US 3255353 A
Description (OCR text may contain errors)
June 1966 s. A. SCHERBATSKOY 3,255,353
APPARATUS FOR NUCLEAR WELL LOGGING WHILE DRILLING Filed Dec. 21, 1962 5 Sheets-Sheet 1 Demodulaior Fm. Z M40 397 407 r I 352 a j l 4 j 3G) 34 a K i W I 4 Modulated E Oscillations 6 2543] I 7 4 Rate i 7 r Mei T 362 i T 3 450 4 Gaie 1 3&0 Networks I I 3 6 3 Ddccfor' 357 I L l l H: k-
INVENTOR. Serge dSclzerbazskQg H4 BY June 7, 1966 S. A. SCHERBATSKOY Filed D60. 1
5 Sheets-Sheet 2 M ,rnan PA5$ PULSE lO2 mar snscrorz comcmEucE g erwozx 109 7 RATE METERING NETWORK 101% LOW PASS PULSE g c HEIGHTSELEcToR I03 I05 2 M INVENTOR. Serge CZ. .fi'cizerbatsko June 1966 s. A. SCHERBATSKOY 3,255,353
APPARATUS FOR NUCLEAR WELL LOGGING WHILE DRILLING Filed Dec. 21, 1962 3 Sheets-Sheet 3 ,L m 4 my AMPLIFIER .96 SCALE/Q NUCLE/m AMPLITUDE DETECTION DISCRIMINATOR SYSTE M PROPORTION/1L GAMMA RAY DETECTOR INVENTOR. Serge CLScherbaiSkry United States Patent This is a continuation-in-part of an application, Serial No,. 832,971, filed on August 11, 1959, by Serge A. Scherbatskoy, now US. Patent No. 3,071,689, which in turn was a continuation-in-part of the present applicants prior application Serial No. 505,086, filed May 2, 1955, on.
which United States Patent No. 2,946,888 issued July 26,
The invention described in this application relates to logging systems employed in conjunction with the drilling of earth boreholes and is adapted to provide information concerning the nature of earth formations adjacent the boreholes. More specifically, this invention relates to a new type of logging-while-drilling operation designated as anticipating logging.
There is an essential difference between the anticipating logging as described in this application and previous systems of logging-while-drilling which were of nonanticipating type. In the non-anticipatory systemof logging-while-drilling, the information obtained by the driller concerned only those formations that were penetrated at the actual time of drilling. These formations were substantially at the same depth as the lower portion of the drill collar or drill bit. Thus, the prior systems utilized logging methods for detecting and measuring certain specific physical characteristics of the formations which are adjacent the drill collar and drill bit while drilling is in progress. These measurements thus obtained were subsequently converted into suitable signals and transmitted to the earths surface for recording graphically on a chart in correlation with depth or time. Some of these non-anticipatory well-logging systems while drilling are described in US. Patents 2,524,031, 2,659,046, 2,755,432, 2,759,143, and 2,787,759.
The anticipatory logging system described in this application is characterized by a certain new and important feature and is adapted to provide the driller with information which he was not able to obtain heretofore. This information provides accurate data concerning not only the formation traversed by the borehole at the time of drilling, but also data concerning the unpenetrated'formations below the bottom of the borehole and toward which the drilling is proceeding. The information provided by such arrangements is obviously of great value to the driller, comprising, as it were, an advance notice of the type of formation to be drilled into. To illustrate, it is always of value and sometimes of very great value to the driller to know what type of formation the bit is approaching as drilling proceeds. Thus, if drilling is proceeding through an oil-bearing sand formation but the bit is approachinga salt-water-bearing formation, it is of great value to know not only the type of formation being approached, but the nearness of the bit to such formation, since it would normally be desirable to stop drilling prior to penetration into the water-bearing formation.
The ability of the novel system of the invention to provide the driller or operator with information in ad- Vance concerning the approach of the bit or the end face of the borehole toward a change in the earth formation through which drilling is proceeding is based upon the fact that different types of formations have different physical characteristics.
