METHOD FOR DETECTING CEMENT VOIDS OR BOREHOLE WASHOUTS

METHODE DE DETECTION DE VIDES DANS DU CIMENT OU DE POCHE DE DISSOLUTION D'UN TROU DE FORAGE

English Abstract

METHOD FOR DETECTING CEMENT VOIDS
OR BOREHOLE WASHOUTS
(D#73,367-F)
ABSTRACT OF THE DISCLOSURE
A fast neutron source is used to irradiate
earth formations in the vicinity of a well borehole.
Dual spaced epithermal neutron detectors are used to
sample the epithermal neutron population at two
different spaced distances from the source. A compen-
sated formation porosity is obtained from the ratio
of counting rates at the dual spaced detectors. An
uncompensated porosity value is obtained from the
count rate at the short spaced detector. Borehole
washout or cement void regions are located by comparing
the compensated and uncompensated values of formation
porosity obtained in this manner.
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Claims

The embodiments of the invention in which an exclu-
sive property or privilege is claimed are defined as follows:
1. A method for locating borehole washouts
or cement voids between the casing and earth formations
in a well borehole, comprising the steps of:
irradiating the earth formations in the
vicinity of the borehole with fast neutrons from a
relatively high intensity neutron source;
detecting essentially only the epithermal
neutron population at a first shorter spaced distance
from said source in the borehole;
detecting essentially only the epithermal
neutron population at a second longer spaced distance
from said source in the borehole;
discriminating against the detection of the
thermal neutron population at said detectors in the
borehole;
combining the epithermal neutron population
measurements made at said two different spaced
distances to derive a first, compensated, indication
of formation porosity;
deriving a second, uncompensated, indication
of formation porosity from said measurement of the
epithermal neutron population at said shorter spaced
distance alone; and
comparing said compensated and said un-
compensated porosity indications to locate the
presence of borehole washouts or cement voids.
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2. The method of Claim 1 wherein the step
combining said epithermal neutron population measure-
ments includes taking a ratio of said measurements to
provide said compensated indication of the porosity of
said earth formations.
3. The method of Claim 1 wherein the two
epithermal neutron detecting steps are performed by
employing cadmium wrapped He3 detectors having effective
sensitive centers at said longer and shorter spaced
distances.
4. The method of Claim 2 wherein said
comparing step includes the step of deriving a percentage
compensation parameter C.
5. The method of Claim 4 wherein said
percentage compensation parameter is defined by the
relationship
<IMG>
where ? (R) is the compensated porosity value and
? (ss) is the uncompensated porosity value.
6. The method of Claim 1 wherein the
steps are repeated at different depth levels in a well
borehole and the compensated and uncompensated porosity
indications are recorded as a function of borehole
depth.
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7. The method of Claim 6 and further
including the step of cross plotting the uncompensated
and compensated porosity indications as an indication
of the presence of borehole washouts or cement voids.
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Description

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1 D#73,367 BACKGROUND OF THE INVENTION
This invention relates to radiological well
logging methods and apparatus for investigating the charac-
teristics of subsurface earth formations traversed by a
borehole and more particularly to apparatus and methods for
measuring earth formations porosities and borehole washouts
or cement voids by means of neutron well logging techni-
ques.
It is well known that oil and gas are more
likely to be found in commercially recoverable quantities
from those earth formations which are relatively porous or
permeable than in more highly consolidated earth forma-
tions. Thus, equipment and methods for accurately
identifying the porosity of earth formations has subs-
tantial industrial importance.
Various methods and apparatus have been proposed
in the prior art for utilizing neutron slowing down and
diffusion through earth formations to measure porosity.
Typically, proposals of this sort have suggested the use of
a pressure housing sonde containing a neutron source and a
pair of neutron detectors spaced at different distances
from the source for transport through a borehole. The
thermal neutron detectors utilized in prior art techniques
have been used with both pulsed and continuous neutron
sources and some combination utilizing the count rate of
the detected thermal neutrons has been related to the-
hydrogen content of the portion of the earth formation
being subjected to the flow of neutrons from the neutron
source. These methods have generally not proven to be as
accurate as desired due to the diameter irregularities of
the borehole wall. The variation of the properties of
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~B~67'8
1 different borehole fluids, the irregular cement annulus
surrounding the casing in a cased borehole, and the pro-
perties of different types of steel casings and earth
formations surrounding the borehole have all tended to
obscure the thermal neutron measurements suggested in the
prior art.
