GB1593877A

GB1593877A - Methods for calibrating radioactivity well logging tools

Info

Publication number: GB1593877A
Application number: GB4387377A
Authority: GB
Original assignee: Schlumberger Ltd USA
Current assignee: Schlumberger Ltd USA
Priority date: 1976-10-28
Filing date: 1977-10-21
Publication date: 1981-07-22
Also published as: AR221832A1; MX3990E; FR2369578A1; FR2369578B1

Description

(54) METHODS FOR CALIBRATING RADIOACTIVITY WELL LOGGING TOOLS (71) We, SCHLUMBERGER LIMITED, a corporation organized and existing under the laws of the Netherlands Antilles, 277 Park Avenue, New York, New York 10017, U.S.A., 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:- The present invention relates to radioactivity well logging and is particularly directed to methods for calibrating radioactivity well logging tools. The invention is particularly useful for calibrating logging instruments that measure the natural radioactivity of earth formations.

The surveying of earth formations by detection of the natural radioactivity of said formations is presently well known and in widespread use. Such surveying is generally accomplished by moving a radiation detector through a well bore and establishing a record of the natural gamma radioactivity as a function of the borehole depth. This record can then be used to determine the interfaces between different formations, to relate formations observed from one borehole with formations in the same field that have been observed from other boreholes, and to provide depth references within a borehole.

It is also presently well known to detect the natural radioactivity of an earth formation within several appropriately selected energy windows and to combine the measured count rates in each window in order to determine the amounts of uranium, thorium and potassium present in the formation. The technique is described in U.S. Patent No.

3,976,878, issued August 24, 1976 to P.

Chevalier et al, and assigned to the assignee of the present invention.

The natural gamma radioactivity of earth formations can further be used to provide quantitative shale indications. Typically, the count rate that reflects the natural radioactivity of a formation is normalized to a standard natural gamma radiation count rate observed in pure shale. The normalized signal is then considered as reflecting the shale concentration. Such a technique is disclosed in U.S. Patent No. 3,786,267 to O. Y. Liu et al., issued January 15, 1974 and assigned to the assignee of the present invention.

The use of natural radioactivity measurements to compare radioactive levels between wells, to determine the concentrations of uranium, thorium and potassium and to provide quantitative shale measurements requires that the instruments for performing such measurements be accurately calibrated.

Before being sent to the field, a natural radioactivity well logging instrument may be calibrated in a pit which simulates an earth formation and is made of concrete or cement blocks containing known amounts of uranium, thorium and potassium. The radioactive zone is thick enough to appear infinite to gamma ray detectors. Typically, the thickness of the pit wall is set at two feet.

Field calibration of the natural radioactivity logging tools is generally accomplished by placing a standard gamma ray source (typically of radium 226) at a standard distance (typically 53 inches) from the detector and adjusting the gain of the system so as to obtain a predetermined reading on a measuring apparatus. Unfortunately, the accuracy of this calibration technique is strongly affected not only by the asymmetry of the detector but also, and chiefly, by a scattering effect due to the tool itself and to its surroundings (drill pipes, catwalks, etc.) that also receive gamma rays which may be redirected toward the detector.As a consequence of these effects, the calibration errors may reach 10/00. Consequently, although natural gamma ray logging has been used by the oil industry for more than thirty years, natural radioactivity logs have never been in widespread use for quantitative measurements. A need exists, therefore, for a more accurate field calibration of the natural radioactivity logging tools. It must also be mentioned that the calibration sources now in use are relatively strong (10011Ci) and can therefore be health hazards.

The calibration pit described hereinbefore avoids both the undesired scattering effect from nearby objects and the errors due to the asymmetry of the detector, but it is obviously not portable and therefore cannot be used as a field calibrator. It must be added that the purpose of a calibration pit is to simulate an earth formation, whereas the purpose of a particular field calibrator is to produce a specified count rate in the tool in which it was designed to check that the detection system is performing satisfactorily.

It is therefore an object of the present invention to provide a method for facilitating calibration in the field of a natural radioactivity logging tool or, more generally, any radioactivity logging tool having a radiation detector.

According to one aspect of the present invention there is provided a method for fieldcalibrating a radioactivity logging instrument having a radiation detector and means connected to said detector for producing a reading of the amount of detected radiation, said method comprising: positioning a portable calibration member in sleeve-like disposition around said detector, said member having a radioactive material distributed therein; and adjusting said producing means to give said reading a predetermined value.

With this technique, the radioactive source is brought very close to the detector and surrounds it completely, so the scattering effect from nearby objects and the errors due to the asymmetry of the detector are avoided. An accurate field calibration of the tool can thus be performed. Furthermore, the source can be made considerably weaker than the calibration sources now in use, which are at a relatively long distance from the detector. Health hazards are there- fore reduced.

Methods for field-calibrating a radioactivity logging instrument in accordance with this invention will now be described, by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of a natural radio-activity well logging tool having a calibrator positioned around its detector; Figs. 2a and 2b show a first version of the calibrator; Figs. 3a and 3b show a second version of the calibrator; Figs. 4a and 4b show a third version of the calibrator; and Figs. 5a and 5b show a fourth version of the calibrator, Fig. 1 showsalogging tool 10formeasuring the natural radioactivity of earth formations, comprising an elongate fluid-and pressuretight housing 12.Within the housing 12, a scintillation detector 14, that includes a scintillator crystal 16 and a photomultiplier 18, responds to the natural gamma radioactivity of the adjacent formation. Other radiation detectors, such as a solid state germanium, cadmium telluride, or mercuric iodide detector, also may be used.