The anticipatory logging system described in the application deals with nuclear radiation measurements and provides informatiton concerning the nuclear structure of various elements which enter into the composition of said formations. More particularly, the following three measurements are made and described in this application: (1) the measurement of the natural radioactivity of earth formations below the bit or the end phase of the borehole, (2) the measurement of gamma rays resulting from the interaction of said formations with neutrons, said neutrons being emitted by an appropriate source positioned within the drill collar, (3) the measurement of gamma rays resulting from the interaction of said formations with gamma rays, said gamma rays being emitted lay an appropriate source positioned within the drill col- Various instrumentalities for making the above measurements of the properties of formations below the drill bit may be housed in a chamber provided in the drill collar. In such a manner normal functioning of the drilling apparently is not affected. Signals representing the desired information obtained by the instrumentalities in the drill collar at the bottom of the borehole may be transmitted to a location outside the borehole and evaluated by the driller. Two systems for transmitting signals are disclosed in this application. In one of these systems, the signals representing the measurements of various properties of earth formations are represented by electric currents and, therefore, an electrical connection is required between the measuring apparatus at the lower end of the borehole and the recording apparatus located outside of the borehole at the earths surface. Such an electric connection may be obtained by means of a conducting liner passing through the entire length of the drill stem and thus provides a transmitting channel between the signaling equipment at the subsurface and the earths surface. The other transmitting system does not require an electrical connection between the apparatus in the borehole and the apparatus located outside the borehole at the earths surface, the information obtained at the bottom of the borehole being transmitted by other means, preferably by pressure-change signals in the downwardly flowing stream of drilling fluid. Such pressure-change signals are produced by means of an appropriate valve system positioned at the lower portion of the drill stem and actuated by a sensing device in accordance with measurements performed. The above second system commonly designated as pulsation transmitting system is described for instance in US. Patent No. 2,787,759, issued to I. J. Arps on April 2, 1957.
Various systems described in this application comprise as one of their essential elements a gamma-ray detector which is positioned within the lower portion of the drill collar. This detector is of a directional type, i.e., it is adapted to respond selectively to those radiations which are emitted by the formations at various distances below the bit and travel vertically upwards toward the bit. The gamma rays incident upon such directional detector may be due to natural radioactivity characterizing the formations, or they may result from the irradiation of the formations with neutrons. In the latter case, a source of neutrons is also placed within the lower portion of the drill collar. The source of neutrons may be of a directional type and in such case it will emit neutrons preferentially in the downward direction so as to irradiate more of the drilling of the borehole, whereby the driller or operator may determine, in advance, the approach of the end face of the borehole toward an interface between earth formations of differing characteristics. It is another object of the invention to provide a method of determining nearness of approach of the end face of an earth borehole to an interface between earth formations of differing physical characteristics during drilling of the borehole.
It is another object of the present invention to provide a method of determining the approach of the end face of an earth borehole toward an interface between earth formations of different characteristics during the drilling of the borehole, whereby the driller may suspend drilling operations, if desired, prior to drilling through such interface.
It is another object of the present invention to provide in an earth borehole logging system adapted for logging during drilling, means for logging the formation ahead of the end face of the borehole whereby approach of the bit toward changes in the earth formation through which an extension of the borehole is directed may be determined in advance.
The above objects, and other objects and features of novelty which will hereinafter be made apparent, are accomplished by the invention, a preferred form of apparatus and mode of operation of which are explained and illustrated in the following description considered in conjunction with the accompanying drawings, in which:
FIGURE 1 is a diagrammatic view illustrating a typical environment and physical arrangement of apparatus according to the invention. This arrangement is characterized by a conducting liner passing through the entire length of the drill stem in order to make an electrical connection between the detecting equipment at the subsurface and the earths surface.
FIGURE 2 shows a directional detector to be used in the arrangement of FIGURE 1.
FIGURE 3 shows a diagrammatic view of another embodiment of my invention comprising subsurface equipment, surface equipment, and including a graphic record of radiation-detector output. In this arrangement, the signals produced in the subsurface equipment are transmitted to the top of the drill hole by means of pulsation of the drilling fluid circulating in the drill hole.
FIGURE 4 shows the subsurface equipment of the well-logging system illustrated in FIGURE 3.
FIGURE 5 shows an alternative type of directional detector which may be used in the FIGURE 1 apparatus as an alternative to the apparatus shown in FIGURE 2.
Referring now to the FIGURE 1, there is illustrated a typical earth borehole 350 being drilled through successive earth strata by means including a drill bit 351 secured to the lower end of a drill collar 352. The drill collar forms the lower section of a drill string 353 comprising one or more sections of a drill pipe, and a kelly.
The kelly and the drill string are suspended from a rotary swivel carried by the traveling block. The kelly, rotary swivel, and traveling block, being conventional, are not shown in the figure. The drill stem is lined with an insulating liner 354 which extends below the bottom limit of the drill stem and engages the drill bit. This liner serves as a bushing to insulate the mud from the drill stem.
