GB1582589A

GB1582589A - Earth formation porosity log using measurement of neutron energy spectrum

Info

Publication number: GB1582589A
Application number: GB50811/77A
Authority: GB
Original assignee: Texaco Development Corp
Current assignee: Texaco Development Corp
Priority date: 1977-04-20
Filing date: 1977-12-06
Publication date: 1981-01-14
Also published as: NO774306L; AU3159577A; CA1103814A; BR7801603A; FR2388269B1; MX4924E; NZ185984A; NO147009B; FR2388269A1; NO147009C; AU506516B2; DE2756328A1

Description

(54) EARTH FORMATION POROSITY LOG USING MEASUREMENT OF NEUTRON ENERGY SPECTRUM (71) We, TEXACO DEVELOPMENT COR PORATION, a Corporation organized and existing under the laws of the State of Delaware, United States of America, of 135 East 42nd Street, New York, New York 10017, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to radiological well logging methods and apparatus for investigating the characteristics of subsurface earth formations traversed by a borehole, and more particularly, to methods and apparatus for measuring the porosity of each formations in the vicinity of a well borehole by means of neutron well logging techniques.

In the search for hydrocarbons beneath the earth's crust one of the parameters which must be known about an earth formation before evaluating potential is the fractional volume of fluid filled pore space, or porosity, present around the rock grains comprising the earth formation. Several techniques have been developed in the prior art to measure earth formation porosity in a borehole environment. One such technique employs a gamma ray source and a single, or multiple, detectors to measure the electron density of the earth formation by gamma ray scattering. This leads to an inferential measurement of the porosity of the formations. Another technique employs an acoustic transmitter and one or more acoustic receivers.

The velocity of sound transmission' through the formation from the acoustic transmitter to the receivers is then measured and this quantity can be related to the porosity since sound travels faster in less porous rocks than in fluid filled pore spaces in the earth formations.

A third commercial technique which has been empoyed in the prior art to measure the porosity of earth formations employs a neutron source and either a neutron or gamma ray detector sensitive to low energy, or thermalized, neutron density. Hydrogen is the principal agent responsible for slowing down neutrons emitted into an earth formation. Therefore, in a formation containing a larger amount of hydrogen than is present in low porosity formations the neutron distribution is more rapidly slowed down and is contained in the area of the formation near the source. Hence, the counting rates in remote thermal neutron sensitive detectors located several inches or more from the source will be suppressed. In lower porosity formations which contain little hydrogen, the source neutrons are able to penetrate further.

Hence, the counting rates in the detector or detectors are increased. This behaviour may be directly quantified into a measurement of the porosity via well established procedures.

All of these commercially employed methods have generally not proven to be as accurate as desirable due to diameter irregularities of the borehole wall, variation of the properties of different borehole fluids, the irregular cement annulus surrounding the casing in a cased well borehole, and the properties of different types of steel casings and formation lithologies which surround the borehole. For example, the thermal neutron distribution 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, lithological properties of the earth formations in the vicinity of the borehole, such as the boron content of these formations, can affect the measurement of thermal neutron populations.The present invention however, rather than relying on a measurement of the thermal neutron population comprises in preferred forms a neutron measurement of the formation porosity which utilizes a measure of the epithermal neutron population at one detector and the fast neutron population at a second detector spaced approximately the same distance from a neutron source. The fast neutron population may be detected directly by means of a fast neutron detector or indirectly by means of an inelastic gamma ray detector. The fast neutron population may be background corrected. Special detectors and other means may be utilized to effectively discriminate against the detection of thermal neutrons or their resultant capture gamma rays as proposed by prior art thermal neutron population measurement techniques.

According to the present invention there is provided a method for determining the porosity of earth formations in the vicinity of a cased well borehole, comprising the steps of: irradiating the earth formations in the vicinity of the cased well borehole with fast neutrons from a source of fast neutrons passed into the borehole; generating a signal representative of the fast neutron population present in the well borehole at a location in the borehole spaced from said fast neutron source; detecting the epithermal neutron population at a location spaced from said neutron source in the borehole and generating a signal representative thereof; and combining said fast and epithermal neutron population representative signals to derive a measurement signal functionally related to the porosity of the earth formations in the vicinity of the borehole.

