Note: Descriptions are shown in the official language in which they were submitted.
CA 02506133 2005-05-02
LOGGING TOOL WITH A PARASITIC RADIATION SHIELD
AND METHOD OF LOGGING WITH SUCH A TOOL
Field of the invention
This invention relates to logging of oil, water or gas well in underground
formations surrounding
a borehole and more particularly to a logging tool with a parasitic radiation
shield such as a
logging-while-drilling gamma ray density measurement tool.
Description of the Prior Art
In hydrocarbon exploration and production, it is of prime importance to
determine if a given earth
formation contains hydrocarbon, and the amount of hydrocarbon within the
formation.
Therefore, formation properties while drilling or in a freshly drilled hole
are measured to predict the
presence of oil, gas and water in the formation. These formation properties
may be logged with
wireline tools, logging while drilling (LWD) tools, or measurement while
drilling (MWD) tools.
One method to predict formation properties is to measure the density of
material in earth
formation using a source of nuclear radiation and a radiation detector. The
density of a material
can be determined either by a transmission or by a scattering measurement. In
a transmission
measurement the material, the density of which needs to be determined, is put
between the
radiation source and the detector. In a scattering measurement the intensity
and energy
distribution of the radiation scattered back to a detector from the material
under investigation
is used to determine the density. Downhole measurements of formation density
are of the
scattering type since it is not usually possible to insert the formation
material directly between
source and detector, with the possible exception of rock samples removed from
the formation.
Gamma-ray scattering systems have been used for many years to measure the
density of a material
penetrated by a borehole. Typically density is measured as a function of
position 25 along the
borehole thereby yielding a "log" as a function of depth within the borehole.
The measuring
tool typically comprises a source of radiation and one or more radiation
detectors, which are in
the same plane as the source and typically, mounted within a pressure tight
container.
Radiation impinges on and interacts with the material, and a fraction of the
impinging
CA 02506133 2005-05-02
radiation is scattered by the material and a traction thereof will return to
the detector. After
appropriate system calibration, the intensity of the detected scattered
radiation can be related to
the bulk density of the material.
The radial sensitivity of the density measuring system is affected by several
factors such as the
energy of the gamma radiation emitted by the source, the axial spacing between
the source and
the one or more gamma ray detectors, and the properties of the borehole and
the formation.
The formation in the immediate vicinity of the borehole is usually perturbed
by the drilling
process, and more specifically by drilling fluid that invades the formation in
the near borehole
region. Furthermore, particulates from the drilling fluid tend to buildup on
the borehole wall. This
buildup is commonly referred to as mudcake, and adversely affects the response
of the system. In
this way, intervening material between the tool and the borehole wall will
adversely affect the
tool response. Intervening material in the tool itself between the active
elements of the tool and
the outer radial surface of the tool will again adversely affect the tool
response by producing
a background of scattered radiation which is independent of the presence of
the borehole
fluid, the mudcake or the formation. Typical sources are isotropic in that
radiation is emitted
with essentially radial symmetry. Flux per unit area decreases as the inverse
square of the
distance to the source. Radiation per unit area scattered by the formation and
back into
detectors within the tool also decreases with increasing distance, but not
necessarily as the
inverse square of the distance. In order to maximize the statistical precision
of the measurement it
is desirable to dispose the source and the detector as near as practical to
the borehole environs,
while still maintaining adequate shielding and collimation.
Prior art logging-while-drilHng systems use a variety of source and detector
geometries to
minimize standoff, such as placing a gamma ray source and one or more gamma
ray detectors
outside the tool body within a drill collar with a stabilizer disposed between
source and detectors
and the borehole and formation; or within stabilizer fins that radiate outward
from a drill collar.
This tends to minimize intervening material within the tool, and positions
source and detectors
near the borehole environs, but often at the expense of decreasing the
efficiency of shielding and
collimation. The signal-to-noise ratio is often degraded by the detection of
particles that have not
probed the earth formation but instead have traveled trough low-density
regions or voids existing
in the tool between source and detectors, and especially through collar and
stabilizer.
2
CA 02506133 2012-05-24
Shielding of source and detectors mounted in the tool body is well known in
the prior art;
chassis is shielded and detectors are mounted in a shielded holder with
windows through
which radiation is detected. Other prior art patents focus on total radiation
shielding of the
tool to the detriment of functionality: EP 0160351 describes a shielded tool
casing with
windows, which receives instrument package, US 6,666,285 describes an
apparatus, which
has a cavity to receive a solid shielded instrument package. Those apparatus,
because they
use a framework totally made of high-density materials, are heavy and brittle,
and in harsh
drilling conditions, can be broken resulting in the destruction and possibly
the loss of the
instrument package and more critically the loss of the radioactive source. The
problem of
providing shielding in the collar and 10 the stabilizer has not been yet
addressed
successfully.
