Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
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1 FIELD OF INVENTION: The present invention relates
to oxygen activation nuclear well logging.
2. DESCRIPTION OF THE PRIOR ART prior U.S. Patent
No. 3,465,151 discloses a technique for oxygen activation
nuclear well logging of formations adjacent a well borehole.
Although it is mentioned and recognized in this prior U.S.
patent that the presence of water and, thus, oxygen in the
well borehole will tend to obscure radiation of interest
from the formations, no effort is made to compensate for
this presence. In fact, it is indicated that the methods of
this prior U.S. patent have greatest utility in empty or oil
filled boreholes. However, typical boreholes contain some
measure of water. Another factor known to be often present
but not compensated for was variations in the Elux intensity
of the neutrons on the oxygen activation readings.
STATEMENT OF INVENTION
.
Briefly, the present invention provides a new and
improved method of nuclear well logging to determine oxygen
concentration of a well formation adjacent a well borehole.
The present invention permits determination of the
oxygen concentration Mo of earth formations while also
compensating for the effects of oxygen present in the well
borehole. A knowledge of Mo can be used to determine if the
formation fluids adjacent the well borehole contain hydro-
carbons. The oxygen content by weight of most water satura-
ted formations is generally substantially constant, from
forty-nine to fifty-five percent, and is essentially
independent of lithology and porosity of the formation.
Where, on the other hand, the formation is porous and is
saturated with hydrocarbons, the oxygen content of the
formation is reduced since water contains oxygen and
hydrocarbons do not contain oxygen. A knowledge of the
oxygen content of an earth formation can thus be used to
delineate hydrocarbon bearing formations from water bearing
formations.
During logging according to the present invention,
compensation for the effects of oxygen present in the well
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borehole is achieved. The formation and borehole consti-
tuents are bombarded with high energy neutrons from a neutron
source in a sollde in the borehole. Gamma radiation resulting
from the O16(n,p)N16 oxygen activation reaction from bombar-
ded oxygen in the formation and borehole is detected with a
~3a~ma ray detector spaced from the neutron source in the
sonde. A measure of detected gâmma radiation is then obtained
in at least two gamma ray energy count windows. A borehole
oxygen ratio of gamma radiation in the gamma ray energy
wlndows from bombarded oxygen in the borehole is obtained,
as well as a formation oxygen ratio of gamma radiation in
the gamma ray energy windows from bombarded oxygen from the
forma-tion in the absence of bombarded borehole oxygen in the
vicinity of the detector. From the borehole oxygen ratio and
the formation oxygen ratio, a measure of the oxygen concen-
tration in the formation may be obtained.
If desired, the techniques of the present invention may
be used -to measure relative changes in oxygen concentration
at various borehole depths and adjacent various formations
rather than obtaining a quantitative measure of oxygen
concentration at the various depths and formations. Also,
the present invention may be performed with plural gamma ray
detectors rather than a single gamma ray detector with
plural gamma ray energy count windows. Additionally, the
present invention may be used to obtain a measure of neutron
output intensity from the neutron source during bombardment
of the formation and borehole constituents.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram of a well loyging system
with portions thereof in a cased jell bore according to the
present invention.
Fig. 2 is a graphical presentation of a typical Nl
gamma ray energy spectrum, having two energy count windows.
Fig. 3 is a graphical representation of gamma ray
energy count ratios as a function of distance of a gamma ray
source from a detector.
Fig. 4A and 4B are schematic diagrams of portions of
the system of Fig. 1 in operation according to the present
nven-tion .
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Fig. 5 is a schematic diagram of an alternative well
logging system according to the present invention
Fig. 6 is a simplified flow chart for the utilization
of the computer shown in Fig. 1.
DESCRIPTION OF PREFERRED EMBODIMENT
Referring to Fig. 1, a well logging system in accor-
dance with the present invention is shown in a well borehole 10
adjacent formations 12 and 14. As is typical, the borehole 10
has a casing 16 held in place by cement 18. A downhole sonde 20
is suspended in the well borehole 10 by an armored well logging
cable 22 and is centralized by centralizers 2~ with respect to
the interior of the well casing 16. The cased borehole 10 is
filled with a well borehole fluid 26 which typically contains as
a constituent thereoE water and accordingly oxygen.
The downhole sonde 20 is provided with a gamma ray
detector 28 and a high energy neutron source 30. Detector 28 is
mounted above the source 30 and the sonde 20, and thus logging
would be performed while the sonde 20 is being lowered into the
borehole 10 from the surface. It should be understood, however,
that the position of the detector 28 and the source 30 in the
sonde 20 may be reversed and logging performed as the sonde 20 is
being raised in the borehole 10 toward the surface.
The detector 28 preferably takes the form of thallium-
activated sodium iodide crystal de-tector provided with suitable
shielding between the detector 28 and the source 30. The detec-
tor 28 may be, for example, of the type described in U.S. Patent
No. 4,032,780. Detector 28 is provided with a power supply,
either in the sonde 20 or at the surfaces. Sonde 20 also contains
suitable electronic circuitry for detector 28 of the type dis-
closed in
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such prior United States patent for transmission of electrical
pulses in response to detected gamma radiation over the cable 22
to a pulse height analyzer 32.