In another aspect the present invention provides apparatus for determining the porosity of earth formations in the vicinity of a cased well borehole, comprising: a fluid-tight, hollow, pressure-resistant body member sized and adapted for passage through a cased well borehole and having contained therein a source of fast neutrons operable to irradiate the earth formations in the vicinity of the borehole with fast neutrons; first detector means disposed at a first location within said body member longitudinally spaced from said source to generate a signal representative of the fast neutron population present at said first location in the well borehole;; second detector means disposed at a second location within said body member longitudinally spaced from said source to detect the epithermal neutron population present at said second location in the well borehole and to generate a signal representative thereof; and means to combine said fast and epithermal neutron population representative signals to derive a measurement signal functionally related to the porosity of the earth formations in the vicinity of the borehole.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing a well logging system according to the present invention, and having two detectors located on one side of a neutron source; Figure 2 is a similar system to Figure 1, but having two detectors equally spaced from a neutron source lying on opposite sides of the neutron source; Figure 3 is a graphical relationship illustrating the variation of the neutron flux ratio of a fast neutron detector to an epithermal neutron detector spaced approximately the same distance from a neutron source for several different porosity formations in the vicinity of a well borehole; Figures 4 and 5 illustrate the neutron energy spectrum at various distances from a neutron source in earth formations of differing porosities;; Figure 6 is a schematic diagram showing a pulsed neutron well logging system according to the present invention, and having two detectors located on one side of a pulsed neutron source; Figure 7 is a timing diagram of the system of Figure 6; Figure 8 is a graphical relationship illustrating the variation of the neutron flux ratio of a fast neutron detector to an epithermal neutron detector spaced approximately the same distance from a 14 MEV neutron source for sandstone and limestone formations of differing porosities; and Figure 9 is a graphical relationship illustrating the fast neutron to epithermal neutron population ratio for several different porosity sandstone and limestone formations as a function of detector distance from a 14 MEV neutron source.

Referring initially to Figure 1 there may be seen a simplified schematic functional representation in the form of a block diagram of a well logging apparatus in accordance with the present invention. A well borehole 11 penetrating earth formations is lined with a steel casing 12 and is filled with a well fluid 14. The steel casing 12 may be cemented in place by cement layer 13 which also serves to prevent fluid communication between adjacent producing formation in the earth.

The downhole portion of the logging system may be seen to be basically composed of an elongated fluid-tight, hollow, body member or sonde 15 which, during the logging operation, is passed longitudinally through the casing 12 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 15. A well logging cable 16 which passes over a sheave wheel 17 supports the sonde 15 in the borehole 11 and also provides a communication path for electrical signals to and from the surface equipment and the sonde 15. The well logging cable 16 may be of conventional armored cable design and may have one or more electrical conductors for transmitting such signals between the sonde 15 and the surface apparatus.

Again, referring to Figure 1 the sonde 15 contains, at its lower end, a neutron source 18.

This neutron source may comprise a typical continuous chemical neutron source such as an actinium-berylium, Californium 252 or Americium berylium which is capable of providing on the order of 10+8 neutrons per second. Alternatively, a deuterium-tritium accelerator type source of the type known in the art, which produces essentially monoenergetic 14 MEV neutrons may be used in a continuous operation mode.

Radiation detectors 20 and 22 are provided in the downhole sonde 15 and or separated from the neutron source 18 by a neutron shielding material 32. The neutron shield material 32 may comprise any highly hydrogenous material which serves to effectively slow down and shield the detectors 20 and 22 from direct neutron irradiation by the neutron source 18. Any suitably highly hydrogenous material such as paraffin or hydrocarbon polymer plastics may be used for this purpose.

While the detectors 20 and 22 are shown slightly separated from each other in the drawing of Figure 1, it will be appreciated that in the present invention, the two detectors 20 and 22 should be located as near the same distance from the neutron source as is practicable. An alternative arrangement in which both detectors may be situated at precisely the same distance from the source will be discussed with respect to Figure 2.

The detector 20 of Figure 1 is a fast neutron detector. This detector may comprise a scintillation type detector which is sensitive to the interaction of the scintillator material with fast neutrons. Such a detector could comprise, for example, a stilbene detector which is sensitive to fast neutron interactions. Such scintillation detectors may also be sensitive to high energy gamma radiation produced by the capture of neutrons from the neutron source in earth formations surrounding the well borehole. However, the pulse shape characteristics of gamma ray interactions produced by such captures may be distinguished from the pulse shape characteristics in such a detector provided by the interaction of a fast neutron with the detector material.Such a stilbene fast neutron detector is described in the publication entitled "A Scintillation counter with Neutron Gamma Ray Discriminators" by F.D. Brooks, published by the Atomic Energy Research Establishment, Harwell, England, 1959, and having laboratory publishing number HL. 59/282 (s.c.9).