A conducting liner consisting of a metallic tube 355 passes the entire length of the drill stem inside of the insulating liner 354 and makes an electrical connection between the subsurface detecting equipment and the earths surface. At the lower end of the drill collar, directly above the bit 351, is positioned a source of radiation 360 and a detecting and signaling means 361, responsive to radiation impinging upon it from below.
To provide an indication of the characteristics of a formation as yet undrilled and situated beyond the face of the borehole two systems are described. These are identified as System A and System B. System A comprises a source of gamma rays adapted to send directionally a beam of radiation downward toward the regions situated below the drill at a considerable distance from the bottom of the borehole, and a gamma-ray detector adjacent said source, said detector being adapted to receive gamma rays scattered and returned from said regions. System B comprises a source of neutrons placed above the drill bit, said source being adapted to transmit neutrons in all directions, in conjunction with a directional gamma-ray detector disposed adjacent the neutron source and adapted to detect gamma rays impinging on the detector as the result of interactions of said neutrons with the formations below the bit.
Consider now System A and let numeral 360 represent a suitable source of gamma rays such as Co or radium. This source is placed in a block 362 of material such as lead or tungsten which strongly absorbs the emitted rays, so that practically the only radiation from the source appearing outside the block is the narrow pencil of parallel rays passing downward through the hole 363. This collimated beam penetrates into the formation below the drill bit and interacts with matter at various depths such as H H H and H shown in the figure. As a result of such interactions some of the gamma-ray photons are absorbed due to the photoelectric effect and others undergo Compton scattering. In the latter case, some of the scattered photons are directed upward in the direction of the arrow Z and are intercepted by the detector 365. The detector 365 is directional, i.e., it is adapted to receive primarily those radiations which impinge upon it from below.
The detector 365 is a proportional gamma-ray counter, of which the well-known scintillation detector is an example, and it yields, across its output terminals, electric current impulses proportional in magnitude to energies of the intercepted photons. The output of detector 365 is connected to gate networks 366, 367, and 368. The gate 366 is arranged to transmit current impulses representing photons having energies below 0.5 mev., the gate 367 transmits impulses representing photons having energy range from 0.5 mev. to 1.5 mev., and the gate 368 transmits impulses corresponding to photons having energies from 1.5 mev. to 2.4 mev. The photons intercepted by the detector are secondary photons originating at various depths below the drill bit and result from scattering at various depths of primary photons radiated by source 360. It is well known that a beam of low-energy photons is more efiectively attenuated than a beam of high-energy photons when passing through various layers of earth. Consequently, the spectral distribution of gamma rays arriving at the detector can be correlated with the depths at which these scattered gamma rays originated. The hard gamma rays produce impulses transmitted through the gate 368, and these gamma rays originate at larger depths such as H shown in FIGURE 1. The medium and soft gamma rays produce impulses in the output of the gates 367 and 366, respectively, and originate at shallower depths H2 and H The values H H H do not represent definite magnitudes of depths, but represent ranges of depths that are in the neighborhood of values H1, H2, and H3.
The outputs of gates 366, 367, 368 are respectively applied to rate meters 376, 377, 378, each of said rate meters producing an output voltage representing the rate of incidence of input impulses. Thus each of said output voltages represents the intensity of photons, within a definite energy range, intercepted by the detector 365, and hence provides information concerning the characteristics of the earth formations at various depths beneath the drill bit, as yet undrilled.
Numerals 386, 387, and 388 designate oscillators, each having its own characteristic frequency and generating output signals controllable in amplitude by the D.-C. voltage derived from the corresponding rate meter by which photons.
it is fed. The modulated outputs of the oscillators are transmitted to the earths surface via the conductor 355 and are there applied to the demodulators 396, 397, 398, which reproduce at their output terminals the original modulating voltages obtained at the outputs of the rate meters 376, 377, and 378, respectively. The Outputs of the demodulators 3'96, 397, and 398 are indicated on the meters 406, 407, and 408, and these meter indications represent characteristics of the formations at progressively greater depths in the undrilled formations below the drill bit.
One can remove from the above System A the source 360 and retain in the subsurface equipment the detector 365 and all the associated equipment for transmitting the output of the detector to the earths surface (such as'the gate networks 366, 367, 368, the rate meters 376, 377, 378, and the oscillators 386, 387, 388). In such case, the detector 365 will respond to gamma rays emitted by the naturally radioactive elements contained in the formation underneath the drill bit, and the resulting logging system will provide anticipatory gamma-ray logging based on the natural radioactivity of the formation-s at various depths below the drill hole.