Summary of the Invention
According to an aspect of the invention there is provided a logging tool for
underground
formations surrounding a borehole, comprising: an elongated body along a major
axis; a
collar disposed peripherally around the body having a collar wall defined by
an inner and
an outer surface. Further, the tool comprises a radiation emitting source
arranged to
illuminate the earth formation surrounding the borehole; at least one
radiation detector
arranged to detect radiation reflected by the earth formation resulting from
illumination by
the source; at least one source collimation window and one detector
collimation window
through which the earth formation is illuminated and radiation is detected;
and at least one
radiation shield located between the inner collar surface and the outer
surface of the tool,
the radiation shield positioned so as to eliminate parasitic radiation that
has not traversed
the outer collar. The radiation shield is located between the emitting
radiation source and the
radiation detector, the radiation shield has a length along the major axis,
which is less than
80% of the distance between the emitting radiation source and the radiation
detector.
In a preferred embodiment, the tool further comprises a stabilizer located at
the periphery
around the outer collar surface, wherein this stabilizer comprises a
stabilizer wall defined
by an inner stabilizer surface and an outer stabilizer surface, and wherein
the radiation
shield is located between this inner collar surface and this outer stabilizer
surface. The
3
CA 02506133 2012-05-24
stabilizer enhances the contact between the tool and the formation by reducing
the space
available for mud between the tool and the formation.
The tool is designed so that the source and the detector are as near as
practical to the
borehole environs. The radiation shields increase the signal to noise ratio.
And the
invention below proposes a robust, secure and functional configuration.
In a preferred embodiment, the radiation shield may have a thickness in the
cross section
perpendicular to the major axis, which is preferably less than 40% of the
width of the tool
at the position of the radiation source. This makes it possible to eliminate a
significant
fraction of the radiation that are coming from source and that have not passed
through the
borehole fluid and the formation, but whose path was entirely inside the
collar and the
stabilizer.
In a preferred embodiment, the radiation shield may have an annular shape
surrounding the
detector window and has a length along the axis, which is less than 40% of the
distance
between the source and the detector. In a preferred embodiment, the radiation
shield may
have a thickness in the cross section perpendicular to the major axis, which
is less than
40% of the width of the tool at the position of emitting radiation source.
This enables
eliminating a part of the radiations passing through the collar to the
detecting window and
not through the window in the collar to the detector window.
In a preferred embodiment, this invention may be directed toward a radiation
density
measurement system in underground formations surrounding a borehole with a
chemical
radioactive source or an electronic radiation source emitting x-ray; or a
chemical or
electronic neutron source.
In a preferred embodiment, this invention may be directed toward a gamma-ray
logging-
while-drilling density tool. The system may comprise a source of gamma
radiation and one
or more gamma detectors. Multiple detectors (2 or more) provide better
efficiency and
allow compensation for the effect of mud and mudcake intervening between the
tool and
the formation. It is clear, however, that the basic concepts of the invention
could be
4
CA 02506133 2013-02-14
employed in other types and classes of logging, logging-while-drilling or
measurement-
while-drilling systems. As an example, the invention can be used in a neutron
porosity
system for measuring formation porosity, wherein the sensor comprises a
neutron source
and one or more neutron detectors.
The gamma-ray radiation shield can be fabricated from a high atomic number
material,
commonly referred to as "high Z" material. High Z material is an efficient
attenuator of
gamma-ray radiation, and permits the efficient shielding, collimation and
optimum
disposition of the source and detectors with respect to the borehole environs.
According to another aspect of the invention there is provided a method for
logging a well
comprising the steps of: lowering a logging tool in a well; and logging the
well using the
logging tool; wherein the logging tool comprising: an elongated body along a
major axis; a
collar disposed peripherally around the body having a collar wall defined by
an inner and an
outer surface; a radiation emitting source arranged to illuminate the earth
formation surrounding
the borehole; at least one radiation detector arranged to detect radiation
reflected by the earth
formation resulting from illumination by the source; at least one source
collimation window
and one detector collimation window through which the earth formation is
illuminated and
radiation is detected; and at least one radiation shield located between the
inner collar surface
and an outer surface of the tool, the radiation shield positioned so as to
eliminate parasitic
radiation that has not traversed the outer surface of the collar and the
radiation shield is located
between the emitting radiation source and the radiation detector, the
radiation shield has a length
along the axis, which is less than 80% of the distance between the emitting
radiation source
and the radiation detector.
