USRE24383E

USRE24383E - Neutron gamma-ray well logging

Info

Publication number: USRE24383E
Authority: US
Publication date: 1957-10-29

Description

Oct. 29, 1957 A. s. MOKAY NEUTRON GAMMA-RAY WELL LOGGING Original Filed Feb. 15, 1952 2 Sheets-Sheet 1' I? ,RDER

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rbhoP n mzZj M w w IN VEN TOR. A1 XA/VDEAPSJVIC /\A r ATTORNEY Oct. 29, 1957 A. s. MOKAY Re. 24,383

NEUTRON GAMMA-RAY WELL LOGGING Original Filed Feb. 15. 1952 2 Sheets-Sheet 2 TJEI-EI- 52 United States Patent" Ofifice Reissued Oct. 29, 1957 NEUTRON GAMMA-RAY WEHLLOGGINGi Alexander S. McKay, Bellaire, Tex.', assignortto The Texas Company, New York, N: Y., a'. corporation of'Dela ware- Originzl N0. 2,752,504,- dated.-J|me 26, 1956, Serial No. 271,755, February; 15-, 1952. A'pplicafiomfonreissue April 1', 1957;SerialNo. 650,059

6 Claims; (Cl; 250 -713 Matter enclosedvin heavy bracketsilappearsin the original patent'but formsmo partofthisireissue specification; matter printed italics indicates the additions made by" reissue. t

Thisinvention relates to a method of determining the natureof earth formations; particularly; those formations traversed by a b'oreholeorwell. Mme-specifically, the inventionrelates to amethod of examining earthforrn'ations in situ by bomliarding 'the formations with neutrons and measuringthe intensity of those gamma=rays induced by the neutron bombardment whiclrareihdiatiireof the fact that they are produced by =the cap'ture of neutrons'by elements other thanhydrogen. The principal objectof the invention is provision of a= method of this" type through the carrying :out "of which certain elements such; for example; as chlorine can'he distinguishedffoni 'hydi'o= gen: Ari-important applicatiomof themetho'dis in the determination as' to whether an earth formation contains salt wateror' hydrocarbon oil.

The fundamental principle involved in this invention is" that when formations" are homh'arde'd by neutrons gamma rays-havingenergies of 'about 232 m. e: v; are emitted by liydrogennuclei on capturing; thermal neu= trons, while chlorine nuclei, for example; emit gamma rays having energie's'up to8 or 9 m; e. v. on capturing thermal neutrons. Ithas been" found that by using a detector-of the type of a proportional crystalscintillation counter; it is possihle todiscrirninate against the pulses causedbythe '2'j2'm: e: v. gamma-rayswhile stillco'unt ing'amappreciahlenumber of the pulses caused By the energy gammarays.

In many formations where oil is found, the nuclei have smallrcro'sssectionsforthecaptureof thermal neutrons thatr has" hydrogen. As a result a large part of the thermalneutr'ons' are captured 5 by; the hydrogen nuclei with" the emissionof 222 m. e: v: gamma rays: Sandi which? is substantially" composed of silicon and oxygen nuclei? is an example of such a" formation" since silicon and especially oxygen'have low'capture cross" sections fhr thermafneutrons: GhiorinB, ontheother'handl has a capture cro'ss"sectionwhich is much larger'than those of silicorr oroxygen'; and thereforedf chlorine is"present in the formation; some of' thermal'neutrorrs wilI'hecap tured by chlorine nuclei which'then' emit gamma rays havr'r'rgenergies that are considerably larger than" the 222 m: e: v-.' gamma rays which arise'throughhydrogen' thermalneutroucapture; In fact; each time a nucleus of the more abundant isotope of" chlorirre captures a thermalneutron; there is" a release of "a total energy of about 8 m: a e: v. and many other nuclei also" emit high encrgygamma' rays under these circumstances;

