
makes its way into the annulus 422 scatter back and is detected by detector
4261. As will
be described in more detail below, analyzing the detected radiation from
detector 4261
can provide information about material disposed within annulus 422. In
contrast,
radiation directed along Path P4 makes its way from the tool 400 and is
directed into the
fluid 418 within the tubing 414. The strategic location of the source 428 and
detector
4264 provide for detection of the radiation that primarily scatters from the
fluid 418.
Thus, in one example embodiment, a study of scattered radiation monitored by
detector
4264 can provide a reference or basis for analysis of those scattered gamma
rays detected
by detector 4261.
[0074] Figure 9A, like Figure 8A, is a sectional view of an example of the
tool 400
disposed in wellbore 402 taken lateral to axis Ax of the wellbore 402 (Figure
8B). In the
example of Figure 9A, the tubing 414 is asymmetrically disposed within the
inner casing
412 so that the thickness Ari of the annulus 422 adjacent where the tool 400
is located is
less than other azithumal portions of the annulus 422. The asymmetric
positioning of the
tubing 414 is shown in a longitudinal sectional view in Figure 9B, along with
the
CA 02799809 2012-12-20
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representative directional paths P1, P4 illustrating example directions of
radiation being
emitted from the source 428.
[0075] Referring now to Figure 10, a simulated count rate as recorded by
detectors
4261, 4264 is graphically provided. More specifically, curves C1, C2
respectively
illustrate situations where count rates obtained by the detectors 4261, 4264
can vary with
changes in the thickness of the annulus 418. Curve C1 illustrates a simulated
response
wherein a liquid is present in the annulus 422. The abscissa of Figure 10
represents a
change in the thickness of the annulus, whereas the ordinate represents a
ratio of the
count rate of detector 4261 over the count rate of detector 4264. As shown,
the ratio of
the count rates of the detectors 4261, 4264 decreases with decreasing
thickness of the
annulus 422. The decrease in count rates may be attributed to more radiation
passing
entirely across the smaller width annulus 422 and into contact with the inner
surface of
the inner casing 412. The higher density of the material making up the casing
412 over
that of the density of the liquid can attenuate the radiation so that
scattering cannot be
observed within the energy level of being detected by the detector 4261.
[0076] Curve C2 of Figure 10 represents a simulated example wherein the
annulus 422
is substantially filled with a particulate matter such as a barite. In this
example, the
response ratio remains fairly consistent in spite of changes in thickness of
the annulus
422. One plausible conclusion is that the rate of scattering of the radiation
from barite is
substantially the same as that of from material making up the inner casing
412. Equipped
with this knowledge, the tool 400 may then be successfully deployed within
concentric
tubulars and used for determining the material within the annulus of
separating the
tubulars. Moreover, changes in coaxial orientation of the tubulars should not
affect the
ability of the tool to identify material within the annulus.
[0077] Figure 11 illustrates a series of plots C3-C15 that represent a
count response
detected by a detector of scattered radiation within various materials having
a different
density. More specifically, the density of the material represented by plot C3
is about 10
pounds per gallon, the density of the material represented by each successive
line
21
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increases by about two pounds per gallon. As such, the density of the material
represented by plot C4 is about 12 pounds per gallon and so on with the
density of the
material represented by plot C14 to be around 32 pounds per gallon. Line C15
represents
barite. In this example, the abscissa represents thickness of the annulus and
the ordinate
represents the detected response. As can be seen from Figure 11, the response
remains
more consistent with changes in annulus thickness for higher density
substances as
opposed to those of lower density. As with the case of the response of Figure
10, the
reduced thickness of the annulus allows radiation to contact the inner surface
of the
casing, thereby attenuating the radiation. A Monte Carlo modeling code MCMP
was
used as the modeling software that produced the data from which Figure 11 was
generated.
[0078] Referring
now to Figure 12A, an example of a tool 400A imaging in a wellbore
402A is shown in a side partial sectional view. In this example a centralizer
430 is
provided on a body 431 of the tool 400A and maintains the tool 400A proximate
the mid
portion of the wellbore 402A. A portion of the wellbore 402A adjacent the tool
400A is
shown having a gravel pack 432 that is made up of a number of pellets 434,
such as a
proppant. Some of the gravel pack 432 is disposed in the annulus 420 and some
extends
radially outward into the surrounding formation 404A through perforations 436
in casing
437 shown lining the wellbore 402A. In the example of Figures 12A and 12B a
single
string of casing is in the wellbore 402A. In an alternate embodiment, the
portion of the
casing 437 having the perforations 436 can be a sleeve coupled with the
remaining string
of casing 437. Moreover, a screen (not shown) can be incorporated into the
production
tubing 414 for filtering sand and other particulates from entering the
production tubing
414. As described in more detail above, the gravel pack 432 can be imaged by
directed
radiation from the source 428 along paths P,-Põ, where scattering of the
radiation is
detected with detector 426. While a single detector 426 is illustrated in the
embodiment
of Figure 12A, multiple detectors 426 could be included. In one example of
use, the
imaging of the gravel pack 432 obtained by the tool 400A, including other
portions of the
22
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wellbore 402A or formation 404A, can provide a ''baseline" image of the gravel
pack
432.