The second detector 22 contemplated for use in the present invention is an epithermal neutron detector. This detector which may comprise, for example, a pressurized He3 detector is sensitive to neutrons in the epithermal energy range from approximately 0.178 electron volts to approximately 1.46 electron volts. This is contrasted to the fast neutron detector 20 which is essentially sensitive to fast neutrons having energies in the range of from roughly .2 x 10+6 electron volts to 12 x 10+6 electron volts. Thus, the two neutron detectors 22 and 20 provide two energy bands of windows in which the neutron population energy spectrum may be observed by the downhole well logging sonde 15.The epithermal neutron detector 22 is embedded in a layer of hydrogenous material 21 and is surrounded by a relatively thin, for example 0.20 inch, layer of thermal neutron absorbing material 19 such as cadmium or the like. The He3 detector 22 is thus shielded from the interaction of thermalized neutrons due to the action of the cadmium layer 19 which, having an extremely large thermal neutron capture crosssection, effectively absorbs all, or most, thermal neutrons in the vicinity of detector 22 before these neutrons can impinge upon the detector and cause any interaction with the detector 22.

In any event, the detectors 20 and 22 provide electrical pulse signals which are representative of the number of neutrons present at their location in the epithermal energy range and in the fast neutron energy range to which the detectors are sensitive. The electrical signals from the detector 20 are amplified in an amplifier 23 and supplied to a multiplexing mixing circuit 25. Similarly, the electrical pulse representations from the epithermal detector 22 are conducted to an amplifier 24 and also supplied to the mixer 25. The mixed signals are supplied via cable 16 conductors to a surface demultiplexer circuit 26. This circuit for example may discriminate signals from the two downhole detectors on the basis of their polarity.Output signals from the demultiplexer 26 comprise pulse signals representative of the fast neutron population in the vicinity of the detector 20 and the epithermal neutron population signals present in the vicinity of detector 22. The fast neutron pulse signals are supplied to a pulse counter 27 and the epithermal neutron population signals are supplied to a second counter 28.

The counters 27 and 28 thus provide counts of the number of fast neutrons present in the vicinity of detector 20 and epithermal neutrons present in the vicintiy of detector 22 in the form of digital counts. These counts may be strobed or synchronized at a predetermined rate, for example 1 per second, into a digital ratio circuit 29. The ratio circuit 29 forms the ratio of counting rates at the two detectors, for example the ratio of fast neutron populattion present at detector 20 to the epithermal neutron population present at detector 22. It will be appreciated by those skilled in the art that it is possible to weight the ratio to compensate for effects produced by any difference in the distance of the two detectors from the neutron source so that a ratio measurement which is normalized to detectors located at the same distance from the neutron source may be obtained.Similarly, the ratio can be corrected to normalize any differences in detector sensitivities.

It has been discovered by the applicants that this ratio signal is a function of the earth formation porosity of the earth formations in the vicinity of the downhole sonde. The output signal from this ratio detector is supplied to a data recorder 30 which may be of the typical strip chart or film recorder type used in well logging. The recorder 30 provides an output trace of the ratio signal on a record medium 31 as a function or borehole depth. The depth information is obtained by mechanically or electrically coupling the recorder 30 to the sheave wheel 17 as indicated by the dotted line 33 of Figure 1 in a conventional manner as known in the art.

Referring, now to Figure 2 a second embodiment of a well logging system in accordance with the concepts of the present invention is illustrated in schematic block diagram form also. In Figure 2 a well borehole 41 is shown penetrating earth formations, lined with steel casing 42 and surrounded by a cement sheath 43. The cased well borehole 41 is filled with a well fluid 44. The downhole sonde 45 is shown suspended from a well logging cable 46 in the borehole, 41, in a manner similar to that illustrated with respect to Figure 1. In the well logging system of Figure 2 the downhole sonde 45 is provided with a neutron source 48 which may be of the continuous chemical type similar to that described with respect to Figure 1.

The downhole sonde is also provided with two neutral detectors 50 and 52 which correspond to the detectors of Figure 1, but which, in the instance of the system of Figure 2, are shown located above and below the neutron source 48.

The detectors 50 and 52 are spaced at the same distance from the neutron source 48. Two neutron shields 53 and 54 separate the neutron source 48 from the detectors and prevent the direct irradiation of the detectors by the neutron source, in the manner of the neutron shield in Figure 1. The detector 52 is sensitive to epithermal neutrons and may comprise a He3 pressurized detector which is surrounded by a neutron moderating hydrogenous material 51 and the outer shell of which is surrounded by a thermal neutron absorber layer 49 such as cadmium or the like in a manner similar to that discussed with respect to Figure 1.