When'a scintillationcounter-wither fairly large sensitive massis used to" detect these thermal neutron induced gamm'as, thereis arelati'on between'themaxirnum size'o't the pulses th'at ar'erau'sed by gammasofa certain'ener'gy', and the sizeof'this-energy; That'is, the-maxi'inummulse siie-thationeohtains ffom a'2l2 rn'. e: v; gamma'isc'om sldrably smaller than the-maximumsi'ze thati'sohtained fronr;-for instanoe;-a= 4 m; e: v. gamma": 'Phuseib'i' seen 2: that if there is a -changein-the relative numbersot fthermal neutrons thatare absorbedb'y the" various nll'clibf io formation; then-inaddition to a possible changeintlie counting rate, this change may'make-itselfapparentirra change inthe pulse height distribution" which could be detected by repulse height analyzer;

Experiments have "been performed which clearly prove theprinciples outlincd'in the foregoing: In-on'e'ofthese experiments a 32 gallon receptacle-was 'filled' witli' wa er; the salinity ofiwhich could *be easily varied oncont'rnlled; A source of neutrons comprisin'ga mixture of radium and beryllium in a container having lead walls"2" thick was immersedhrr" the water and i an additional" 4" of lead shielding was 1 placed between L the source and" a" sciittillti ti'omdtecror- [scintillemeterfl having a fair'ly largedumim ophor comprising" a sodium iodide; thallium activated; crystalin' orderto minimize thedirect radium radiation With the crystal When the" receptacle was; filled' with pure water; it was found that the hydrogen nucleifirst served a's the moderator; slowing; the fast neutrons down to thermal vel'ocities; andthen proceeding to 'capturemo'st of-"the thermalneuttons, producing 212m": e: v. photons and duteriumin the -process: A's chlorine ill the form-of NaCl was added to the water, the fast' neutrons'were slowed Riown'asbefor'eby thtrhydrogen nuclei which were present in approximately the same number per unit volumeof liquid a'sbefore; Howevenithe' chlorine nuclei then competed with the hydrogen nuclei for. the capture ofthethermal neutronsand since the chlorine capturecross sectiondsahout- 106 times that'of the hydrogen, chlorine-nucleicapture-as manytherrnal neutrons-as the hydrogen when the relative-numherof" chlorine" and hydrogen nuclei penuuit volume of liquid is only /ibb which corresponds to a salinity of 45 grams per litre: This means that" when the salinity, reaches" about? grams per litter; the chlorine nuclei are already capturing most= of the therrnal neutrons and the addition of" more chlorine nuclei by increasing the salinity" still fiirther will have but little effect on=the nature of ther're'dtron capture gamma rays; The that the experimental curve of Figure 2 flattens outat thesame" salinityas' the calculatedrate of thermal neutron capture by chlorine nuclei versus salihityshows thatonecanhaveaquantita tive=k nowled ge of the physical processes involved.

Iii accordancewith one embodiment of the invention; the induced gamma'raysaredetect'ed hya' scintillation detectorlfscintillorneterjfcomprising; asw'ell known, a lumino'phor and a-"photo-multiplier tube and'the electrical pulses produced are transmitted to' the surface where they'are-amplifiedand dividedinto two groups by means of a conventional discriminator which, for example; can be set toblock a'llpulses" smallenthan' those which cor= respond-to the-maximum'pulse from a' 2:2 m; e; v. gamma ray. The'counting ratecf the'lar'ger'pulsesis then sepja= rately' recorded in addition" to the usual. recording of the counting rate'of'a'llpulses which provides the more or'less usual neutron gamrna ray log"provid'ed; of course;

' Certain considerations are of importancein the design of a chlorine logging tool or instrument. Since the discriminator bias is set quite high, there is comparatively l ttle interference from the thermal noise of the photomultiplier. It is quite possible that suitable photo-multiplier tubes will be developed for higher temperatures than the type 5819 tube, the present limit of which is about 75 C. When and if this improvement is achieved, there will be less or no necessity for refrigeration of the detector when used in a bore hole. Furthermore, the high discriminator bias also makes the source shielding requirements less stringent than in the case of the present neutron-gamma ray logging instrument. Thus, with the smaller amount of source shielding, the detector may be placed as little as 4 or 5 inches from the source and with this geometry it is known from past experience that the resulting log will be substantially insensitive to the amount of hydrogen in the formations. At the same time, the log will still be sensitive to the presence of chlorine and, thus, by operating with the detector close to the source, changes on the log due to the presence of chlorine can easily be distinguished from changes due to hydrogen content of the formations. If, in this case one wished to measure the hydrogen content simultaneously, a second detector at a greater distance from the source would be required together, of course, with means for conducting its output to a recorder at the surface. One also gains in the provision of higher counting rates when the detector is disposed closer to the source.