Also disclosed is a logging tool for underground formations surrounding a
borehole, comprising:
an elongated body along a major axis; a collar disposed peripherally around
the body having
a collar wall defined by an inner and an outer surface; a radiation emitting
source arranged to
illuminate the earth formation surrounding the borehole; at least one
radiation detector arranged
to detect radiation reflected by the earth formation resulting from
illumination by the source; at
least one source collimation window and one detector collimation window
through which
the earth formation is illuminated and radiation is detected; at least one
CA 02506133 2013-02-14
radiation shield located between the inner collar surface and an outer surface
of the tool, the
radiation shield positioned so as to eliminate parasitic radiation that has
not traversed the outer
surface of the collar; and another radiation shield having an annular shape
surrounding the
detector collimation window.
Brief description of the drawings
Further embodiments of the present invention can be understood with the
appended
drawings:
= Figure 1 illustrates a logging-while-drilling tool according to the
invention.
= Figure 2a is a side view on the major axis of the tool of figure 1 with
the
radiation shield localized between source and first detector.
= Figure 2b is a side view on the major axis of the tool of figure 1 with
the
radiation shield localized closed to first detector.
= Figure 2c is a side view on the major axis of the tool of figure 1 with
both
radiation shields.
= Figure 3 shows pulse-height spectra obtained by numerical modeling of the
logging-while-drilling tool of figure 2a and 2c as well as a case in which
neither of the shields 30 and 31 is present.
Detailed description
Figure 1 illustrates a logging-while-drilling tool, identified as a whole by
the numeral 20,
disposed by means of a drill string within a well borehole 18 defined by a
borehole wall 14
and penetrating an earth formation 16. The upper end of the collar element 22
of the tool
20 is operationally attached to the lower end of a string of drill pipe 28.
The stabilizer
element of the tool 20 is identified by the numeral 24. A drill bit 26
terminates the lower
end of logging tool 20. It should be understood, however, that other elements
can be
disposed on either end of the tool 20 between the drill pipe 28 and the drill
bit 26. The
5a
CA 02506133 2013-02-14
upper end of the drill pipe 28 terminates at a rotary drilling rig 10 at the
surface of the earth
12. The drilling rig rotates the drill pipe 28 and cooperating tool 20 and
drill bit 26 thereby
advancing the borehole 18. Drilling mud is circulated down the drill pipe 28,
through the
axial passage in the collar 22, and exits at the drill bit 26 for return to
the surface 12 via the
annulus defined by the outer surface of the drill string and the borehole wall
14.
Figures 2a, 2b and 2c illustrate conceptually radiation shields on the tool 20
of figure 1
shown in
5b
CA 02506133 2005-05-02
side view on the major axis of the tool. In a first embodiment, the tool is a
logging- while-drilling
gamma-ray scattering tool with a chemical radioactive source. The tool 20 is
made of an elongated
tool body 21 and a drill collar 22 disposed peripherally around the tool body
21. In the illustrated
tool, a stabilizer 24 is disposed peripherally around the drill collar 22; the
stabilizer is optional
and reduces the amount of mud between the tool and the formation wall and
therefore the
influence of the borehole fluid on the measurement. The tool 20 receives one
source collimation
window 202 through which the earth formation 16 is illuminated by the
radiation emitted from
the radioactive source, and two detector collimation windows 212 and 222
through which the
radiation coming from the outside of the tool 20 is detected. In the
illustrated tool, a source of
gamma radiation 201 illuminating the earth formation 16 and affixed to a
source holder 200, is
mounted in the collar wall 22. Though this is the preferred way, other
locations for the source
201 are in the tool body 21 or in the stabilizer 24. The source 201 is
preferably cesium-137
('Cs) which emits gamma radiation with an energy of 0.66 million electron
volts (MeV).
