Note: Descriptions are shown in the official language in which they were submitted.
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FOR~,AT10~ DE~SITY LOGGING USI~G
_ O DETECTORS AND 50URCES
Back~round of the Invention
1. Field of the Inven~ion
The present invention relates to logging of subterranean formations for
determination of density using gamma rays. Particularly, this ;nvention
relates to determination of formation density without positioning the logging
probe against the wall of the borehole traversing the earth formation. More
particularly, this invention is useful for measurement of density while
lû drilling.
2. Setting of the ]nvention
Wireline gamma ray density probes are devices incorporating a gamma ray
source and a gamma ray detector, shielded from each other to prevent counting
of radiation emitted directly from the source. During operation of the probe,
lS gamma rays (or photons) emitted from the source enter the formation to be
studied, and interact with the atomic electrons of the material o~ the forma-
tion by photoelectric absorption, by Compton scattering, or by pair production.
In photoelectric absorption and pair production phenomena, the particular
photons involved in the interacting are removed from the gamma ray beam.
Ir the Compton scattering process, the involved photon loses some sf its
energ~ while changing its original direction of travel, the loss being a
function of the scattering angle. Some of the photons emitted from the source
into the sample are accordingly scattered toward the detector. Many of these
never reach the detector, since their direction is changed by a second Compton
scattering, or they are absorbed by the photo-electric absorption process of
the pair production process. The scattered photons that reach the detector and
interact with it are counted by the electronic equipment associated with the
detector.
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The major difficulties encountered in conventional gamma ray density
measurements include definition of the sample size, limited efSective depth and
sampling, disturbing effects of undesired, interfering materials located
between the density probe and the sample and the requirement that the probe be
positioned against the borehole wall. The chemical composition of the sample
also affects the reading of conventional gamma ray density probes.
One prior art wireline density probe disclosed in U.S. Pat. No. 3,2U2,822
incorporates two gamma ray detectors, one collimated gamma ray source and
ratio-building electronic circuits, and is useful as long as the interfering
1û raterials, located between the detectors of the probe and the formation sam?le,
are identical in thic~ness and chemical composition along the trajectories of
emitted and received gamma rays. Non-uniformities in the wall of the borehole
will interfere with the proper operation of the probe. Such non-uniformities
can be caused by crooked holes, by cave-ins, and by varying thicknesses of the
mudcake on the wall of the hole.
The prior art also includes U.S. Pat No. 3,846,631 which discloses a
wireline density probe which functions regardless of the thickness and the
chemical composition of materials that are located between the density probe
and the sample. The method comprises passing of two gamma ray beams from two
2C intermittently operated gamma ray sources into the sample, receiving the radiation
- backscattered from each of the two sources by two separate detectors, and
building ratios of products of the four separate counting rates in such a
manner that the numerical result is an indication of the density of the sample.
The critical dimension of the two-detector probe is the spacing between
the detectors. If the interfering materials are non-uniform over distances
comparable to the spacing of the two detectors, the measured density will be
erroneous.
Neither of the wireline probes described above is disclosed as being
useful for measurement while drilling and incorporation into a rotating drill
string.
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SUMMARY OF ~HE INVENTION
It is a primary object of this inYentiOn to provide a method and apparatus
for measuring the density of a subterranean formation while drilling a borehole
traversing the formation.
~his object and other objects are realized and the limitations of the
prior art are overcome in the apparatus of the invention which includes a
device for use in a borehole traversing an earth formation including two gamma
- ray emitting means spread 180 apart about the device, the means emitting
collimated gamma ray beams along two trajectories, the trajectories projecting
1û in an azimuthally symmetric pattern about the axis of the device, intersecting
at a first point on the axis of the device, and intersecting d first circle
located in a sample of the formation to be measured, a first gamma ray detect-
ing means oriented to received emitted gamma rays scattered from two locations
within the formation sample along a first two trajectories, the trajectories
projecting in an azimuthally symmetric pattern about the axis of the device,
~ntersecting a second point on the axis of the device and intersecting the
first circle, and a means for determining the product of the counting rate of
gamma rays received by the detecting means from each of the two trajectories as
scattered from the two locations within the formation sample, wherein, the
product is indicative of the average density of the formation sample.
The objects of this invention are realized further by the method of
determining the average density of a sample of earth formàtion surrounding a
borehole including the steps of lowering a de~ice into the borehole to a
location adiacent to the sample; emitting gamma rays into the formation from
the device along two trajectories projecting in an azimuthally symmetric
pattern about the axis of the device, intersecting at a first point on the axis
of the device and intersecting a first circle located in the formation sample;
counting the emitted gamma rays scattered from the formation sample back to the
device along a first set of two trajectories projecting in an azimuthally
symmetric pattern about the axis of the device, intersecting at a second point
on the axis of the device and intersecting the first circle; and determining
the product of the two count measurements, wherein the product is indicative of
the average density of the formation sample.