Consider now the System B and refer to FIGURE 1. Now the numeral 360 designates a source of neutrons such as a radium-beryllium mixture. These neutrons are slowed down by the surrounding earth formations until they reach thermal energies and are eventually captured by various elements in the formations, each neutron capture being accompanied by the emission of one or more It is well known that the energies of various gamma rays of capture characterize particular elements which participate in the process of capture and the rate of occurrence of gamma rays of particular energy represents the relative abundance of the corresponding element in the earth formations.
In the immediate neighborhood of the neutron source 360 is placed the directional gamma-raydetector 365 which produces across the output terminals pulses representing the energies of gamma rays intercepted by the detector. These gamma rays result from the interaction of neutrons with the underlying formations at various depths underneath the drill bit 351. These gamma rays undergo multiple collisions in the formations, and the scattered gama rays resulting from these collisions are emitted in various directions. A portion of these scattered gamma rays and a portion of gamma which did not undergo any scattering travel upwards in the direction of the arrow Z. These gamma rays directed upward may be intercepted by the detector 365 and in such case one obtains across the output terminals of the detector 'current pulses representing energies of the intercepted gamma rays. These pulses are transmitted to the gate network 366, 367, 368. The gate 366 transmits pulses corresponding to soft gamma rays, gate'367 transmits pulses corresponding to gamma rays of intermediate energy, and gate 368 transmits pulses corresponding to hard gamma rays. The outputs of the gate networks are applied to the rate-metering networks 376, 377, 378 and the DC. voltage from the outputs of the rate-metering network are arranged to modulate the amplitudes of oscillators 386, 387, 388, as previously described with respect to System A. These modulated outputs are in turn applied to the metallic conductor 355, are transmitted to the earths surface, and are demodulated by demodulators 396, 397, 398 in the same manner as previously noted, the indications on meters 406, 407, and 408 providing information as to the characteristics of the formations at various depths within the undrilled formations beneath the drill bit. Thus, the driller is continually informed during the drilling process regarding the character of the formations about to be penetrated.
The pass-band characteristics of the gate networks may be adjusted to yield information as to the relative abundance of one or more specific elements of particular interest. For example, a gate network may be adapted to selectively transmit the pulses representing the energy of 2.3 mev., corresponding to gamma rays of capture emitted by hydrogen. In such case the output of the rate meter fed by such gate network will represent the relative abundance of hydrogen in the formations underneath the drill bit.
The detector shown in FIGURE 2 is illustrative of a directional gamma-ray detector suitable for use as detector 365 in the FIG. 1 apparatus. In FIGURE 2, the directional detector consists essentially of an assembly comprising a scintillating phosphor 430 operatively engaged to a photomultiplier 431 and surrounded by a lead shield 432. The phoshor may be of anthracene, sodium iodide, or any of the other materials adapted to produce light as a result of interaction with photons. The shield 432 is of a material such as lead or tungsten which will strongly absorb all incident photons except those arriving along the directional axis LK designated as the axis of the directional receiver. The phosphor 430 is suflicently large to absorb all the energy of incoming photons so as to produce across the output terminals 434 of the photomultiplier 431 a succession of impulses having magnitudes proportional to the energies of incident photons intercepted by the detector.
Another embodiment of my invention is illustrated diagrammatically in FIGURE 3. This figure shows a typical earth borehole it} being drilled through successive earth strata or formations, including formations 11, 12, 13, 14, 15, 16, and 17. The drilling equipment comprises a drill bit 18 secured to the lower end of a drill collar 19 which forms the lower section of an otherwise conventional drill string 20 comprising one or more sections of drill pipe, and a kelly 21. The kelly and drill string are suspended from a rotary swivel carried by a traveling block 22 supported for vertical movement by a cable 23 rigged in conventional manner about a crown block 24 supported by a suitable derrick 25, the cable extending to and being operated by. a draw works 26 from which draw works power is provided through bevel gearing 27 for rotating the kelly, drill string, and drill bit. The drill bit, drill string, kelly, and swivel are provided with one or more suitable internal passages through which is pumped, under pressure, a stream of drilling fluid supplied to the swivel through a rotary hose 28 and a conduit 29 by a pump 34), the hose permitting travel of the swivel, kelly, drill string, and drill bit as the borehole is extended during the drilling operations.