[0079] In one example the baseline image can be obtained before any fluid
production
through the gravel pack 432 has taken place. Referring now to Figure 12B, a
side partial
sectional view of an example of how flowing fluid over a period of time
through the
gravel pack 432B from the formation 404B can introduce an amount of detritus
438 into
the gravel pack 432B. In one example, the detritus 438 includes fines and
other solid
particles that occupy interstices between the pellets 434B that make up the
gravel pack
432B thereby restricting flow of fluid from the formation 404B through the
gravel pack
432B and into the wellbore 402B. By deploying the tool 400A into the wellbore
402B
and directing radiation from the source 428 along paths 13,-13,,, and to the
detector 426, as
shown in Figure 12B, an image of the detritus 438 in the gravel pack 432B can
be
obtained. Thus the presence of the detritus 438 can be identified or confirmed
by
comparing a baseline image of the gravel pack 432 with the later obtained
image.
[0080] In one example, the baseline image can be obtained prior to
remediating or
repairing the gravel pack 432, where one example of remediation/repair is an
acidizing
procedure. Referring now to Figure 12C, the wellbore 402C is being imaged with
the
tool 400A after operations for repairing or remediating the gravel pack 432C
have taken
place. In the example of Figure 12C, the gravel pack 432C continues to contain
detritus
438C that was not removed during the attempted repair. As such, by imaging the
gravel
pack 432C with the tool 400A as shown, radiation from the source 428 scatters
from
within the gravel pack 438C and detected by detector 426. Thus analysis of the
detected
scatter can reveal the presence of and a location of the remaining detritus
438C. The
imaging the wellbore 402C with the tool 400A after a repair or remediation of
the gravel
pack 432C can verify the repair procedure was successful, and if not, can
reveal at what
depth and azimuth detritus 438C remains in the gravel pack 432C. Based on this
information, decisions for future or additional repair/remediation can be
made.
23
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100811 In Figure
13, a side sectional view of an example embodiment of the 400A is
shown disposed in a wellbore 402 where different types of and/or weight
densities of
cement are in place between casing 437 and the surrounding formation 404. More
specifically, a portion of the cement bond is made up of a light weight cement
440 shown
disposed in the annulus 420. In one example, light weight cement describes
cement
between a casing and formation having a density of up to around 12
pounds/gallon. Also
illustrated in the example of Figure 13 is a standard weight cement 442
disposed in the
annulus 420 above the light weight cement 440. For the purposes of discussion
herein,
standard weight cement can include cements between a casing and formation
having a
density greater than around 12 pounds/gallon. The tool 400A of Figure 13 is
configured
so that radiation from its source 428 is directed along paths and that
scatters of the
radiation that occur from within the annulus 420 having the cement 440, 442
are sensed
by the detector 426. Because of the sensitivity and resolution of the tool
400A, the
radiation will scatter from the light weight cement 440 differently from how
it will scatter
from the standard weight cement 442. Moreover, an analysis of the differences
in
sensing of the detector 426 when the tool 400A is disposed adjacent the
different cements
440, 442, can identify the respective locations of these different cements
440, 442.
10082] With
reference now to Figure 14, an example of the tool 400A is depicted in a
side partial sectional view disposed in a wellbore 402 wherein asphaltenes 444
are being
produced from the formation 404. In this example, the asphaltenes 444 can
become
lodged in the gravel pack 432 as well as the perforation 436 in the casing
437. Similar to
the example of operation of Figure 13, radiation from the source 428 in the
tool 400A is
directed radially outward from the tool 400A so that some of the radiation
scatters from
asphaltenes 444 in the gravel pack 432 or screen 436. Sensing the radiation
scatter with
the detector 426 and analysis the results of the sensing can indicate the
presence and/or
location of asphaltenes 444 in the gravel pack 432 or screen 436. In one
example, the
presence of asphaltenes are detected by limiting an energy level(s) of the
radiation sensed
with the detector 426 to be consistent with an energy level(s) of radiation
known to
scatter from asphaltenes 444.
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[0083] In Figure 15, an example embodiment of the tool 400A is shown in a
side
partial section view. In this example the tool 400A is equipped with a
centralizer 430 that
positions the tool 400A towards the mid-portion of the wellbore 402. Radiation
is
directed from the source 428 along paths P, ¨P,, so that the radiation that
scatters from
annulus 420 can be sensed by receiver(s) 426. In one example, strategically
forming
collimator 328 (Figure 3B) and spatially locating the source 428 and
detector(s) 426
enables sensing of desired radiation scatter by the detector(s) 426. Also
optionally,
monitoring scatter in a selective energy range can indicate the material
disposed in the
annulus 420. In the example of Figure 15, precipitate 424 is detected in the
annulus 420
by analyzing the counts of scattered radiation sensed by the sensor(s) 426,
where the
precipitate 424 can include barite.
100841 The present invention described herein, therefore, is well adapted
to carry out
the objects and attain the ends and advantages mentioned, as well as others
inherent
therein. While a presently preferred embodiment of the invention has been
given for
purposes of disclosure, numerous changes exist in the details of procedures
for
accomplishing the desired results. These and other similar modifications will
readily
suggest themselves to those skilled in the art, and are intended to he
encompassed within
the spirit of the present invention disclosed herein and the scope of the
appended claims.