The thermal neutron population surrounding a
source and detector pair sonde as proposed in the prior art
can be affected by the chlorine content of the borehole
fluid. Similarly other lithological factors such as the
boron content of the earth formations surrounding the cased
borehole affect thermal neutron populations. Measurements
of thermal neutron captures are utilized in neutron life-
time logs or thermal neutron population die away logs of
various types as contemplated in the prior art. The
present invention, however, rather than reIying on a
measure of the thermal neutron population, utilizes a
measurement of the epithermal neutron population by means
of two spaced neutron detectors each longitudinally spaced
from a neutron source having a relatively high intensity
neutron flux. Special detector means are utilized in the
invention to effectiveIy discriminate against the detection
of thermal neutrons as proposed in the prior art. More-
over, by comparing the compensated porosity measurement
using the dual speed detectors with an uncompensated
porosity measured by the use of only one of the detectors,
the location of borehole washouts or cement voids can be
found.
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1~)8867~3
In accordance with the invention, there is provided a method for
locating borehole washouts or cement voids between the casing and earth
formations in a well borehole, comprising the steps of: irradiating the
earth formations in the vicinity of the borehole with fast neutrons from a
relatively high intensity neutron source; detecting essentially only the
epithermal neutron population at a first shorter spaced distance from said
source in the borehole; detecting essentially only the epithermal neutron
population at a second longer spaced distance from said source in the bore-
hole; discriminating against the detection of the thermal neutron population
at said detectors in the borehole; combining the epithermal neutron
population measurements made at said two different spaced distances to
derive a first, compensated, indication of formation porosity; deriving a
second, uncompensated, indication of formation porosity from said measure-
ment of the epithermal neutron population at said shorter spaced distance
alone; and comparing said compensated and said uncompensated porosity
indications to locate the presence of borehole washouts or cement voids.
The present invention has been found to give improved results
over prior techniques in that there is less sensitivity to disturbing
environmental parameters.
For a better understanding of the present invention, together with
other and further objects, features and advantages, reference is made to
the following detailed description thereof, when taken in conjunction with
the appended drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic diagram showing a well logging system
for carrying out the method of the present invention;

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FIGURE 2 is a graph illustrating the relationship of the ratio
of the counts in the two epithermal neutron detectors of a well logging
system such as that of Figure 1 to the porosity of earth formations
surrounding a borehole;
FIGURE 3 is a graphical relationship showing the count rate at
the short spaced detector as a function of porosity; and
FIGURE 4 is a cross plot of the short spaced count rate vs. the
ratio of count rates at the two detectors illustrating the location of
washout or cement void regions in a South Texas well.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring initially to Figure 1 there may be seen a simplified,
schematic, functional representation in the form of a block diagram of well
logging apparatus for carrying out the method of the present invention. A
well borehole 2 penetrating earth formations 3 is lined with a steel casing
4 is cemented in place by a cement layer 6 which also serves to prevent
fluid communication between adjacent producing formations in the earth 3.
The downhole portion of the logging system may be seen to be
basically composed of an elongated, fluid tight, hollow, body member or
sonde 7 which, during the logging operation, is passed horizontally through
the casing 4 and is sized for passage therethrough. Surface instrumentation,
whose function will be discussed in more detail subsequently, is shown for
processing and recording electrical measurements provided by the sonde 7.
A well logging cable 8 which passes over a sheave wheel 9 supports the
sonde 7 in the borehole
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1 and also provides a communication path for electrical
signals to and from the surface equipment and the sonde 7.
The well logging cable 8 may be of conventional armored
design having one or more electrical conductors for trans-
mitting such signals between the sonde 7 and the surface
apparatus.
Again referring to Figure l, the sonde 7 contains
a neutron source ll. One neutron source contemplated for
use herein comprises a Californium 252 continuous neutron
source. Such a source has the meritorious feature of
producing a very high intensity of neutrons essentially
having an average energy of 2.3 MEV. Alternatively, an
actinium beryllium neutron source having an intensity of
approximately l x l08 neutrons/sec. may be used to ad-
vantage as will be discussed in more detail subsequently.
However, it will be understood by those skilled in the art
that the invention is not limited to the use of a con-
tinuous neutron source. It is contemplated that a pulsed
neutron source of suitable intensity could be used if
desired, provided that suitable source to detector spacings
are also provided. For the purposes of the preferred
embodiments of the present invention, however, the high
intensity of the Californium 252 or AcBe neutron sources is
desirable.
