CA1226377A - Borehole compensated oxygen activation nuclear well logging - Google Patents
Borehole compensated oxygen activation nuclear well loggingInfo
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- CA1226377A CA1226377A CA000473470A CA473470A CA1226377A CA 1226377 A CA1226377 A CA 1226377A CA 000473470 A CA000473470 A CA 000473470A CA 473470 A CA473470 A CA 473470A CA 1226377 A CA1226377 A CA 1226377A
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Abstract
I.
BOREHOLE COMPENSATED OXYGEN ACTIVATION
NUCLEAR WELL LOGGING
(D#76,104 -F) ABSTRACT OF THE DISCLOSURE
Formations adjacent a well borehole are activated with neutrons from a pulsed neutron source in a sonde at an energy level, such as 14 MeV, to energize oxygen present in the formation and borehole, giving rise to gamma radiation from 016(n,p)N16 reaction. Gamma radiation in the energy windows 3.25 - 4.00 MeV and 4.75 - 7.20 MeV is detected in a time gated detector and counted. Count rate contributions from the borehole and from the formation are differentiated on the basis of radial distance from the center of the sonde. In this manner, compensation for adverse effects of borehole oxygen activation on formation oxygen activation measurements of interest is achieved.
BOREHOLE COMPENSATED OXYGEN ACTIVATION
NUCLEAR WELL LOGGING
(D#76,104 -F) ABSTRACT OF THE DISCLOSURE
Formations adjacent a well borehole are activated with neutrons from a pulsed neutron source in a sonde at an energy level, such as 14 MeV, to energize oxygen present in the formation and borehole, giving rise to gamma radiation from 016(n,p)N16 reaction. Gamma radiation in the energy windows 3.25 - 4.00 MeV and 4.75 - 7.20 MeV is detected in a time gated detector and counted. Count rate contributions from the borehole and from the formation are differentiated on the basis of radial distance from the center of the sonde. In this manner, compensation for adverse effects of borehole oxygen activation on formation oxygen activation measurements of interest is achieved.
Description
~2~63~7 BACKGROUND OF THE INVENTION
-1 FIELD OF INVENTION: The present invention relates to oxygen activation nuclear well logging.
-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 63~
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 .
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 ;
~2;21~37~
-4a-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 !, ~63 7~
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 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:
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 63~
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 .
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 ;
~2;21~37~
-4a-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 !, ~63 7~
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 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
Substituting equations (3) and ~4) into (l) yields
(5) C = KBCB + KFCF
~2~63~t7 Solving equations (2) and (5) simultaneously yields
~2~63~t7 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
-E
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
can be determined from equations (6) and (7) once KB and KF
can be determined.
DETERMINATION OF K
-E
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.
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,
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.
637~7 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 37~
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.
637~7 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 37~
-10-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) 1 ~Z63~77 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.
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) 1 ~Z63~77 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.
Claims (22)
1. A method of nuclear well logging to determine oxygen concentration of a formation adjacent a well borehole while compensating for the effects of oxygen present in the well borehole, comprising the steps of:
(a) bombarding the formation and borehole con-stituents with high energy neutrons from a neutron source in a sonde;
(b) detecting with a gamma ray detector spaced from the source in the sonde gamma radiation resulting from the 016(n,p)N16 reaction from bombarded oxygen in the formation and borehole;
(c) obtaining a measure of detected gamma radia-tion in at least two gamma ray energy count windows;
(d) obtaining a borehole oxygen ratio of the gamma radiation in the gamma ray energy windows from bombar-ded oxygen in the borehole;
(e) obtaining a formation oxygen ratio of the gamma radiation in the gamma ray energy windows from the formation in the absence of bombarded borehole oxygen in the vicinity of the detector; and (f) obtaining from the borehole oxygen ratio and the formation oxygen ratio a measure of the oxygen concen-tration in the formation.
(a) bombarding the formation and borehole con-stituents with high energy neutrons from a neutron source in a sonde;
(b) detecting with a gamma ray detector spaced from the source in the sonde gamma radiation resulting from the 016(n,p)N16 reaction from bombarded oxygen in the formation and borehole;
(c) obtaining a measure of detected gamma radia-tion in at least two gamma ray energy count windows;
(d) obtaining a borehole oxygen ratio of the gamma radiation in the gamma ray energy windows from bombar-ded oxygen in the borehole;
(e) obtaining a formation oxygen ratio of the gamma radiation in the gamma ray energy windows from the formation in the absence of bombarded borehole oxygen in the vicinity of the detector; and (f) obtaining from the borehole oxygen ratio and the formation oxygen ratio a measure of the oxygen concen-tration in the formation.