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BRIEF DESCRIPTIO~ OF THE DRAWINGS
Other features and intended advantages of the invention will be more
readily apparent by reference to the following detailed description in connec-
tion with the accompanying drawing in which Figure 1 is a cross-sectional
representation of a device in accordance with the present invention for logging
densities in a formation traYersed by a rotating drill string, in which the
de~ice may be located, and Figure 2 is a schematic representation of the
electronic circuitry required to detect, count and process the scattered
ph~tons.
PREFERRED EMBODIMENT OF THE INVENTION
The gamr,a density sub 10 of this invention is shown in Figure 1 as inter-
cor,nected between the upper drill string 12 and the lower drill string 14.
Rotation of the drill string 12, 14 causes the drill bit 16 to form borehole 18
traversing earth formation 20.
The sub lO includes a first gamma ray source 22~ and a second gamma ray
source 24. The two sources are situated about the sub in an azimuthally
sym"etric pattern i.e. 180 apart. The sources are collima~ed to form trajec
to ies which are also azimuthally symmetrical. The trajectories are oriented
to pass through a first point 28 located on the axis 29 of the sub 10. The
ter" trajectory as used herein indicates not only the actual path of travel of
the gamma ray but also a line of extension behind the source as well as beyond
the detector.
The plurality of sources may be a single primary source from which the
emitted gamma rays are collimated to form the two symmetrical gamma ray beams.
The sub 10 further includes a first set of detectors including a first
gamma ray detector 30 and a second gamma ray detector 32. The detectors are
situated about the sub 10 in an azimuthally symmetrical pattern which is in
axial and azimuthal alignment with the first and second sources 22 and 24. The
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detectors are collimated to receive gamma rays scattered from the formation
along trajectories which are also azimuthally symmetrical. The trajectories
are oriented to intersect the axis 29 of the sub 10 at a second point 36.
~; ~he trajectories from the sources will intersect the first set of detector
trajectorieS at a first circle 38 about the sub 10. The first circle falls in
a plane which is perpendicular to the axis of the sub 10, the plane
intersecting the axis 29 at a third point 39. The second point 36 is posi-
tioned an axial distance away from the first point 28 and the first and second
points 28, 36 are preferably on opposite sides o~ the third point 39.
The sub 10 includes a second set of detectors including a third gamma ray
detector 40, and fourth gamma ray detector 42. This second set of detectors is
situated about the sub 10 in an azimuthally symmetrical pattern which is also
in axial and azimuthal alignment with the first and second sources, 22, 24 and
the first set of detectors 30 and 32.
The second set of detectors will receive gamma rays along a third set of
two trajectories which are azimuthally symmetric about the sub 10 and are
oriented to intersect the axis 29 at a fourth point 45 and to intersect a
second circle 72. Preferably, the fourth point 45 and the first point 28 are
on opposite sides of the third point 39. Each trajectory of the third set
2û should be parallel to a corresponding trajectory of the second set.
The first and second sets of detectors are shielded from the sources to
prevent the emitted gamma rays from reaching the detectors directly from the
sources.
~ he firs* circle 38 and the second circle 72 formed in the formation 2û
will be the center of the formation samples 46 and 47 respectively which are to
be measured for density.
In the rethod of this invention the sub 10 rotates about its axis 29 as
gamma rays 48 are emitted into the sample by the first source 22 and gamma rays
50 by the second source 24. The emitted collimated beams of gamma rays form a
first cone-shaped region of formation which is irradiated.
In the formation 20, some of the gamma rays 48 and 50 are scattered by the
sample formation 46 toward the first set of detectors. Gamma rays 54 are
scattered at location 56 in formation sample 46 toward and received by the
first detector 30. Gamma rays S8 are scattered at location 62 in formation
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sample 4~ to~ards and received by the second detector 32. Since the two
collimated sources 22, 24 are symmetrically located, there is only one right
conical resion irradiated during the sub's rotation. The two collimated
detectors 30, 32 receive emitted gamma rays scattered from the formation sample
46 bac~ to the sub 10 along trajectories forming a second cone, inverted with
respect to the first cone.
The thickness of the cones is determined by the collimator's diameter.
~he circle 38 formed by the intersection of the cones has as its center point
39 on the axis 29 of sub 10. At a given instant of time, two small sectors 66,
68 of the formation sample, each 180~ apart, will be sampled.
The received gamma rays 54, 58 will react with the first set of detectors
30, 32 and cause electrical pulses. The pulse amplitudes are proportional to
the energy of the received gamma rays. If it is desired to provide counting
rates indicative of only those rays which have been scattered only once in the
samFle 46, these p~llses would be amplified by preamplifiers and amplifiers, andfed to discriminators (not shown), which are set to pass only those pulses
having energy levels of gamma rays that were scattered at the location 56,
to~ards the detector 30, and at location 62 towards the detector 32. Gamma
rays that underwent multiple scattering prior to entering the detectors 30, 32
will be rejected by the discriminators. The output of the detectors and, if
used, the discriminators, leads to the gates, which provide individual counting
rates of received gamma rays from the two detectors 30, 32. This arrangement
is shown generally in Figure 2.