Suitable radiation detectors 12 and 13 longi-
tudinally spaced from each other and from the neutron
source ll are provided in the downhole tool. These de-
tectors are operated as neutron detectors. In the present
invention it is contemplated that detectors 12 and 13 are
neutron detectors of the He3 type. These are gas filled
counting tubes filled with He3 gas under pressure. The He3
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1 respond to neutrons scattered back to the detectors 12 and
13 from the surrounding earth formations. Charge pulses
~stablished from nuclear reactions between the back
scattered neutrons and the filling gas within the detectors
12 and 13 produce a satisfactory measure of the epithermal
neutron population. Detectors 12 and 13 are connected to
amplifiers 14 and 15 respectively which are connected to
cable driving circuitry 16 for transmission of the electri-
cal pulse signals from the detectors to the surface. Cable
driving circuitry 16 may comprise, for example, an ampli-
fier and means for encoding the pulses from the two de-
tectors for tranmission on the cable. This could be done, -~ -
for example, by transmitting pulses from the detectors as
opposite signed electrical pulses on the cable in order
that they may be distinguished and separated at the sur-
face. Of course, other means could be used as desired.
The portion of the sonde 7 between the neutron source ll
- and detectors 12 and 13 is provided with a shield 17 of a
neutron moderating material, for example, steel and lucite.
This is provided in order to prevent the direct interaction
of neutrons from the source with the two detectors since it
is desired to measure the slowing down effect of the
formations surrounding the borehole on the epithermal
population.
~'' Neutron detectors 12 and 13 of Figure l are
surrounded and enclosed by a cadmium shield 18 which is
designed to screen out the entry of thermal neutrons to the
interior of the detector structure. The higher energy ~ ~
epithermal neutrons penetrate the cadmium shield 18 more
readily. Inside the cadmium shield 18 the neutron de-
tectors are surrounded by and embedded in lucite plastic
I layer l9 (or any other suitable high hydrogen content
material). The cadmium layer surrounding the lucite
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1~88678
1 ~hield is approximately .020 inches in thickness which
is adequate to effectively attenuate the thermal neutron
flux entering the detectors from the borehole. More-
over, the detectors are spaced from the neutron source
at an optimum distance to provide good counting statistics
and formation porosity signal. The short spaced neutron
detector 12 is preferably sized about 1 inch in diameter
and has about 4 inches of effective sensitive length
and contains He3 at 1 atmosphere pressure. The long
spaced detector 13 is sized about 2 inches in diameter,
about 4 inches in effective sensitive length and contains
He3 at about 8 atmospheres pressure. Using a 700 micro-
gram Californium 252 neutron source which emits about
1.69 x 109 neutrons per second, it has been found that
optimum source to detector spacings for these detectors
are approximately 19 inches from the source to the
center of the short spaced He3 detector 12 and about
31 inches to the center of long ~pacèd detector 13.
These dimensions are applicable with the detectors
configured as shown in the drawing of Figure 1 when
surrounded by the cadmium sleeve 18. The portion of
the cadmium sleeve 18 interposed between the two
detectors, as shown at 20 of Figure 1, serves to limit
solid angle response to the detectors to the approxi-
mate borehole level opposite each detector.
In the configuration shown in Figure 1 for
the neutron detectors, it has been found that the
neutron count rate is reduced by about a factor of 4
when the detectors are surrounded by the cadmium sleeve
18 as illustrated, from the count rate obtained with
the same He3 detectors not surrounded by the cadmium
sleeve. This reduction in count rate would render the
spacing from the source to detectors too great to
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1 obtain good counting statistics when used with a lesser
intensity neutron source than Californium 252 or
actinium beryllium. Moreover, it will be noted that
the sensitivity of the neutron detectors 12 and 13 are
unequal as the short spaced detector 12 contains He
at one atmosphere of pressure while the long spaced
detector 13 contains He at eight atmospheres pressure.
Since the epithermal neutron count rate in a He
detector is proportional to the He pressure, the long
spaced detector 13 is more sensitive to epithermal
neutrons than the short spaced detector. This
cooperative arrangement is optimized for the use of
the l.69 x l~ neutron/sec. Californium 252 neutron
source in the configuration shown. It will be
appreciated by those skilled in the art that other
source-detector spacings of optimum design could be
used with other neutron sources, if desired, and still
remain within the inventive concepts of the present
invention.
Voltage pulses from the neutron detectors "
12 and 13 are amplified by the amplifiers 14 and 15
as previously discussed and transmitted to the surface
via the well logging cable 8 and cable driver circuits
16. At the surface, circuitry is shown schematically
in block diagram form for interpreting the ratio of
the counts in each of the two spaced neutron detecotrs
and for recording a log of this ratio. A log of the
count rate from the short spaced detector is also made.
Signals from the longer spaced detector 13 which may be
encoded in any manner desired as previously described,
are detected as counts in the counter 21. Counts from
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1 the short spaced detector 12 are detected in a second
counter 22. Counters 21 and 22 may be of any of the
well known types of digital or proportional analog
counters known in the art. Outputs from counter 21
and counter 22 are sampled and supplied to a ratio
circuit 23 which produces an output signal proportional
to the ratio of counts in the long spaced detector 13
to the counts occurring in the short spaced detector 12.