For a better understanding of the invention, reference may be had to the accompanying drawing in which:

Fig. 1 is a vertical sectional elevation through a portion of a bore hole in which an instrument is disposed for carrying out the method of the invention; [and,]

Fig. 2 is an experimental curve showing variation in cognting rate with changes in salt concentration[.] on

Fig. 3 is a vertical sectional elevation through a portion of a bore hole having suspended therein an instrument for carrying out a further embodiment of the method of the invention.

Referring to the drawing, a bore hole is shown as traversing several sub-surface earth formations such as those indicated at 12, 14 and 16. The bore hole may or may not be provided with a steel casing since it is well known that the neutrons and gamma rays will pass through the conventional casing with but very little loss. Shown within the bore hole is a logging instrument or tool indicated generally by the housing 18. This instrument is suspended from a conductor cable 20 which passes over a suitable cable measuring reel or drum 22 and then to an amplifier 24.

Shown within and preferably in the lower portion of the instrument 18 is a source 26 of neutrons such as a mixture of radium and beryllium. Also within the instrument housing and above the source 26 is a scintillation detector [scintillometer] indicated generally by the arrow 28 and comprising essentialy a luminophor 30 and a photo-multiplier tube 32 which may be of the 5819 type having a cathode 34. The output of the tube 32 passes through a suitable preamplifier 36, the output of which is led to the lower end of the cable 20.

The luminophor 30 and the photo-multiplier tube 32 of the scintillation detector [scintillometer] areshown as enclosed in a receptacle 38 which may comprise a Dewar flask. If desired, the receptacle 38 may be packed with ice or some other cooling agent at the surface before the instrument is run into the hole so as to maintain the temperature of the tube 32 below 75 C. above which the sensitive surfaces of the photo-multiplier tube would be ruined. It is understood that any other suitable means can be used for refrigerating the scintillation detector [scintillometer] when it is to be used in a bore hole hav in; temperatures higher than 75 C.

Between the source 26 and the luminophor 30 is a 4 shield member 40, the purpose of which is to absorb a large portion of the direct radiation from the source 26 which otherwise would strike the luminophor and cause spurious indications. As has been stated hereinabove, when the logging instrument is used for making a chlorine log, the discriminator bias can be set quite high and the distance between the source 26 and the scintillation detector [scintillometer] can be as little as 4 or 5 inches. Neutrons such as the one indicated by the dotted line 42 pass outwardly into the surrounding formations wherein gamma rays may be induced, some of these such as the one indicated at 44 passing back into the bore hole to penetrate the luminophor 30. The luminophor scintillates upon gamma ray bombardment with the resulting production of photons, and the nature of the substance is such that the number of photons produced in unit time is a function of the product of the number of impinging gammas and their energies. The photons are converted into electrons by a photo-sensitive device such as the photo-multiplier 32 and each detected gamma ray produces an electron pulse, the amplitude of which is a function of the energy of that gamma ray. It is understood that the luminophor 30 will be of such size that the electrons produced therein dissipate all their energy in the luminophor, which then gives off a certain portion of this energy as photons which are in turn detected by the photo-multiplier tube.