The product of the counts in the near detectors 30 and 32 and in the far
detectors 40 and 42 and the quotient of the products is produced using the
electronics schematically shown in Figure 2. The counters 80-83 convert the
current pulses produced in the detectors into digital voltage pulses by means
of amplifiers and voltage discriminators (not shown) and then store the counts.
~he counts from the near detectors 30, 32 are stored in counters 80 and 81,
the counts from the far detectors 40, 42 are stored in counters 82, 83.
Inputs into the counters are voltage counts from the detectors and voltage
levels from the clock 75.
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The clock 75 is preset to produce d pulse at regular intervals, for
example every 30 seconds. When it sends a pulse to the counters 80-83 and to
the multipliers 84, 85 the counts in 80-81 and the counts in 82-83 are ~Jlti-
plied together by the multipliers 84 and 85 respectively. ~,ultiplying device
84 computes the produc~ of counts in counters 8û, 81; multiplying device 85
computes the product of counts in counters 82, 83. The dividing device 86
computes the quotient of the products produced by devices 84 and 85 once every
time a pulse is signaled from the clock 75. The output of divider 86, i.e.,
the ratio of the outputs of multipliers 84, 85 may then be plotted versus time
by a suitable plotting device 87.
~he individual counts from the detectors 30, 32, 40 and 4Z may vary with
time due to the sub's location within the borehole as caused by rotatior,of the
drill string off of the axis of the borehole.
In the method of this invention, the two instantaneous counts from the
first set of detectors 30, 32 are multiplied by multiplier 84 resulting
in a constant value thus indicating elimination of variables with time, such as
the thic~ness of mud through which the emitted gamma rays must pass to be
received at the detectors, and the movement of the sub in relation to the
borehole wall.
The sub, in an off-axis position, will recei~e gamma rays 48, which have
scattered from the formation sample 46, at detector 30. ~hese rays 48 will
have traveled thru a different amount of mud and formation than gamma rays 50
from source 24. However, the sum of the path lengths through mud, and the sum
of the path lengths through the formation are constant provided that the
diameter of sub 10 is substantially similar to the diameter of borehole 18.
A density log for measurement while drilling applications should be
accurate to within about 0.1 g/cm3. Since formation density is typically 2.5
g/cm3, the accuracy required is about 4%. If vertical resolution required for
the log i5 0.5 foot, a required counting rate may be estimated as follows:
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S
~r ' 0.04
N~N2
where
c~ is the statistical variation of the produce ~lN2
Nl is the total counts at detectors 30, and
N2 is the total counts at detectors 32.
assuming Nl~ N2 = N
then, from the field of statistics
-
~r = ~ 2 N3
~ .,
and cr ,,, cr = ~ 2 ~ O.04
NlN2 N2 N
Solving for N
N ~ 1250 counts
Each density log measurement should detect an average of 1,250 counts per
measurement and there should be a measurement every 1/2 foot. At 60 feet per
hour drilling rate, each measurement will therefore be completed in 30 seconds.
Therefore, each detector 30, 32, 40, 42 should have sufficient sensitivity
such that about 43 counts per second are registered. Alternatively, each
source may be adjusted to emit at a rate such that the detectors receive at the
required rate of 43 counts per second.
To compensate for borehole effects Dn the measurement of the average
density for the formation samples 46 and 47, the method of this invention would
include use of the counts from the second set of detectors 40, 42. The product
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of these two counts (output of multiplier 85) would be used to form a ratio
(output of divider 86) between the product of the first set of detectors and
the product of the second set of detectors. Alternatively, the prDduct of the
two ratios of a detector of the first set to a corresponding detector of the
second set may be used to determine the average density. This is shown sener-
ally in Figure 2.
A similar arrangement for the second set of detectors 40, ~2 may be
included in the sub 10 for receiving, discriminating, counting, storing and
using the gamma rays received by the second set, as shown in Figure 2.
The type of gamma ray sources is also not an object of the invention,
since different types are preferred for different applications. Capsule type
sources containing the radioactive isotopes such as cobalt 60 and cesiu 137,
are the types of gamma ray sources most frequently used in gam'"a ray der,sity
probes.
The diameters of the borehole 18 and the sub 10 should be substantially
equivalent. This can be accomplished by the use of stabili2ers on the exterior
of the sub which are then part of the relative diameter determination.
Various other alterations in the details of construction and the sequence
of computations can be made without departing from the scope of the invention,
which is indicated in the appended claims.
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