This ratio is shown in Figure 2 as a function of forma-
tion porosity for three different types of formations
in cased boreholes. It will be observed from Figure 2
that the higher the ratio in the lower is the porosity
of the formations surrounding the borehole. This is
because a high porosity formation will generally contain
more hydrogen bearing compounds such as oil or water
in its pores and will therefore more rapidly attenuate
the neutron flux emanating from the neutron source 11
than a lower porosity formation. The dual spaced epi-
thermal neutron log of the present invention makes an
accurate measure of the hydrogen index of the formation.
The hydrogen index is defined as the quantity of hydrogen
per unit volume of formation.
The output signals from the detectors 12 and
13 in the downhole tool, when taken in ratio form by
the ratio circuit 23 which may be of conventional analog
or digital design, are supplied to a recorder 24, which
as indicated by dotted line 25, is driven either electri-
cally or mechanically by the sheave wheel 9 as a function
of borehole depth. Thus, a log is produced on a record
medium 26 as a continuous recording of the ratio of counts
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~Q138678
1 of the long spaced to short spaced detectors and the
count rate at the short spaced detector 12 as a function
of borehole depth. This ratio may be interpreted as
illustrated by Figure 2 in terms of a borehole compensated
porosity of the earth formations surrounding the bore-
hole. The count rate at the short spaced detector may
be interpreted in terms of an uncompensated porosity,
similarly, by reference to the calibration curve of
Figure 3.
It will be understood b~ those skilled in -
the art that the power supply circuits 27 may be used
to furnish electrical power for the downhole portion of
the equipment via the well logging cable 8 and that
downhole power circuits tnot shown) are utilized to
power the electronic circuitry shown in the downhole
tool.
Although the sonde 7 shown in the drawing of
Figure l is suspended freely in the borehole 2,
characteristics of the earth formation surrounding the
borehole and the borehole environment itself, may make
it advisable to centralize the housing of the sonde 7
within the borehole by means of bow springs or the like
(not shown). Alternatively, a backup pad (not shown)
can be used to urge the housing of the tool against
the borehole wall. However, with the operating para-
meters as described herein for the source to detector
spacing, the source composition, and intensity and
the geometry shown in the drawing of Figure l good
sensitivity has been accomplished. Similarly the
apparatus as shown and described minimizes the effect
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1 of neutron absorbers such as boron in formations
surrounding the borehole.
If the percentage compensation, C, for bore-
hole effects is defined as
C =(~R) - ~(55)) x lO0 (l)
~(R)
where ~ (R) represents the compensated porosity as
measured by the ratio of count rates at the dual
spaced detectors 12 and 13 and ~ (ss) represents the
uncompensated porosity as determined from Figure 3
from the short spaced detector 12 count rate alone,
then C can be used as an indicator of regions of
borehole diameter variations, borehole washout, or
cement void spaces. Such an indicator can be very
valuable in the interpretation of other types of well
logs which may be highly sensitive to borehole diameter
variations. The indicator C can also be very valuable
in locating cement voids which can lead to the un-
desired communication of fluids from one formation to
another along the borehole.
Referring now to Figure 4 a plot of the count
rates Cs at the short spaced detector vs. the ratio R
of the count rates (R = ~ss /CLs) at the two detectors
is shown for actual measurements taken in a South Texas
well. The general trend of points in the well is
shown by the plain dot points in Figure 4. The circled
dot points in the depth interval 4590 feet to 4610
feet and the squared dot points in the interval 4770 feet
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1 to 4790 feet illustrate the effect of a large value
of the compensation percentage C parameter. These points
are indicative of borehole washout or cement void in
these two depth intervals.
It will be appreciated by those skilled in
the art that the calibration curves of Figures 2 and 3
could be entered in tabular or analytic form in the
memory of a small general purpose digital computer
(not shown) and the computation of ~ (R), ~(SS) and C
made and logged directly as a function of depth, if
- desired. Moreover, magnetic tape recording of primary
and secondary data can be performed. A small general ~ -
purpose computer suitable for this purpose could be
a model PDP-ll computer as supplied by the Digital
Equipment Corporation of Cambridge, Mass. In this
manner a direct record of the washed out or cement
void regions of a well can be made at the well site.
The foregoing description may make other
alternative arrangements apparent to those skilled
in the art. The aim of the appended claims is, there-
fore, to cover all such changes and modifications as
fall within the true spirit and scope of the invention.
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