Alternately, cobalt-60 (60Co) emitting gamma radiation at 1.11 and 1.33 MeV
can be used as
source material. The tool 20 receives a first or "short spaced" gamma ray
detector 211 disposed
at a first axial distance from the source 201, and a second or "long spaced"
gamma ray detector
212 disposed at a second axial distance from the source, where the second
spacing is greater than
the first spacing. In the illustrated tool, the detectors are mounted in the
tool body 21 in holders:
210 for the first detector and 220 for the second detector. Though this is the
preferred way, other
locations for the detectors 211, 221 are in the collar wall 22 or in the
stabilizer 24. The detectors
are preferably scintillation type such as sodium iodide (Nat) or Gadolinium-
oxy-ortho-silicate
(GSO) to maximize detector efficiency for a given detector size.
Insertion of high-density materials in the collar is often undesirable since
the collar supports the
stresses inherent to logging conditions, in figure 2a, a side view of the tool
illustrates a
radiation shield 30 located in the collar 22 whose shape is optimized to
reduce leakage
through the collar without affecting its mechanical strength.
The trajectories of gamma rays traveling from the source to the detector are
like broken
lines, on which each break corresponds to a collision with an electron within
the surrounding
material Gamma radiations lose energy by means of the most pertinent reaction
here:
Compton scatter reaction. After undergoing one or more Compton scattering
events, a small
fraction of the emitted with reduced gamma-ray energy returns to the tool and
is detected by the
gamma radiation detector. The function of the radiation shield 30 is to
intercept and attenuate
by photoelectric absorption or by Compton scattering and subsequent
photoelectric
6
CA 02506133 2005-05-02
absorption, a significant fraction of those gamma rays that travel through the
collar or/and
stabilizer and that might otherwise go back to the detector after being
scattered in the collar
or/and stabilizer.
Figure 2b illustrates a side view of the tool with a radiation shield 31
located on the inner collar
surface in the collimation window 212 of the first detector 211. The function
of the radiation
shield 31 is to intercept and attenuate gamma rays traversing the collar to
the detecting window.
Figure 2c illustrates a side view of the tool with both radiation shields 30
and 31.
To estimate the amount of gamma ray leakage that is effectively removed by the
radiation
shields, a Monte-Carlo N-Particle model is built based on the tool plan of
figures 2. A
compromise is found between the effective shielding and the mechanical
strength of the tool.
The model of source used is a mono-energetic 0.662 million electron volts
(MeV) cesium-137
radiation. Pulse-height spectra for energies between 0.1 and 0.5 MeV for the
first Nal detector
are computed for three different configurations: (1) tool without extra
radiation shield, (2) tool
with radiation shield 30 as in figure 2a, (3) tool with radiation shields 30
and 31 as in figure 2c.
One or more pieces of a high-density material, i.e. a material with a high
atomic number
(more than Z-70) and a high density (more than 15 g/cc) like tungsten, gold or
depleted uranium,
are inserted in the collar in a particular locations where their shielding
efficiency will be
maximal and their influence on the mechanical stmigth will be minimal. High Z
materials are
efficient attenuators of gamma radiation, and permit the efficient shielding,
collimation and
optimum disposition of the source and detectors with respect to the borehole
environs.
The radiation shield 30 of figure 2a is in a preferred embodiment, placed into
a cavity in the outer
surface of the collar, wrapped in a rubber envelope and then compressed
underneath a cover
plate screwed onto the collar between the source and the detector. In a
preferred
embodiment, better efficiency is obtained when length along the axis of this
radiation shield is
less than 80% of the first axial distance between source and detector; and
when thickness of this
radiation shield in the cross section perpendicular to the major axis is less
than 40% of the width
of the tool at the position of the source. In a second preferred embodiment,
best efficiency is
obtained when length along the axis of this radiation shield is less than 60%
of the first axial
distance between source and detector; and when thickness of this radiation
shield in the cross
section perpendicular to the major axis is less than 20% of the width of the
tool at the position of
the source. The radiation shield is disposed circumferentially around the
collar outer surface, and
preferably covering less than 180 of this surface. The effectiveness of the
radiation shield 30 is
7
CA 02506133 2005-05-02
maximized when its edge is brought closer to that of the collimation window of
the first detector.
The effectiveness is also increased when the thickness of the radiation shield
is increased or an
extension towards the source is made, but at the expense of a lower mechanical
strength. As an
example of optimization, for a circular part of a tungsten patch, the length
along the axis is 58
mm whereas the first axial distance is 170 mm, and the thickness is 7 mm. and
for the circular
part, the internal radius is 78 mm and the opening angle is 90 .