Pump 30 is provided with a suitable surge bell or tank 31 serving to reduce pulsations in the discharge of the pump, and draws the drilling fluid from any suitable source such as a supply sump 32, through an intake pipe 33. The drilling fluid forced downwardly through the drill string and out orifices in the drill bit returns to the surface through the annular space encircling the drill string, carrying with it the drill chips, and is discharged from the upper cased portion of the borehole through a suitable means such as pipe 34, through which the fluid is passed to a convenient location for screening, settling and return to sump 32.
The thus-far enumerated structures, With the exception of drill collar 19, are conventional, and may be of any suitable construction and arrangement and are shown to illustrate the environment of the invention and aid in a clear explanation of the operation of the preferred embodiment of apparatus hereinafter described.
The embodiment illustrated in FIGURE 3 utilizes changes of mud pressure in order to transmit the logging information from the subsurface to the earths surface. Thus the measurements performed at the bottom of the drill hole are translated into equivalent series of signals in the form of pressure changes created in the downwardly flowing stream of drilling fluid within the drill string, which signals are quickly transmitted therein and thereby upwardly and out of the borehole to a suitable transducer mechanism which serves to retranslate the pressure changes into sense-perceptive indications suitable for interpretation and utilization by the driller or operator. In the preferred embodiment of apparatus of the system of the invention herein disclosed, the pressure changes are chosen to be increases in pressure to a value somewhat above normal, followed in each case by a decrease in pressure to a normal value.
Referring again to FIGURE 3, the pressure-change signals thus propagated in the stream of drilling fluid within drill collar 19 are transmitted by and through the drilling fluid to the top of the drill string through the swivel and hose to the conduit 24 and pump 30. The pressure changes are also transmitted through drilling fluid contained in a conduit connected to conduit 29 as indicated, and are transmitted past a throttling valve 36 interposed in the conduit 35 and to a transducer means indicated diagrammatically and generally at 40. The transducer may assume a variety of different structural forms, but preferably and as illustrated, comprises a box 41 divided by a flexible diaphragm 42 into two chambers interconnected by a small bore tube 43. Conduit 35 passes through a sealed opening in the left-hand chamber and terminates in a sealed bellows 44. The right-hand chambar is provided with a bellows 45 sealed around an opening in box 41 and carrying at a sealed aperture a recorder rod 46 which is attached to diaphragm 42 for actuation thereby as indicated. Rod 46 supports and moves a pen 47 arranged to form a graph on a strip of graph paper 48 supplied from a roll 49 and taken up by a take-up roll 50 and wound on a roll 51 by suitable driving means such as a clockwork 52 in a manner and by means as indicated. Long duration or slow changes in drilling fluid pressure in conduit 35, such as are incidental to increasing depth of the borehole, are equalized at diaphragm 42 by the action of tube 43 which permits slow movements of fluid from chamber to chamber in box 41, but which is incapable of passing the fluid rapidly enough to more than slightly damp the pressure-change signals received through conduit 35. Thus, a pressure-change signal is translated by diaphragm 42 into a corresponding movement of rod 46 and pen 47, which latter forms a sense-perceptive indication in the form of a graphical representation or record of the signal as received. As noted, the record will be, in the disclosed embodiment of the invention, a graph comprising repetitive series of indications or signals. The time intervals between these signals are translated by uniform movement of graph paper 43, by means of roll 50 and clockwork 52, into equivalent and proportional distance intervals, the latter being designated on FIGURE 4 as D D D and D A time-log of borehole depth may be secured by a conventional depth vs. time meter; or the equivalent information may be supplied as indicated by a depth indicator 53 which comprises a depth measuring device provided with a printer drum rotated by a drum and cable connection to the traveling block as diagrammatically indicated, and an actuator operated periodically through a mechanical drive including a drive shaft 53d driven by clockwork 52, and arranged to print or otherwise provide depth indications at intervals on paper 48. A preferred arrangement of the apparatus is such that the depth indications are numerical indications of the actual borehole depth. The transducer and signal-recording means, and the cooperating depth-indicating means, are not per se of the present invention and may be of any suitable and convcntional construction.