The source 30 is preferably of the deuteriu~-tritium
reaction accelerator type and generates a relatively high intensi-
ty of neutrons having an energy sufficient to induce the
O (n,p)N reaction, such as approximately fourteen
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V. Preferably, the source 30 is pulsed and the detector
2~ is time gated in the manner of U.S. Patent No. 4,032,780
to minimize adverse effects of thermal neutron capture gamma
radiation on the gamma radiation dejected by detector 28.
When the source 30 is activated, the formation 12 and
the borehole constituent fluid 26 are bombarded with high
energy neutrons. The neutrons emitted by the source 30
interact with oxygen nuclei in water present in fluid in the
formation 12 and the borehole constituent fluid 26, producing
thé radioactive isotope N16 through the O16(n,p)N16 reaction.
Radioactive isotope N16 decays with a half-lifç of 7.3
seconds, emitting 7.12 and 6.13 MeV gamma radiation. The
intensity of this gamma radiation is detected by the detector
28 and is transmit-ted through electronics in the sonde 20
over the cable 22 to pulse height analyzer 32 at the surface.
The intensity of the gamma radiation detected by the
detector 28 and counted in the pulse height analyzer 32 is
affected not only by the oxygen content of the formation 12,
which is the quantity of interest during logging, but aLso
by the oxygen in the borehole constituent fluid 26 whenever
the fluid ~6 contains water. Thus, the neutron source 30
induces N16 activity within water in the borehole fluid 26.
As the sonde 20 moves through the borehole 10 with the
source 30 preceding the detector 28, the activated borehole
water is forced passed the detector 28 contributing to the
total recorded ~16 activity.
The present invention permits determination, such as in
a computer 34, of the oxygen concentration Mo of earth
formations while also compensating for the effects of oxygen
present in the well borehole. The values Mo determined in
computer 34, as well as other results determined in the
computer 34 in a manner set forth below, may be plotted as a
function of borehole depth with a recorder 35.
A knowledge of Mo can be used to determine if the
forma-tion fluids adjacent the well borehole 10 contain
hydrocarbons. The oxygen content by weight of most water
satllrated formations is generally substantially constant,
from forty-nine to fifty-five percellt, and is essentially
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independent of lithology and porosity of the formation.
Where, on the other hand, the forma-tion is porous and is
saturated with hydrocarbons, the oxygen content of the
formation is reduced since water contains oxygen and hydro-
carbons do not contain oxygen. A knowledge of the oxygencontent of an earth formation can thus be used to delineate
hydrocarbon bearing formations from water bearing formations.
The pulse height analyzer 32 may be either a multi-
channel analyzer or plural single channel analyzers. Pulse
height analyzer 32 accumulates gamma radiation counting
rates CL and CH falling within two suitable energy windows,
such as the energy windows extending from about 3.25 MeV to
4.00 MeV and from about 4.75 MeV to 7.20 MeV, respectively
(Fig. 2~.
The measured counting rates CL and CH in pulse height
analyzer 32 represent the sum of contributions from oxygen
activation in the borehole fluid 26 and formation 12, as
expressed in equations (l) and (2) below, where the sub-
scripts B and F represent the borehole and formation com-
ponents, respectively.
(l) O = CB + CF
(2~ cH = cH + CF
.
Two constants KF and KB, developed in a manner set forth
below, are defined as follows:
(3) KF - CF/CH
(4) K - CL/CH
Substituting equations (3) and ~4) into (l) yields
(5) C = KBCB + KFCF
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Solving equations (2) and (5) simultaneously yields
(6) CF = (C -KBC )/(KF-KB)
(7) CB = (C KFC )/(KB~KF)
C and C are, of course, measured quantities. CF and CB
can be determined from equations (6) and (7) once KB and KF
can be determined.
DETERMINATION OF K
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Figure 3 shows a curve relating to C /C to R, the
radial distance from the center of the sonde 20 to a center
of origination of distribution of radioactive N16 nuclei.
The general development of a curve of this type is discussed
in detail in U.S. Patent No. 4,032 t 778.
Basically, a gamma ray spectral degradation technique
is performed in test pit formations using a suitable source
of suitable energy, such as 6~13 MeV, at various radial
distances R from the detector 28 and a calibration measure
count rate ratio CH/CL is measured. A graph of the form of
Fig. 3 is then formed for the count ratios as a function of
various distances R.
Recalling from equation (4) above that KB is equal to
the ratio of the borehole components CB/CB, it is apparent
that KBl can be obtained from the curve of Fig. 3.
Specifically, RB, the radial distance from the center of the
sonde 20 to the center of the N16 activity in the borehole
26, is
(8) RB = (RADIUS OF SONDE + RADIUS OF CASING)/2
Since the radii are known, RB and thus KB can be determined
using the above equation (8) and the curve of Figure 3.