Shown surrounding the luminophor 30 is a short cylinder 46 of a substance such as cadmium capable of ab sorbing thermal neutrons which might otherwise strike the luminophor. Within the cylindrical shield 46 is another cylindrical shield 48, preferably of lead for absorbing at least a major portion of the gamma rays which might be induced in the cadmium 46 due to the impingement of thermal neutrons thereon. If desired, the two

shields

46 and 48 can be replaced by a single cylindrical shield of boron since very little gamma radiation is produced on capture of thermal neutrons in boron and the 4 m. e. v. alpha particles are easily stopped before they reach the luminophor.

It has been found that for the luminophor 30 a crystal of sodium iodide, thallium activated is very satisfactory. Other luminophors which can be used are crystals of thallium activated potassium iodide, calcium tungstate and cadmium tungstate, and as is well-known, these crystals are now available in sutficiently large sizes to operate successfully in a proportional scintillation detector [scinti1 lometer].

At the surface, the output from the amplifier 24 passes to a suitable discriminator 50. The discriminator 50 is in efiect an adjustable filter which passes only pulses of a predetermined amplitude to the recorder 52 in which the pulses are recorded as traces against time. It is also desired that all of the pulses, regardless of their size, be simultaneously recorded in log form and to this end these pulses are led to a second recorder 54. It will be apparent that the log or record obtained at the recorder 54 will comprise more or less what is known as a neutron-gamma ray or induced gamma ray log.

In operation, and assuming that a chlorine log is to be made, the instrument 18 will be either lowered or raised past the formations surrounding the bore hole 10. Neutrons from the source 26 cause gamma rays to be induced in the formations and some of these gamma rays strike or penetrate the crystal 30. The photons produced in the crystal strike the cathode 34 of the photo-multiplier 32 producing electrical pulses which are preamplified at 36 and conducted upwardly over the cable 20 to the surface. These pulses are preferably again amplified at 24 and pass to the discriminator 50, the bias of which is adjusted to a fairly high value, say volts. The smaller pulses, for example, those due to thermal noise in the tube 32 and the pulses produced by the 2.2 m. e. v. gamma rays induced in hydrogen containing formations are blocked out in the discriminator and the pulses of larger. amplitude such as those: producedv aby- 8 01*9 m: e. v: gamma r'ays from' chlorine are recorded in log format the deviceSZ; At the same time,.aconespondingrecord is made bythe device' -54 but in this case all of the pulses, both large-andsmall;arerecorded, the discriminator fbr' recorder 54""preferably being set to re move: the small pulses caused by thermal noise. Assum ing that theinstrurn'ent' 18 is being pulled-upwardly past the formations 1'6 and- 14-if-'the counting rate of the large pulses at*2is small; an indication will-be had to theeffect that the formation Iii-contains little if any chlorine"-or"saltwa'ter. If the counting rate recorded-at 54 is'fairly low, this would' indicateithat formation 16 contains hydrogen which in all probability'willbe in the formof 051.. Ini'movinginto=the"forrnation' 14 if the counting .rateat 1 54 remains" aliout the? same. as before but 'the. counting rate of the large pulses at 52. increases, an :indication will-be :hadthat the-formation 14- contains chlorine or? salt water; It is: believed. obvious thatthe interface. between: salt: water and. oiliwithin a single formation-can be determined and locatedxin the same manner.

Figure 2- isea curve which is the. result of experiments mentioned hereinaboveand'shows that as the salt concentration is: increased up i to .from- 100. to. 150 grams per liter, the counting rate-ofithe large pulses increases very materially.

Figure 3 shows an instrument 18', generally like the instrument 18 of: Figure 1; but which further includes a second detector together with" means for conducting its output to a recorder at the'surfa'ce The instrument 18' provides means for conducting a simultaneous chlorine andih'ydrogen log in accordance with the presentinvention. The elements shown in this-figure corresponding to those of Figure I'are identified with similar reference numbers. In the instrument 18' Off Figure 3, the receptacle 38 corresponds to the receptacle 38 of Figure 1, except" that it contains two' separate detectors 2 8" and 28a. Asia thecase of the'instrument18 of Figurel, the detectof28, togetherwith the associated equi ment for transmitting its signal to the recorder 52, is employed to detect chlorine. The detector 28a, which is positioned further from the source 26 than the detector 28, includes a luminophor 30a and a photomultiplier 32a and is em played to detect hydrogen. The output of the detector 28a is coupled through a preamplifier 36a to an amplifier 24a, a discriminator 50a and a recorder 52a. The signals from the respective detectors Z8 and 28a may be transmitted to the surface equipment, either through the use of a multi-conductor cable or through other known signal transmission techniques.