2. The method of claim 1, wherein said step of obtaining a measure of the oxygen concentration includes:
obtaining a measure of the formation oxygen activation gamma radiation in one of the gamma ray energy windows.
obtaining a measure of the formation oxygen activation gamma radiation in one of the gamma ray energy windows.
3. The method of claim 2, wherein said step of obtaining a measure of the formation oxygen activation gamma radiation comprises obtaining a measure of the gamma radia-tion in the higher energy gamma ray energy window, wherein the higher energy gamma ray energy window is from about 4.75 MeV to about 7.20 MeV.
4. The method of claim 1, wherein said step of obtaining a measure of the oxygen concentration includes:
obtaining a measure of the formation oxygen activation gamma radiation and the borehole oxygen activation gamma radiation in one of the gamma ray energy windows.
obtaining a measure of the formation oxygen activation gamma radiation and the borehole oxygen activation gamma radiation in one of the gamma ray energy windows.
5. The method of claim 4, wherein said step of obtain-ing a measure of the formation oxygen activation gamma radiation and the borehole oxygen activation gamma radiation comprises obtaining a measure of the gamma radiation in the higher energy gamma ray energy window, wherein the higher energy gamma ray energy window is from about 4.75 MeV to about 7.20 MeV.
6. The method of claim 1, further including the step of:
(a) obtaining a measure of the formation oxygen activation gamma radiation in one of the gamma ray energy windows; and (b) obtaining from the borehole oxygen ratio and the formation oxygen ratio a measure of the neutron output during said step of bombarding.
(a) obtaining a measure of the formation oxygen activation gamma radiation in one of the gamma ray energy windows; and (b) obtaining from the borehole oxygen ratio and the formation oxygen ratio a measure of the neutron output during said step of bombarding.
7. The method of claim 1, further including the step of:
forming a record of the measure of oxygen concentration in the formation as a function of borehole depth.
forming a record of the measure of oxygen concentration in the formation as a function of borehole depth.
8. The method of claim 1, wherein said step of obtain-ing a formation oxygen ratio comprises the steps of:
(a) bombarding a formation of interest and bore-hole fluids at an activation spot adjacent the neutron source with high energy neutrons;
(b) moving the sonde so that the detector is located at the activation spot;
(c) displacing the bombarded borehole fluids during said step of moving the sonde so that essentially only formation gamma radiation is present at the activation spot;
(d) detecting with the detector the formation gamma radiation in the gamma ray energy windows; and (e) forming a ratio of the detected gamma radiation in the gamma ray energy windows.
(a) bombarding a formation of interest and bore-hole fluids at an activation spot adjacent the neutron source with high energy neutrons;
(b) moving the sonde so that the detector is located at the activation spot;
(c) displacing the bombarded borehole fluids during said step of moving the sonde so that essentially only formation gamma radiation is present at the activation spot;
(d) detecting with the detector the formation gamma radiation in the gamma ray energy windows; and (e) forming a ratio of the detected gamma radiation in the gamma ray energy windows.
9. The method of claim 1, wherein said step of obtaining a borehole oxygen ratio comprises the steps of (a) forming a calibration measure of count rate ratios as a function of borehole radius in a test facility;
(b) obtaining an average borehole radius reading comprised of the average of the sum of the radius of the sonde and the radius of the borehole casing; and (c) obtaining from the calibration measure and the average borehole radius the borehole oxygen ratio.
(b) obtaining an average borehole radius reading comprised of the average of the sum of the radius of the sonde and the radius of the borehole casing; and (c) obtaining from the calibration measure and the average borehole radius the borehole oxygen ratio.
10. A method of nuclear well logging to determine oxygen concentration of a formation adjacent a well borehole while compensating for the effects of oxygen present in the well borehole, comprising the steps of:
(a) bombarding the formation and borehole constituents with high energy neutrons from a neutron source in a sonde;
(b) detecting with plural gamma ray detectors spaced from the source and from each other in the sonde gamma radiation resulting from the 016(n,p)N16 reaction from bombarded oxygen in the formation and borehole;
(c) obtaining a measure of detected gamma radiation from each of the plural gamma ray detectors in a gamma ray energy count window; and (d) obtaining from the detected gamma radiation from the plural detectors a measure of the oxygen concentration of the formation.