The radiation shield 30 of figure 2a can be associated with another radiation
shield 31 of figure
2b, located at the base and very close to the collimator window of the first
detector, this radiation
shield 31 has an annular shape surrounding this collimator window and with a
trapezoidal
section. Both radiation shields in this embodiment are illustrated on figure
2c. The efficiency is
maximized with specific angular aperture of the trapezoidal section just as
the dimension of
the annular shield. Nevertheless, these dimensions of the annular shield are
dictated by the
requirements for mechanical strength. Therefore, in a preferred embodiment,
better
efficiency for the radiation shield 31 is obtained when this radiation shield
is located between the
first detector and the outer stabilizer surface facing the first detector, and
when this radiation
shield has an annular shape with a length along the axis or a diameter, which
is less than 40% of
the distance between source and first detector. In a second preferred
embodiment, best efficiency
for the radiation shield 31 is obtained when this radiation shield has an
annular shape with a
length along the axis or a diameter, which is less than 20% of the distance
between source and
first detector. In a preferred embodiment this radiation shield has a
thickness in the cross section
perpendicular to the major axis, which is less than 40% of the width of the
logging-while-drilling
tool at the position of emitting radiation source. In a second preferred
embodiment, this radiation
shield has a thickness in the cross section perpendicular to the major axis,
which is less than 20%
of the width of the logging-while-drilling tool at the position of emitting
radiation source.
Figure 3 shows the pulse-height spectra obtained by numerical modeling of the
tool with
optimized radiation Welds 30 and 31 for the three configurations already
described above. In
order to determine the amount of gamma-radiation passing through the tool to
the detectors,
without interacting with the materials in the borehole or the formation, the
earth formation is
assumed to be very dense like tungsten (17.4 g/cm3) so that practically no
gamma-rays will
return from the formation and the signal is entirely due to gamma-rays
traveling through the
collar and the stabilizer. From those data and for an energy range between
0.15 and 025 MeV,
corresponding to the principal energy used for logging-while-drilling density
measurements with a
cesium-137 gamma ray source, the percentage of total gamma-ray leakage removed
from the
8
CA 02506133 2012-05-24
total signal by the radiation shields is evaluated. For a stabilizer diameter
of 8 inches, the
percentage of gamma-ray leakage removed is of 45% with the radiation shield 30
alone
and of 54% with both radiation shields 30 and 31; for a stabilizer diameter of
9 3/8 Inches,
this percentage is 43% and 51% respectively.
In a second model, the earth formation is assumed to be made of an aluminum
alloy (2.805
g/cm3) so gamma-rays will return in this model also from the formation. The
percentage of
gamma-ray leakage removed from the signal by the radiation shields is
evaluated in this
model as well and the results are comparable to those obtained with the first
model. For a
stabilizer diameter of 8 inches, the percentage of gamma-ray leakage removed
is 43% with
the radiation shield 30 alone and of 57% with both radiation shields 30 and
31; for a
stabilizer diameter of 9 3/8 inches, this percentage is 38% and 46%
respectively.
The radiation shield 30 removes almost 50% of gamma-ray leakage and the
radiation
shield 31 removes an additional 10% of gamma-ray leakage. These radiation
shields 30
and 31 mounted offer therefore various mechanical, operational and technical
advantages.
Radiation shields between first and second detectors or in the collimation
window of the
long spaced detector are possible; nevertheless this second detector is less
sensitive to
gamma-ray leakage and a reduction of the leakage is less important.
In a second embodiment, the tool 20 is a logging-while-drilling density tool
with an
electronic radiation source. The source 201 is an x-rays generator. The
shielding materials
need to be inserted into the structural materials of the tool body, collar or
stabilizer with
the intent to optimize shielding with a minimal impact on the structural
strength of the tool.
Shielding materials for lower energy gamma-rays or x-rays could be lighter
materials.
In a third embodiment, the tool 20 is a logging-while-drilling neutron
scattering tool with a
chemical or electronic neutron source. The source 201 is a chemical source, as
Radium-
Beryllium source or an electronic source like pulsed neutron generator. The
shielding
materials need to be inserted into the structural materials of the tool body,
collar or
stabilizer with the 10 intent to optimize shielding with a minimal impact on
the structural
9
CA 02506133 2012-05-24
strength of the tool. Shielding materials for neutrons will typically be
hydrogenous
materials and/or neutron absorbing materials, like boron or cadmium for slow-
neutrons;
and will typically be high atomic number materials like tungsten and/or
hydrogenous
materials for fast neutrons.
0