It may be mentioned that the probability of slow neutron capture is about 100 times as great, atom for atom, in chlorine as in hydrogen. The rate of production of these neutron-capture gamma rays remains essentially constant in a mixture of neutron-capturing atoms. The addition of 1000 chlorine atoms to 100,000 hydrogen atoms causes enough additional absorption that the neutron population is reduced and the net result is that the rate of production of capture gamma rays stays about the same. These 1000 chlorine atoms, however, having 100 times the capturing power produce as many capture gamma rays as the 100,000 hydrogen atoms. Thus, a mixture of one chlorine atom and 100 hydrogen atoms gives off equal numbers of chlorine-capture gammas and hydrogen-capture-gamma rays. For these reasons, it is possible to detect quite small brine concentrations by using a neutron-gamma ray scintillation detector proportionally and observing or recording the high energy gamma rays. I

While the invention has been described as having particular application to the location of chlorine-containing formations, it is to be understood that considerable other information can be obtained by carrying out this method. Thus, the presence of shales would also be indicated by an; increased large pulse counting rate and these readings indicatingthighporosity; on the standard or conventional neutron-gamma my log: could then. be dismissed more authoritatively-on this-sbasisathan byt depending-.oma-natm ralt gamma. ray; lo'gc'to 'pickhoutth'e shales; as has been donein the-pasts Likewise thisrmethod can also'be used to. locatecertain meta-ls,particularly where. they occur in: a quartz matrix; For. example, manganese emits a 5 3 or. 7,2vm. e; v. gammalrays superimposed on-a continuous-backgroundon capture of=thermal neutronst If. desired, a differential pulse= discriminator can be utilized 'which aecepts=only-. the pulses :which. lie ina-com parativelyvsmall amplitude range. This can be-very'useful whentdealing-with .gamnra rays of sufficient energy to largely: produce electronv pairs on. inter-'actionwith the detector; Under these: conditions, the pulses :will have i a definitesize corresponding to a certain discrete gamma energy; By;- using anrinstrument': such as 'a' multichannel differential pulse discriminator, it is possible that one could differentiate between some of the gammasrthat are induced in nuclei such as. iron;,c-hlori'ne and E silicon.

Although the scintillatiomdetector [scintillometer] has been described as having a sodiumriodide-crystal activated. with thallium improvements arebeing made which indicate that-.th'e liquidiluminophors may also be used successfully in the-rnethod described. Likewise, luminophors-may be". used of the: typedescribed hi the U: S. Letters Patent Now 2,5 59,219; granted? July 3, 1951,. to C. G. Ludeman.

Obviously, .many. modifications. and variations of the invention ,as hereinbefore. setforthmay. be. made without'departing from the spirit=and scope thereof and, therefore, only such limitations should be imposed as are indicated'in the appended claims.

ll The methodflofexamininganea'rth formation traversed by a bore noteto determine whether said formationcontains: chlorine or oil which comprises bombard ing said formation withueut'ronsfrom a point within said bore hole refinance gamma rays therein, causing those inducedtgamma rays-reaching a zone inthe bore hole spaced vertically from said source by a distance of approximately 4 to 6 inches to impinge upon a body with the resultant transfer of gamma ray energy to electrons which in turn produce photons while causing substantially all of the energy of the electrons to be dissipated in said body, causing said photons by electron cascade to produce electrical pulses proportional in amplitude to the energy of the activating gamma rays, and recording those pulses having high amplitudes of the order of 8 m. e. v., the presence of any substantial number of these pulses being indicative of the presence of chlorine in said formation.