(a) bombarding the formation and borehole constituents with high energy neutrons from a neutron source in a sonde;
(b) detecting with plural gamma ray detectors spaced from the source and from each other in the sonde gamma radiation resulting from the 016(n,p)N16 reaction from bombarded oxygen in the formation and borehole;
(c) obtaining a measure of detected gamma radiation from each of the plural gamma ray detectors in a gamma ray energy count window; and (d) obtaining from the detected gamma radiation from the plural detectors a measure of the oxygen concentration of the formation.
11. A method of measuring neutron output intensity from a neutron source in a sonde which bombards formation and well borehole constituents with neutrons during well logging, comprising the steps of:
(a) bombarding the formation and borehole con-stituents with high energy neutrons from a neutron source in a sonde;
(b) detecting with a gamma ray detector spaced from the source in the sonde gamma radiation resulting from the 016(n,p)N16 reaction from bombarded oxygen in the formation and borehole;
(c) obtaining a measure of detected gamma radia-tion in at least two gamma ray energy count windows;
(d) obtaining a borehole oxygen ratio of the gamma radiation in the gamma ray energy windows from bombarded oxygen in the borehole;
(e) obtaining a formation oxygen ratio of the gamma radiation in the gamma ray energy windows from the formation in the absence of bombarded borehole oxygen in the vicinity of the detector; and (f) obtaining from the borehole oxygen ratio and the formation oxygen ratio a measure of the neutron output intensity during said step of bombarding.
(a) bombarding the formation and borehole con-stituents with high energy neutrons from a neutron source in a sonde;
(b) detecting with a gamma ray detector spaced from the source in the sonde gamma radiation resulting from the 016(n,p)N16 reaction from bombarded oxygen in the formation and borehole;
(c) obtaining a measure of detected gamma radia-tion in at least two gamma ray energy count windows;
(d) obtaining a borehole oxygen ratio of the gamma radiation in the gamma ray energy windows from bombarded oxygen in the borehole;
(e) obtaining a formation oxygen ratio of the gamma radiation in the gamma ray energy windows from the formation in the absence of bombarded borehole oxygen in the vicinity of the detector; and (f) obtaining from the borehole oxygen ratio and the formation oxygen ratio a measure of the neutron output intensity during said step of bombarding.
12. The method of claim 11, wherein said step of obtaining a measure of neutron output intensity includes:
obtaining a measure of the borehole oxygen activa-tion gamma radiation in one of the gamma ray energy windows.
obtaining a measure of the borehole oxygen activa-tion gamma radiation in one of the gamma ray energy windows.
13. The method of claim 12, wherein said step of obtaining a measure of the borehole oxygen activation gamma radiation comprises obtaining a measure of the gamma radiation in the lower energy gamma ray energy window, wherein the lower energy gamma ray energy window is from about 3.25 MeV to about 4.00 MeV.
14. The method of nuclear well logging to determine oxygen concentration of a formation adjacent a well borehole while compensating for the effects of oxygen present in the well borehole, comprising the steps of:
(a) bombarding the formation and borehole con-stituents with high energy neutrons from a neutron source in a sonde;
(b) detecting with a gamma ray detector spaced from the source in the sonde gamma radiation resulting from the 016(n,p)N16 reaction from bombarded oxygen in the formation and borehole;
(c) obtaining a measure of detected gamma radia-tion in at least two gamma ray energy count windows, (d) obtaining a borehole oxygen ratio of the gamma radiation in the gamma ray energy windows from bombarded oxygen in the borehole;
(e) obtaining a formation oxygen ratio of the gamma radiation in the gamma ray energy windows from the formation in the absence of bombarded borehole oxygen in the vicinity of the detector; and (f) obtaining from the borehole oxygen ratio and the formation oxygen ratio a measure of relative changes in the oxygen concentration in the formation.
(a) bombarding the formation and borehole con-stituents with high energy neutrons from a neutron source in a sonde;
(b) detecting with a gamma ray detector spaced from the source in the sonde gamma radiation resulting from the 016(n,p)N16 reaction from bombarded oxygen in the formation and borehole;
(c) obtaining a measure of detected gamma radia-tion in at least two gamma ray energy count windows, (d) obtaining a borehole oxygen ratio of the gamma radiation in the gamma ray energy windows from bombarded oxygen in the borehole;
(e) obtaining a formation oxygen ratio of the gamma radiation in the gamma ray energy windows from the formation in the absence of bombarded borehole oxygen in the vicinity of the detector; and (f) obtaining from the borehole oxygen ratio and the formation oxygen ratio a measure of relative changes in the oxygen concentration in the formation.