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DETERMINATION OF KF
KF for a given borehole condition can be measured by
using an activation-count technique. The sonde 20 is posi-
tion d such that the source 30 is opposite a formation, such
as 36 (Fig. 4A), containing at least some water and whose
porosity is somewhat similar to that of the formation to be
logged. The formation is then activated for approximately
thirty seconds to allow the Nl6 activity to approach satura-
tion. Next the sonde 20 is moved such that the detector 28
is positioned opposite the original activation spot (Fig.
4B ) and the quantities CL and CH are measured. When the
sonde 20 is moved, the Nl6 activity induced within the
borehole water 26 is displaced away from the general area of
the detector 28 which is now opposite the activation spot,
as indicated schematically by the arrows above the sonde 20.
thus, the recorded counting rate CL and CH contain substan-
tially no borehole component. Mathematically, wherefore,
(9) CB = CB =
From equations (l) and (2), we have
( 10 ) CL/CH = (c~LfcF)/(cB~cF)
Substituting equation (9) into equation (10) yields
(ll) CL/C = CF/CF KF
KF is therefore, determined for a given borehole condition
through equation if
Having now determined the constants and KF, equations
l6) wind (7) can now be used in the compu-ter 34 to compute
O and CB, respectively, from known or measured quantities.
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O the counting rate froM N from the formation only
in the energy range 4.75 - 7.~ MeV, is related to Mo, the
concentration of oxygen in the formation, through the
equation
(12) H
Mo = ~NQCF
where ON = the 14 MeV neutron output of the neutron
generator
Q = a constant for a given son~e geometry,
detector efficiency, and logging speed
Mo, the quantity of interest, can be determined prom the
computed quantity CH if ON remains constant. In practice,
however, ON can vary and should be monitored.
I r the borehole conditions remain constant, (i.e. the
oxygen content of the borehole fluid and the casing size
remain constant ON is related to CH through the eq~lation
(13) ON = pCH
where P is a constant for a given sonde geometry, detector
efficiency, and logging speed. Substituting equation (12)
and (11) yields
(14) Mo = HCFCB
where H - P.Q is again a calibration constant for a given
sonde geometry, detector efficiency, and logging speed.
Equation ~14) relates M , the yuantity of interest, to
C~I and CH which can be computed in the computer 34 in the
foregoing manner from known or measured quantities. Computer
AL may also determine ~N~ the neutron output intensity, as
well. In field operations, it sometimes suffices -to measure
relative changes in Mo to delineate hydrocarbon and water
bearing formations. It is, therefore, in these situations
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not necessary to Snow Mo explicitly. If, however, quantita
tive values of Mo are desired, Mo can be obtained by cali-
brating the sonde 20 in test formations containing known
values of M and with known borehole conditions. In this
procedure, CF and CHB are measured, and since Mo is known,
Equation (14) can be solved for the calibration constant H
in computer 34.
The adverse effects of the activated borehole fluid can
also be eliminated by using a dual gamma ray detector oxygen
activation logging sonde 40 (Fig. 5). A neutron source 42
is again pulsed in the manner set forth above and gamma ray
detectors 44 and 46 are time gated to minimize contributions
from thermal capture gamma radiat.ion. Detectors 44 and 46
are spaced from source 42, and from each other, a distance
S. The counting rates recorded in a single energy window
extending, for example, from 3.25 to 7.2 MeV in detectors 44
and 46 can be expressed as
(15~ Cl = Cl,B~Cl,F
(16) C2 = C2,B+C2,F
where the subscripts 1 and 2 designate the detector sub-
script for detectors 44 and 46, respectively, in the draw-
ings and F and B designate formation and borehole components,
respectively. However,
(17~ 2,B l,B
where A is the decay constant of N16 and S i5 the spacing
be-tween the two detectors, and
(18) f = vAB/ JAB A20)
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where v is the logging speed, Ago is the cross section area
of the sonde 20, and AB is the cross sectional area of the
borehole. Also:
5(19) 2,F l,F
Substituting equations (19) and ~17) into equation (16)
yields
10~20) C2 = C ~-AS/f~ C e-AS/v
Solving equations (20) and (15) simultaneously yields
(21) C2-Cle
15, e~AS/V _e-AS/f
(2~) Cle AS/v -C2
Z0Cl,B -AS/v e-AS/f
All of the terms on the right hand side of equations ~21)
and (22) are either known (A,v,S), are measured (Cl, C2), or
are computed from known quantities (f). These equations
can, wherefore, be solved in conputer 34 to determine Cl F
and Cl B which are in kurn, substituted into equation (14)
to determine M , the concentration of oxygen within the
f~r~ation.
The foregoing disclosure and description of the in-
vention are illustrative and explanatory thereof and various
changes in the size, shape and materials as well as the
details of the illustrated construction may be made without
departing from the spirit of the invention.