Similarly, the detector 50 of Figure 2 may be a fast neutron detector comprising stilbene detector similar to that described with respect to Figure 1. Output signals from the fast neutron detector 50 are amplified in an amplifier 55 and supplied as one input to a mixer circuit 57. Output signals from the epithermal neutron detector 52 are supplied via an amplifier 56 to the opposite input of the mixer amplifier driver circuit 57. The signals from the two detectors may be discriminated from each other by multiplexing, or on the basis of their polarity as desired, in a manner known in the art.

The electrical output signals from the mixer driver circuit 57 are conducted to the surface via well logging cable 46 and supplied to a demultiplexing circuit 58 which serves to separate the signals into that from each of the downhole detectors 50 and 52. The fast neutron representative signals are supplied to a counter 64. The epithermal neutron population signals are supplied to a counter 59. The counters 64 and 59 serve to digitally count the number of fast neutrons present at the detector 50 and epithermal neutrons present at the detector 52 and to provide output signals which may be strobed or synchronized into a digital ratio circuit 60 in the manner discussed previously. The ratio signal output from the ratio circuit 60 is supplied to a recorder 61 which again records this information as a function borehole depths on an output record medium 62.Again, the recorder 61 is electrically or mechanically coupled to the sheave wheel 47 to provide the depth information so that these signals may be recorded as a function of borehole depth.

While not shown in Figures 1 and 2, it will be appreciated by those skilled in the art that conventional electrical power supplies situated, for example, at the surface, supplying operating voltages for the circuit components in the downhole sondes 45 and 15 in a manner known in the art.

Turning now to Figure 4 and 5, the basis for the porosity measurement of the well logging system according to the present invention is illustrated. Figures 4 and 5 each illustrate the neutron energy spectrum as a function of distance from a neutron source. These figures were derived according to calculations performed theoretically by a monte carlo type neutron transport computer program. The illustration of Figure 4 shows the neutron energy spectra for a water filled sand of 36% porosity at different distances from an americium-berylium neutron source. The illustration of Figure 5 shows the neutron energy spectra at different distances from the same source in a water filled sand of 3% porosity. It will be observed from Figures 4 and 5 that these neutron energy spectra in the energy range from .2 x 10+ to 12 x 10+6 electron volts differ considerably from one another.

If desired, the total count rate in each of the detectors integrated over any desired energy range may also be used as a conventional porosity indicator. Similarly, if desired, an energy threshold can be set to exclude any thermal or epithermal neutron count rates which may be present in the fast neutron detector. As previously mentioned these counts may also be discriminated against on the basis of their different pulse shape characteristics in the stilbene detector.

Referring, now to Figure 3, the neutron flux ratio of fast neutrons to epithermal neutrons in the previously mentioned energy ranges plotted as a function of distance from the source is illustrated as a function of several different porosities. It will be observed from Figure 3, that at reasonable source detector spacings of from less than 40 to more than 80 centimeters that this neutron population ratio exhibits a distinctive separation as a function of porosity. It can also be observed that in the high porosity range from 18% to 36% that, at source to detector spacings of from approximately 40 to 80 centimeters, a quite large separation is provided by the measurement of the fast neutron to epithermal neutrons population ratio.This is a vast improvement over other neutron type porosity logs such as those made using accelerator type neutron sources and detectors spaced relatively close to such sources. Such logs lose sensitivity in the higher porosity ranges, such as the range from 18-36%. Also, the present invention, by measuring only neutron populations above the thermal energy range remains less sensitive to formation lithology effects than other logs which measure neutron population in the thermal energy range. Small concentrations of boron or other strong thermal neutron absorbers do not effect the measurement of the present invention.

It is possible, by placing calibrating charts such as that of Figure 3 in the memory of a small general purpose digital computer, to compute and record the porosity of earth formations directly as a function of depth, utilizing a well logging system in accordance with the concepts of the present invention. A small general purpose digital computer such as the Model PDP-l l which is sold by the Digital Equipment Corporation of Maynard, Massachusetts, would be suitable for this purpose. It would also be apparent given the disclosure of the present invention present herein, for a programmer of ordinary skill to program such a small general purpose computer using a common compiler language such as Fortran and utilizing conventional mathematical interpolation procedures to perform this porosity calculation in the manner described.

Referring now to Figure 6 there may be seen a simplified schematic functional representation in the form of a block diagram of a further well logging apparatus in accordance with the present invention, similar parts being identified by similar reference numerals to Figure 1.