15. The method of claim 14, wherein said step of obtaining a measure of the oxygen concentration includes:
obtaining a measure of the formation oxygen activation gamma radiation in one of the gamma ray energy windows.
obtaining a measure of the formation oxygen activation gamma radiation in one of the gamma ray energy windows.
16. The method of claim 15, wherein said step of obtaining a measure of the formation oxygen activation gamma radiation comprises obtaining a measure of the gamma radia-tion in the higher energy gamma ray energy window, wherein the higher energy gamma ray energy window is from about 4.75 MeV to about 7.20 MeV.
17. The method of claim 14, wherein said step of obtaining a measure of the oxygen concentration includes:
obtaining a measure of the formation oxygen activation gamma radiation and the borehole oxygen activation gamma radiation in one of the gamma ray energy windows.
obtaining a measure of the formation oxygen activation gamma radiation and the borehole oxygen activation gamma radiation in one of the gamma ray energy windows.
18. The method of claim 17, wherein said step of obtaining a measure of the formation oxygen activation gamma radiation and the borehole oxygen activation gamma radiation comprises obtaining a measure of the gamma radiation in the higher energy gamma ray energy window, wherein the higher energy gamma ray energy window is from about 4.75 MeV to about 7.20 MeV.
19. The method of claim 14, further including the step of:
(a) obtaining a measure of the formation oxygen activation gamma radiation in another of the gamma ray energy windows; and (b) obtaining from the borehole oxygen ratio and the formation oxygen ratio a measure of the neutron output during said step of bombarding.
(a) obtaining a measure of the formation oxygen activation gamma radiation in another of the gamma ray energy windows; and (b) obtaining from the borehole oxygen ratio and the formation oxygen ratio a measure of the neutron output during said step of bombarding.
20. The method of claim 14, further including the step of:
forming a record of the measure of oxygen concen-tration in the formation as a function of borehole depth.
forming a record of the measure of oxygen concen-tration in the formation as a function of borehole depth.
21. The method of claim 14, wherein said step of obtaining a formation oxygen ratio comprises the steps of:
(a) bombarding a formation of interest and borehole fluids at an activation spot adjacent the neutron source with high energy neutrons;
(b) moving the sonde so that the detector is located at the activation spot;
(c) displacing the bombarded borehole fluids during said step of moving the sonde so that essentially only formation gamma radiation is present at the activation spot;
(d) detecting with the detector the formation gamma radiation in the gamma ray energy windows; and (e) forming a ratio of the detected gamma radiation in the gamma ray energy windows.
(a) bombarding a formation of interest and borehole fluids at an activation spot adjacent the neutron source with high energy neutrons;
(b) moving the sonde so that the detector is located at the activation spot;
(c) displacing the bombarded borehole fluids during said step of moving the sonde so that essentially only formation gamma radiation is present at the activation spot;
(d) detecting with the detector the formation gamma radiation in the gamma ray energy windows; and (e) forming a ratio of the detected gamma radiation in the gamma ray energy windows.
22. The method of claim 14, wherein said step of obtaining a borehole oxygen ratio comprises the steps of:
(a) forming a calibration measure of count rate ratios as a function of borehole radius in a test facility;
(b) obtaining an average borehole radius reading comprised of the average of the sum of the radius of the sonde and the radius of the borehole casing; and (c) obtaining from the calibration measure and the average borehole radius the borehole oxygen ratio.
(a) forming a calibration measure of count rate ratios as a function of borehole radius in a test facility;
(b) obtaining an average borehole radius reading comprised of the average of the sum of the radius of the sonde and the radius of the borehole casing; and (c) obtaining from the calibration measure and the average borehole radius the borehole oxygen ratio.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000473470A CA1226377A (en) | 1985-02-04 | 1985-02-04 | Borehole compensated oxygen activation nuclear well logging |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000473470A CA1226377A (en) | 1985-02-04 | 1985-02-04 | Borehole compensated oxygen activation nuclear well logging |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1226377A true CA1226377A (en) | 1987-09-01 |
Family
ID=4129744
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000473470A Expired CA1226377A (en) | 1985-02-04 | 1985-02-04 | Borehole compensated oxygen activation nuclear well logging |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1226377A (en) |
-
1985
- 1985-02-04 CA CA000473470A patent/CA1226377A/en not_active Expired
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