Consider now FIGURE 4. This figure shows certain details of the arrangement of FIGURE 3, and more particularly the subsurface equipment contained within the drill collar. As shown in FIGURE 4, the drill collar comprises an upper section 73, an upper sub 74, a special section whose construction will hereafter be more fully described, and a lower section 76 to which drill bit 18 is secured. The downwardly flowing stream of drilling fluid is delivered by upper section 73 of the drill collar tothe specially shaped internal bore 77 of sub 74. The previously mentioned pressure changes in the drilling fluid stream representing the logging information are effected within the bore 77. This bore 77 has a substantially uniform diameter in its upper portion. However, it diverges at its lower port-ion in a section of increasing diameter reaching a maximum diameter in the .region where the interior bore of the special section 75 joins the bore of sub 74. The special section 75 of the drill collar has a relatively large internal diameter, whereby there may be positioned within said drill collar and supported in the drillingfluid stream a generally cylindrical apparatus case 78. The apparatus case 78 is provided with upper and lower sets of supporting legs 80 and 81, respectively, which legs are suitably formed to snugly engage the interior wall of section 75. The respective supporting legs are circumferentially spaced apart around the exterior periphery of the apparatus case 78 and are preferentially of smooth streamlined form, whereby the stream of drilling fluid may flow without undue interference. The drilling-fluid stream passing downwardly around the exterior periphery of the apparatus case 18 flows into the bore of the lower sub 76 from which it is delivered to the drill bit 18. The direction of flow of the drilling mud is indicated by arrows.
The apparatus case 78 comprises a radiation detecting or measuring means for developing electrical signals representing the physical characteristics of various formations below the drill bit and a valve means controllable by said signals, said valve means being adapted to effect corresponding pressure changes within the drilling-fluid stream. The radiation detecting or measuring means comprises a radiation source which may emit gamma rays, or neutrons, and a nuclear detecting system, said system comprising various instruments included schematically within the dotted line 95. The valve-controllable means comprises various elements including an electromagnet 94 energized by the signals derived from the nuclear detecting system and valve head 86 suitably attached to a suitable valve rod, said valve being actuated by the electromagnet.
The above-mentioned valve may assume a variety of physical forms. The form chosen to illustrate the preferred embodiment of apparatus according to the invention comprises a lower shaped portion of the wall of the bore 77, of the sub 74, and a complementary shaped valve head 86 fixedly mounted on the upper end of a valve rod 87. The valve rod 87 is mounted as indicated for vertical reciprocation in the upper piece 79 of apparatus case 78. This valve rod comprises three sections: an upper section 88, a lower section 89, and an intermediate section 90. Both the upper section and the lower section of the rod are made of a non-magnetic material, and these sections are rigidly secured to the central section of ferro-magnetic material. Upper section 88 is adapted to be guided by suitable bearing and packing means 79 of case 78; while lower section 89 is arranged to be guided in conventional bearing means provided in a suitable valve rod guide disc M secured in the interior of case 78 as indicated. The lower end of rod section 89 is surrounded by a compression spring 92 abutting against disc 91 and pressing against a disc 93 suitably fixed to section 89. The spring 92 is adapted to hold the valve rod 87 and the valve head 86 in approximately the position illustrated in FIGURE 5 with a normal stream of drill fluid flowing.
The motion of the valve 86 is actuated by an electromagnet 94 which is energized by the electric signals obtained in the subsurface equipment from the sensing or measuring means said signals representing the logging information Which has to be transmitted to the driller at the earths surface. In the arrangement shown in FIGURE 4, the electromagnet Q4 is not energized, and therefore a normal fiow of drilling fluid flows through the drill pipe and the drill collar.
The valve-actuating electromagnet 94 comprises a ferromagnetic outer case and a coil or winding within the case. This electromagnet is secured in the upper interior of the casing 7 8 and it encircles the section 88 of the valve rod. The ferromagnetic section 90 of valve rod 88 is partly positioned within the electromagnet as a readily slidable core piece.
The aforedescribed arrangement of valve structure is such that when the electromagnet 94 is not energized spring 92 and the drilling-fluid stream act to position valve head 86 to provide no restriction to the flow of the drilling fluid. However, when the electromagnet 94 is energized, valve rod section 90 is drawn upwardly, forcing valve head 86 upwardly to increasingly restrict flow of drilling fluid therepast to cause a pressure increase in the drilling-fluid stream thereabove. It is thus seen that pressure changes may be produced in the drilling stream by the selective operation of the valve.