The sonde 15 contains, at its lower end, a pulsed neutron source 18'. This neutron source may comprise a deuterium-tritium accelerator tube which can be operated in pulsed mode to provide repetitive pulses or bursts or essentially monoenergetic 14 MEV neutrons and capable of providing on the order of 10+8 neutrons per second. A pulsing circuit 18'A provides electrical pulses which are timed in a manner to be described subsequently to cause the neutron generator 18' to repetitively emit neutron pulses of approximately 10 microseconds duration.

Radiation detectors 20 and 22 are provided in the downhole sonde 15 and are separated from the neutron source 18' by a neutron shielding material 32. The neutron shield material 32 may comprise any highly hydrogenous material which serves to effectively slow down and shield the detectors 20 and 22 from direct neutron irradiation by the neutron source 18. Any suitable highly hydrogenous material such as paraffin or hydrocarbon polymer plastics suitable for this purpose While the two detectors 20 and 22 are shown slightly separated from each other in the drawing of Figure 6, it will be appreciated that in the preferred mode of the present invention, the two detectors 20 and 22 should be located at approximately the same distance from the neutron source as is practicable.An alternative arrangement in which both detectors may be situated at precisely the same distance from the source would be to place one detector above and one detector below the neutron source 18 at the desired distance.

As with the embodiment of Figure 1, output signals from the demultiplexer 26 comprises pulse signals representative of the fast neutron population in the vicinity of the detector 20 and the epithermal neutron population signals present in the vicinity of detector 22. The fast neutron pulse signals are supplied to two time gate circuits 70 and 71. The epithermal neutron population signals are supplied to a third time gate 72.

Time gates 70, 71 and 72 are supplied with clocking signals from a clock circuit 73. A control circuit 74 in the downhole tool supplies control signals to the neutron generator pulser circuit 18 A each time the neutron generator tube 18' is to emit a pulse of 14 MEV neutrons of 10 microsecond duration. This pulse occurs once every 50 microseconds in the embodiment of the invention illustrated in Figure 6. Simultaneously this neutron generator firing pulse is provided by the control circuit 74 to a conductor of the logging cable 16 for transmission to the surface of the earth. The surface demultiplexer 26 separates these firing pulses out and provides an output pulse corresponding to each one to the clock circuit 73.

Clock circuit 73 provides a conditioning pulse immediately upon receipt of the generator fire pulse to time gate 70, this pulse being of 10 microseconds duration. After a delay of 38 microseconds the clock circuit 73 then provides conditioning pulses to time gates 71 and 72, these pulses also being of 10 microseconds duration. This timing sequence is illustrated schematically in the timing diagram of Figure 7.

The effect of this timing sequence is for time gate 70 (labelled Gate 1) to allow passage of fast neutron population count pulses from downhole fast neutron detector 20 to pass to a counter circuit 75 only during the burst of neutron of 10 microseconds duration emitted by the neutron generator 18' in the downhole sonde 15.

Similarly the counts occurring in the epithermal neutron detector 22 in the downhole sonde 15 are only permitted to enter a counter 76 via the time gate 72 (labelled Gate 3) for a 10 microsecond duration interval beginning 38 microseconds after the initiation of a downhole neutron burst. Likewise only during this same 10 microsecond interval are counts from the fast neutron detector 20 permitted to enter background counter circuit 77 at the surface via time gate 71 (labelled Gate 2).

The counters 75, 77 and 76 thus provide counts of the number of fast neutrons present in the vicinity of detector 20, background counts due to capture gamma rays resulting from lingering thermal neutrons in the vicinity of detector 20 and epithermal neutrons present in the vicinity of detector 22 in the form of digital counts. A background corrected count of the fast neutron population is formed in background correction circuit 78 by subtracting the background count in counter 77 from the fast neutron population count in counter 75.

Since the background time gate 71 occurs just prior (2 microseconds) to the next neutron pulse this provides a good approximation to the background gamma rays due to lingering thermal neturons present in the vicinity of detector 20 from the previous pulse. Thus any response of detector 20 to thermal neutrons is minimized. Counts from background correction circuit 78 and epithermal counter 76 may be strobed or synchronized at a predetermined rate, for example once per second, into a digital ratio circuit 79. The ratio circuit 79 forms the ratio of counting rates at the two detectors, for example the ratio of fast neutron population present at detector 20 to the epithermal neutron population present at detector 22.It will be appreciated by those skilled in the art that it is possible to weight the ratio to compensate for effects produced by any difference in the distance of the two detectors from the neutron source so that a ratio measurement which is normalized to detectors located at the same distance from the neutron source may be obtained. Similarly, the ratio can be weighted to normalize and differences in detector sensitivities.