2. The method of examining an earth formation traversed by a bore hole to determine whether said formation contains a substance having a cross section for capture of thermal neutrons which is appreciably higher than the cross section of a hydrogen-containing substance which comprises bombarding said formation with neutrons from a point within the hole to induce gamma rays therein, causing those induced gamma rays reaching a zone in the bore spaced vertically from said source by a distance of approximately 4 to 6 inches to impinge upon a body with the resultant transfer of gamma ray energy to one or more electrons which in turn produce photons While causing substantially all of the energy of the electrons to be dissipated in said body, causing said photons by electron cascade to produce electrical pulses proportional in amplitude to the energy of the activating gamma rays, and recording those pulses having high amplitudes of the order of 8 m. e. v.

3. The method of examining an earth formation traversed by a bore hole to determine whether said formation contains chlorine or oil which comprises bombard ing said formation with neutrons from a point within said r -7 bore hole. to induce gamma rays therein, causing these! gamma rays reaching a zone in the bore hole spaced vertically from said source by a distance such that the resulting log will be substantially insensitive to the amount of hydrogen in the formation to impinge upon a first detector comprising a body which results in the transfer of gamma my energy to electrons which in turn produce photons while causing substantially all of the energy of the electrons to be dispersed in said body, causing said photons by electron cascade to produce electrical pulses proportional in amplitude to the energy of the impinging gamma rays, recording those pulses having amplitudes above 2.2 m. e. v. to provide a log that is indicative of the presence of chlorine in said formation, running a separate log with a separate detector and recording from said separate detector pulses indicative of the hydrogen content of said formation, thereby obtaining quantitative knowledge distinguishing between oil and salt water in said formation.

4. The method in accordance with claim 3 in which the two logs are run simultaneously.

5. The method according to claim 4 in which the spacing between said second mentioned detector and the neutron source is greater than the spacing between the first mentioned detector and the neutron source.

6. The method of examining an earth formation traversed by a bore hole to determine the quantities of hydrocarbon oil and salt water contained therein as evidenced by the quantities of hydrogen and chlorine therein which comprises the steps of passing a source of neutrons through the bore hole in order to irradiate the formation whereby gamma rays are induced therein through capture of neutrons by chlorine and hydrogen nuclei present in the formation, simultaneously passing a first scintillation detector through said bore hole having a luminophor of sufiicient mass to detect gamma rays in excess of 2.2 m. e. v., simultaneously passing a second scintillation detector through said bore hole having a luminophor of suflicient mass to detect gamma rays of 2 .2 in. e. v., said two detectors being spaced from the neutron source along the vertical axis of the bore hole with the first detector being positioned closer to the n'eutro'u source than said second detector whereby the first detector is sensitive to chlorine capture gamma rays in excess of 2.2m. e. v. and relatively insensitive to hydrogen capture rays of 2.2 m. e. v. and whereby said second detector is relatively more sensitive to hydrogen capture gamma rays of 2.2 m. e. v., employing the photon output produced by the. luminophor of said first detector to provide a first signal that is proportional to the rate-of-occurrenceof the detected gamma rays in excess of 2.2 m. e. v. and plotting said first signal against the position of the logging instrument in the bore hole to provide a first log indicative of the rate-of-occurrence of chlorine nuclei at successive positions along said formation, emplaying the photon output produced by the luminophor of said second detector to provide a second signal that is proportional to the rate of occurrence of the detected gamma rays of 2.2 m. e. v. and plotting said second signal against the position of the logging instrument in the bore hole to provide a second log indicative of the rate-of-occurrence of hydrogen nuclei at successive positions along said formation, thereby obtaining quantitative knowledge distinguishing between oil and salt water in said formation.

References Cited in the file of this patent or the original patent UNITED STATES PATENTS 2,469,460 Fearon May 10, 1949 2,508,772 P'ontecorvo May 23, 1950 Scherbotskoy Aug. 4, 1953 OTHER REFERENCES The Detection of Gamma-Rays with Thallium-Activated Sodium Iodide Crystals, Hofstadter, Physical Review, vol. 75, No. 5, March 1949, pages 796-798.