Consider now the radiation detecting or measuring means for developing electrical signals which energize the electromagnet 94. This means comprises a radiation source 115 and the nuclear detecting system 96. The radiation source positioned within the lower portion of the apparatus case 78 is placed in a block 116 of a material which strongly absorbs the radiation emitted by said source so that practically the only radiation emitted by this source which appears outside the block is the narrow pencil of parallel rays which pass in the downward direction through the hole. This collimated beam penetrates into the formation below the drill bit and interacts with matter at various depths such as H H H shown in FIGURE 3. As a result of such interactions gamma rays are emitted by said formations in various directions from the points at which said interactions occ-ur. Some of such gamma rays directed upwards from such depths as H H and H are intercepted by the nuclear detecting system 96,. said system being adapted to produce across its output terminals 95, appropriate electric pulses representing the energies of said gamma rays. The nuclear detecting system 96 comprises a proportional gamma-' ray detector 110 having its output terminal applied to an amplitude discriminator 111. The amplitude discriminator is in turnconnected to a scaler 112 which has its output terminal connected to an amplifier 113. The signals obtained across the output terminal 95 of the amplifier are applied to the valve-energizing electromagnet 94.
The proportional gamma-ray detector may be. of the type shown in FIGURE 2., i.e., it consists of a scintillating phosphor 430 operatively engaged to a photomultiplier 431, said phosphor and photomultiplier being surrounded by a lead shield 432. The lead shield is adapted to strongly absorb all photons except those arriving upward along the vertical direction. Across the output terminal of the detector 110 electrical signals are produced which have magnitudes representing the respective energies of photons intercepted by detector 110. The amplitude discriminator 111 connected to the output terminal of said detector is provided with an upper and a lower threshold and is adapted to transmit only those signals the magnitude of which exceed the lower threshold and are below the upper threshold. Consequently, the output of the discriminator 111 transmits only those pulses which represent photons within a limited energy range. These pulses are in turn applied to the scaler 112. The pulses obtained in the output of the scaler occur at a rate which is a sub-multiple of the pulses derived from the discriminator 111.
These pulses are in turn transmitted through the amplifier 113 to the lead 95 and thence to the electromagnet 94, the magnet being energized momentarily by each amplified pulse.
Thus each pulse applied to the terminal 95 produces a corresponding pressure pulse in the mud stream, resulting in transmission to the earths surface of a pressure signal. Each such pressure signal actuates the recording system as explained hereinabove and illustrated in FIG- URE 3, thus providing on graph paper 48 a repetitive 10 series of indications or signals, each of which represents a current pulse'obtained from the output terminal of the amplifier 113. The time intervals separating said signals represent the flux intensity of gamma rays intercepted by the detector 110, and hence provide an index of the character of formations below the drill bit.
In the arrangement of FIGURE 3 the source 115 may be omitted, the remainder of the subsurface equipment contained within the drill stem being retained. In such case the detector 110 will respond to gamma rays emitted by the naturally radioactive elements contained in the formations underneath the drill bit and hence will provide an anticipatory gamma-ray logging system based on the natural radioactivityof the formations below the drill hole.
The particular type of directional detector shown in FIGURE 2 may of course be replaced by other directional radiation-detecting systems, such as that shown in FIG- URE 5.
The apparatus therein shown comprises a pair of scintillation phosphors 101 an 102, having respectively associated with them photomultipliers 103 and 104. In accordance with conventional practice, these photomultipliers are of the type which develop electrical pulses re- .sponsively to the appearance of light flashes in the respective phosphors, such flashes being produced by gammaray interactions. The electrical pulses have magnitudes which are respectively proportional to the brightness of the flashes.
The line defined by the respective phosphors 101 and 102 is designated in FIGURE 5 by the arrow A. Scintillation phosphor 101 is a relatively small radiation-sensitive element, while phosphor 102 is substantially larger or more dense than phosphor 101, or both.
The photomultiplier 103 has its output connected to a low-pass pulse-height selector 105, .a device which is per se conventional and adapted to transmit only output impulses from photomultiplier 103 having magnitudes be-' low a predetermined threshold value. The photomultiplier 104 has its output connected to a high-pass pulseheight selector 107, also per se a conventional device, which is characterized by transmitting only electrical impulses from photomultiplier 104 having magnitudes above a predetermined threshold value.
The outputs of pulse-height selectors 105 and 107 are fed to a coincidence network 106, which is in turn con nected to a rate meter 108, the output of which is indicated on the meter 109.