It has been discovered by the applicants that this ratio signal is functionally related as will be described subsequently to the earth formation porosity of the earth formations in the vicinity of the downhole sonde. The output signal from the ratio circuit 79 is supplied to a data recorder 80 which may be of the typical strip chart or film recorder type used in well logging. The recorder 80 provides an output trace of the ratio signal on a record medium 81 as a function of borehole depth. The depth information is obtained by mechanically or electrically coupling the recorder 80 to the sheave wheel 17 as indicated by the dotted line 82 of Figure 6 in a conventional manner as known in the art.

While not shown in Figure 6, it will be appreciated by those skilled in the art that conventional electrical power supplies can be situated, for example, at the surface, to supply operating voltages for the circuit components in the downhole sonde 15 in a manner known in the art.

Turning now to Figures 8 and 9, the basis for the porosity measurement of the embodiment of Figures 6 and 7 is illustrated. Figure 8 illustrates graphically the ratio of fast neutron population in the energy range from 0.2 MEV to 12 MEV to epithermal neutron population in the energy range from 0.178 EV to 1.46 EV at source to detector spacings of 60 cm. from a 14 MEV deuterium-tritium neutron accelerator.

Values of the ratio are shown for both sandstone and limestone lithologies. It can be observed that no large lithology effect is present in this measurement of porosity and that the ratio varies nearly linearly over the entire porosity range of from 3% to 36%. This represents a dramatic improvement over prior art porosity measurements made with 14 MEV neutron sources and detectors spaced at shorter distances than 60 cm. of the system shown in Figure 8. Such prior art measurements suffered from nonlinear response and lack of sensitivity to porosity changes in the high porosity range from 18% to 36% and also from lithology effects due to the measurement of thermal neutron capture reactions.

Figure 9 illustrates the fast neutron to epithermal neutron population ratio over the same energy ranges as Figure 8, but plotted as a function of source to detector spacing from a 14 MEV deuterium-tritium accelerator source.

The sandstone and limestone lithology response for porosities in the range of from 3% to 36% is shown in Figure 9 also. It can be observed from Figure 9 that greater sensitivity of a system according to the present invention can be achieved with source to detector spacings in the range greater than 40 cm. Such spacings are preferred for porosity logging systems according to the present invention.

The present invention, by measuring only neutron populations above the thermal energy range remains less sensitive to formation lithology effects than other logs which measure neutron population in the thermal energy range. Small concentrations of boron or other strong thermal neutron absorbers do not adversely effect the measurement of the present invention.

It is possible, by placing calibration charts such as that of Figures 8 and 9 in the memory of a small general purpose digital computer, to compute and record the porosity of earth formations directly as a function of depth, utilizing a well logging system as illustrated in Figure 6. The calibration chart such as Figure 8 could be, for example, entered in the memory of a computer in a tabular form. The neutron population measurements can be taken from the borehole instrument and supplied as input to a small computer such as the Model PDP-l 1 supplied by the Digital Equipment Corporation of Maynard, Massachusetts.It would also be apparent given the disclosure of the invention present herein, for a programmer of ordinary skill to program such a small general purpose digital computer using a common compiler language such as FORTRAN and utilizing conventional mathematical interpolation procedures to perform this porosity calculation from the calibration charts in the manner described.

A further embodiment of well logging apparatus in accordance with the present invention will now be described, with reference again to Figures 6 to 9. In this further embodiment the first detector 20 of Figure 6, which is a scintillation type fast neutron detector, is replaced by an inelastic scattering gamma ray detector. This detector may comprise a scintillation type detector which is sensitive to the interaction with the scintillator material of gamma rays produced by the inelastic scattering of fast neutrons by the material comprising the earth formations. Such a detector could comprise, for example, a thallium-doped sodium iodide crystal detector which is sensitive to high energy gamma ray interaction.

Such scintillation detectors may also be sensitive to high energy gamma radiation produced by the capture of neutrons from the neutron source in earth formations surrounding the well borehole. However, the background correction techniques described above in connection with Figure 6 and 7 can be used to correct for such thermal neutron capture gamma rays. This technique is based on separating the inelastic scattering gamma rays from capture gamma rays by time separation. This is possible because the inelastic gamma rays will generally only exist during a neutron burst, while the capture gamma rays will persist after the neutron burst and thus can be sampled subsequently.

It will be appreciated that the electrical signals from the inelastic gamma ray detector are similar to the electrical signals from the fast neutron detector 20 and are processed by the remainder of the illustrated system in the same manner as described above in connection with Figures 6 to 10. The respective counters 75, 77 and 76 in this embodiment thus provide counts of the number of inelastic scattered gamma rays caused by the fast neutrons present in the vicinity of the detector 20, background counts due to capture gamma rays resulting from lingering thermal neutrons in the vicinity of the detector 20, and epithermal neutrons present in the vicinity of detector 22, in the form of digital counts. It will be appreciated that the detected inelastic gamma rays are representative of the fast neutron population.

WHAT WE CLAIM IS:- 1. A method for determining the porosity of earth formations in the vicinity of a cased well borehole, comprising the steps of: irradiating the earth formations in the vicinity of the cased well borehole with fast neutrons from a source of fast neutrons passed into the borehole; generating a signal representative of the fast neutron population present in the well borehole at a location in the borehole spaced from said fast neutron source; detecting the epithermal neutron population at a location spaced from said neutron source in the borehole and generating a signal representative thereof; and combining said fast and epithermal neutron population representative signals to derive a measurement signal functionally related to the porosity of the earth formations in the vicinity of the borehole.

2. A method according to claim 1, wherein said fast neutron population location in respect of which said representative signal is generated and said epithermal neutron population detection location are at substantially the same distance from said neutron source in the borehole.

3. A method according to claim 1 or claim 2, wherein said combining step is performed by forming a ratio of said fast neutron and said epithermal neutron representative signals.

4. A method according to claim 3 and further including the step of calibrating said ratio signal according to a predetermined functional relationship to derive a porosity signal quantitatively representative of the porosity of the earth formations in the vicinity of the borehole.

5. A method according to any one of claims 1 to 4, wherein said irradiating step comprises continously irradiating the earth formations with fast neutrons from a continous neutron source.

6. A method according to claim 5, wherein said irradiating step is performed with a chemical type continuous neutron source.

7. A method according to claim 5, wherein said irradiating step is performed with a deuteruim-tritium accelerator type continous neutron source.

8. A method according to any one of claims 5 to 7, wherein said fast neutron representative signal generating step includes detecting fast neutrons by means of a stilbene scintillation detector for fast neutrons, and wherein said detector response to fast neutrons is distinguished from said detector response to gamma rays by pulse shape discrimination.

9. A method according to any one of claims 1 to 4, wherein said irradiating step comprises: repetitively irradiating the earth formations with relatively short duration bursts of fast neutrons; and wherein said fast neutron representative signal generating step comprises: detecting, substantially only during each said neutron burst, the fast neutron population present in the well borehole and generating said signal representative thereof.

10. A method according to claim 9 including the steps of: detecting, substantially during each time interval between said repetitive neutron bursts, the background radiation present at said fast neutron detection location due to lingering

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. present herein, for a programmer of ordinary skill to program such a small general purpose digital computer using a common compiler language such as FORTRAN and utilizing conventional mathematical interpolation procedures to perform this porosity calculation from the calibration charts in the manner described. A further embodiment of well logging apparatus in accordance with the present invention will now be described, with reference again to Figures 6 to 9. In this further embodiment the first detector 20 of Figure 6, which is a scintillation type fast neutron detector, is replaced by an inelastic scattering gamma ray detector. This detector may comprise a scintillation type detector which is sensitive to the interaction with the scintillator material of gamma rays produced by the inelastic scattering of fast neutrons by the material comprising the earth formations. Such a detector could comprise, for example, a thallium-doped sodium iodide crystal detector which is sensitive to high energy gamma ray interaction. Such scintillation detectors may also be sensitive to high energy gamma radiation produced by the capture of neutrons from the neutron source in earth formations surrounding the well borehole. However, the background correction techniques described above in connection with Figure 6 and 7 can be used to correct for such thermal neutron capture gamma rays. This technique is based on separating the inelastic scattering gamma rays from capture gamma rays by time separation. This is possible because the inelastic gamma rays will generally only exist during a neutron burst, while the capture gamma rays will persist after the neutron burst and thus can be sampled subsequently. It will be appreciated that the electrical signals from the inelastic gamma ray detector are similar to the electrical signals from the fast neutron detector 20 and are processed by the remainder of the illustrated system in the same manner as described above in connection with Figures 6 to 10. The respective counters 75, 77 and 76 in this embodiment thus provide counts of the number of inelastic scattered gamma rays caused by the fast neutrons present in the vicinity of the detector 20, background counts due to capture gamma rays resulting from lingering thermal neutrons in the vicinity of the detector 20, and epithermal neutrons present in the vicinity of detector 22, in the form of digital counts. It will be appreciated that the detected inelastic gamma rays are representative of the fast neutron population. WHAT WE CLAIM IS:-

1. A method for determining the porosity of earth formations in the vicinity of a cased well borehole, comprising the steps of: irradiating the earth formations in the vicinity of the cased well borehole with fast neutrons from a source of fast neutrons passed into the borehole; generating a signal representative of the fast neutron population present in the well borehole at a location in the borehole spaced from said fast neutron source; detecting the epithermal neutron population at a location spaced from said neutron source in the borehole and generating a signal representative thereof; and combining said fast and epithermal neutron population representative signals to derive a measurement signal functionally related to the porosity of the earth formations in the vicinity of the borehole.

neutrons from a previous neutron burst, and generating a background signal representative thereof; and compensating said fast neutron representative signal for the background radiation by combining said fast neutron representative signal and said background representative signal.