When the apparatus of FIGURE 5 is placed in a gamma-ray field, an incident photon arriving along the direction AC may undergo Compton scattering in the phosphor 101, thus producing therein a flash of light. Because phosphor 101 is small in mass, no subsequent scattering of the photon will normally take place within that phosphor. The scattered photon will leave the phosphor 101 at an angle dependent on its original energy and the quantity of energy it has given up as the result of its interaction within the phosphor 101. If only a small amount of energy has been given up in phosphor 101, the direction of the scattered photon will be practically unchanged; i.e., it will leave the phosphor- 101 along the general direction CM and will henceenter the second phosphor 102.
Because phosphor 102 is large and massive compared to phosphor 101, it will normally absorb completely photons scattered by the phosphor 101.
When a gamma ray arrives along the direction AC, and gives up only a small amount of its energy in the phosphor 101, it will proceed along a substantially unchanged direction and will enter phosphor 102, giving up the remainder of its energy thereto. This results in the production in the phosphor 101 of a relatively lowintensity flash and the production in the phosphor 102 of a substantially higher-intensity flash. These flashes produce electric pulses of corresponding intensity in the respective photomultipliers 103 and 104, both of which pass through the pulse-height selectors 105 and 107 and actuate the coincidence network 106. Since the two impulses reach the coincidence network 106 at substantially the same instant of time, they produce an output pulse from network 106 which is counted by the rate meter 108 and indicated on the meter 109.
The only gamma rays that will thus actuate the instrument of FIGURE 5 are those arriving substantially along the direction AC. A gamma ray arriving at phosphor 101 from some other direction, such as along the direction indicated by the arrow M, may undergo a scattering reaction in phosphor 101 which will result in a scattered ray along the direction C-M. Should this occur, however, the scattering angle 0 will be relatively large, corresponding to substantial energy loss in the phosphor 101. This will result in an output pulse from photomultiplier 103 of magnitude too great to be transmitted by the low-pass pulse-height selector 105. In most cases, moreover, the reduced-energy scattered ray, when it strikes and is absorbed by phosphor 102, will not produce an output pulse from photomultiplier 104 of sufficient intensity to be transmitted by the highpass pulse-height selector 107.
From the foregoing it will be understood that the only photons having any significant probability of being detected and counted by the FIGURE 5 apparatus will be those arriving at phosphor 101 along the direction AC.
The apparatus of FIGURE 5 is essentially unresponsive to rays arriving in the direction indicated by the arrow X-i.e., rays aligned along the direction AC but directly opposite in sense.
Such rays, on striking the large, dense phosphor 102, will normally be completely absorbed therein and will not produce any scattered ray which is intercepted by phosphor 101. Hence the coincidence of impulses necessary to produce an output from network 106 will not occur.
The width of the beam or cone of arriving gammaray photons that will be detected by the FIGURE 5 apparatus may be controlled by suitable adjustment of the threshold of low-pass pulse-height selector 105; plainly, the higher the threshold of selector 105, the greater will be the acceptance angle of rays arriving generally along the line AC that will be detected and counted by the FIGURE 5 apparatus.
Other types of directional radiation detectors known to the art, apart from those disclosed in FIGURES 2 and 5, may of course be used in the apparatus of this invention.
It is to be understood that the foregoing description is illustrative only, and that the invention is not to be limited thereby. The scope of the invention is to be determined primarily by reference to the appended claims.
1. A system of logging-while-drilling adapted for logging of earth formations beyond the bottom of an earth bore hole, comprising in combination a drill string, a drill bit, a nuclear radiation source within said drill string for irradiating the hitherto undrilled formations .lying beneath said drill bit, a directionally sensitive radiation detector within said drill string positioned to respond preferentially to gamma rays emitted from below the drill bit and directed upwardly, and means fed by said detector for transmitting to a point external of the bore hole information derived from the output of said detector.
2. The apparatus defined in claim 1 comprising also means operatively engaged with said nuclear radiation source of confining said irradiation to a direction downward and into the undisturbed formations beyond the end of the bore hole.
3. The apparauts defined in claim 2 wherein the nuclear radiation source is a source of gamma rays.
References Cited by the Examiner UNITED STATES PATENTS 2,349,366 5/1944 Moon 25083.6 2,825,044 2/1958 Peterson 250-836 2,849,530 8/1958 Fleet 250--83.6 2,905,826 9/1959 Bonner 25083.6 2,934,652 4/1960 Caldwell 250-83.6 2,978,634 4/1961 Arps 73-152 3,019,338 1/1962 Monaghan 25083.3 3,041,454 6/1962 Jones 25083.6
RALPH G. NILSON, Primary Examiner.
JAMES W. LAWRENCE, Examiner.