11. A method according to claim 9 or claim 10, wherein the fast neutron detecting step is performed using a stilbene scintillation detector for fast neutrons.

12. A method according to any one of claims 1 to 4, wherein said irradiating step comprises: repetitively irradiating the earth formations with relatively short duration bursts of fast neutrons; and wherein said fast neutrons representative signal generating step comprises: detecting, substantially only during each said neutron burst, inelastic gamma radiation produced by the scattering of fast neutrons, said inelastic gamma radiation being representative of the fast neutron population in the vicinity of the borehole, and generating said signal representative thereof.

13. A method according to claim 12 including the steps of: detecting, substantially during each time interval between said repetitive neutron bursts, the background radiation present at the location where said inelastic scattering gamma radiation is detected due to lingering neutrons from a previous neutron burst, and generating a background signal representative thereof; and compensating said inelastic scattering gamma radiation representative signal for the background radiation by combining said inelastic scattering gamma radiation representative signal and said background radiation representative signal.

14. A method according to claim 12 or claim 13, wherein the inelastic gamma ray detecting step is performed by using a thallium doped sodium iodide scintillation detector.

15. A method according to any one of claims 9 to 14, wherein said irradiating step is performed with a pulsed deuterium-tritium reaction accelerator tube which emits substantially monoenergetic 14 MEV neutrons.

16. A method according to any one of claims 9 to 16, wherein said step, of detecting performed by emitting approximately 10 microsecond duration bursts of fast neutrons at a burst repetition rate of approximately 20,000 bursts per second.

17. A method according to any one of claims 9 to 16, wherein said step of detecting the epithermal neutron population in the borehole is effected substantially only during each time interval between said repetitive neutron bursts.

18. A method according to any one of claims 1 to 17, wherein said fast neutron population location in respect of which said repretative signal is generated is at a distance in the range of from 40 to 80 centimeters from said neutron source.

19. A method according to any one of claims 1 to 18 including recording the signal functionally related to the porosity of the earth formations as a function of borehole depth.

20. Apparatus for determining the porosity of earth formations in the vicinity of a cased well borehole, comprising: a fluid-tight, hollow, pressure-resistant body member sized and adapted for passage through a cased well borehole and having contained therein a source of fast neutrons operable to irradiate the earth formations in the vicinity of the borehole with fast neutrons; first detector means disposed at a first location within said body member longitudinally spaced from said source to generate a signal representative of the fast neutron population present at said first location in the well borehole; second detector means disposed at a second location within said body member longitudinally spaced from said source to detect the epithermal neutron population present at said second location in the well borehole and to generate a signal representative thereof; and means to combine said fast and epithermal neutron population representative signals to derive a measurement signal functionally related to the porosity of the earth formations in the vicinity of the borehole.

21. Apparatus as claimed in claim 20 wherein said first detector is longitudinally spaced between 40 and 80 centimetres from said source.

22. Apparatus as claimed in claim 20, wherein both said detectors are longitudinally spaced substantially the same distance from said neutron source.

23. Apparatus as claimed in claim 22, wherein said detectors are each spaced between 40 and 80 centimeters from said neutron source.

24. Apparatus as claimed in any one of claims 20 to 23, wherein said first detector comprises a stilbene scintillation detector to detect fast neutrons.

25. Apparatus as claimed in any one of claims 20 to 24, wherein said source comprises a chemical type continous neutron source.

26. Apparatus as claimed in any one of claims 20 to 24, wherein said source comprises a deuterium-tritium accelerator type continous neutron source.

27. A method for determining the porosity of earth formations in the vicinity of a cased well borehole substantially as described herein with reference to Figures 1, 3, 4 and 5; or Figures 2 to 5; or Figures 6 to 9; of the accompanying drawings.

28. Apparatus for determining the porosity of earth formations in the vicinity of a cased well borehole substantially as described herein with reference to Figures 1, 3, 4 and 5; or Figures 2 to 5; or Figures 6 to 9; of the accompanying drawings.