The output pulses of the photomultiplier 18 are first amplified in an amplifier 20 before being applied to a pulse height discriminator circuit 22 that passes only pulses above a selected amplitude in order to eliminate most of the spurious signals caused, for example, by "dark current" within the photomultiplier 18. These pulses are then prepared in a pulse output circuit 24 for transmission, through a conductor 26 in an armoured cable 28, to surface equipment 30.

In the surface equipment 30, the incoming pulses are received by an amplifier and discriminator circuit 32 and applied to a count rate meter 34 that converts the received pulses into an output signal corresponding to the natural gamma radio-activity registered at the detector 14. This signal is applied simultaneously to a meter 36 and a trace recorder 38.

During field operations, the abovedescribed logging tool has to be periodically calibrated at the well site by adjusting the gain of the detection system so as to keep a constant count rate reading on the meter 36 when a calibrator containing a standard gamma ray source is placed at a reference position with respect to the detector 14.

The gain adjustment can be performed by adjusting with a gain control 39 either the high voltage source of the photomultiplier 18 or the amplifier 20 or, as shown in Fig.

1, the discriminator 22.

As can be seen from Fig. 1, the calibrator is in the form of a sleeve 40 that has a radioactive material distributed therein and envelops the portion of the housing 12 containing the detector 14. The sleeve extends above and below the scintillator 16 and has an internal diameter which is substantially the same as the housing diameter (typically 351, inches). If L represents the length of the scintillator (typically 12 inches) the sleeve length is preferably between 1 SL and 2L.

Figs. 2a-b, 3a-b and 4a-b illustrate three versions of the calibrator. In each case the sleeve 40 is made of metal in two semicylindrical parts which are conveniently hinged to allow positioning around the tool.

Hinges 42 and fasteners 44 have been therefore schematically represented in the figures.

As can be seen from Figs. 2a and 2b, the radioactive material is in the form of several wires 46 which extend along generatrices of the sleeve 40. The wires 46 can be introduced into predrilled axial holes, but as the drilling of such holes in the sleeve wall may not be very easy, each half-sleeve is preferably made of two concentric half-tubes 48 and 50, the internal tube having grooves 52 which are distributed circumferentially about its outer surface and are slightly shorter than the sleeve. The radioactive wires 46 are thus placed inside the grooves 52 before the two half-tubes are welded or otherwise fastened together. The wires 46 are preferably embedded in a sheath of epoxy 54 inside the grooves. This sheath protects the wires against oxidation. Alternatively, the source material could be placed inside the grooves 52 in the form of a powder dispersed within epoxy.

In another version of the calibrator represented in Figs. 3a and 3b, the radioactive material is in the form of circular wires 56 which are now distributed longitudinally inside the sleeve 40. Each half-sleeve is also made up of two concentric half-tubes 58 and 60. The internal tube has circular grooves 62 that contain the radioactive wires 56, preferably embedded in a protective sheath of epoxy 64. Alternatively, the source material can be placed inside the grooves 62 in the form of a powder dispersed within epoxy.

In a third version of the calibrator represented in Figs. 4a and 4b, the source material 66 is uniformly distributed inside the sleeve 40. In this case, each half-sleeve is made of two concentric half-tubes 68 and 70, which are welded together. The internal tube has a hollow 72 which is slightly shorter than the sleeve and contains the source material 66 in the form of either a powder dispersed within a suitable medium, such as epoxy, or a chemical dissolved in a paint. Alternatively, the source material can be deposited electrolytically in the hollow 72.

The amount of radioactive material introduced into the sleeve is determined, in each case, so as to produce the desired count rate reading on the meter 36 when the tool is functioning properly.

As the calibrator is close to the detector, a very small amount of radioactive material is needed. As an example, an 18-inch long sleeve containing six wires of uranium 238 (as depleted uranium), each with a 0 01 inch diameter and a strength of lyCi will provide approximately the same reading as a l00Ci pill of the same material used at 53 inches from the detector. There is practically no health hazard from a luCi source.

In operation the sleeve 40 is simply clamped around the housing 12 and the gain control 39 is adjusted so as to obtain the desired reading on the meter 36. Marks 74 can be advantageously provided on the housing to show the exact position of the sleeve around the detector. Since the calibrator is as close as possible to the detector and surrounds it completely, the scattering effects from nearby objects and the errors due to the asymmetry of the detector are mitigated. An accurate calibration of the tool can thus be performed.

The calibrator may have a slight screening effect on the background radiation that is normally received by the detector. Although the percentage of background screened by the sleeve is generally very low (2%), it may be suitable to measure the screening effect.

For that purpose, a "passive" sleeve, exactly identical to the radioactive sleeve but containing no source, can be used.

The sleeve, which is advantageously made of steel or aluminium, must be thin enough to have a relatively low weight that makes it convenient to manipulate, but thick enough not to be easily damaged. Typically, though not necessarily, the sleeve thickness is between 1/20 and 1/10 of its internal diameter for materials such as steel or aluminium.

The radioactive material distributed inside the sleeve has preferably an energy spectrum that approximates the energy spectrum of the formations which will be investigated by the tool. Uranium 238 constitutes a most appropriate source material, as its half-life is 105 years and its radiation spectrum is similar to that seen by the detector in rocks. Howver, several other source materials can be used, such as radium 226, cobalt 60, cesium 137 or any radioactive substance emitting in the same range as the formations of interest.

In a fourth version of the calibrator, the sleeve 40 is formed by a flexible rectangular flat pad which is wrapped around the housing 12. The pad is of sufficient size so as to completely encircle the tool and extend for about one tool diameter above and one tool diameter below the scintillator 16. As an example, for a housing diameter of 35/8 inches and a 1 0-inch long scintillator, the pad will be 11 3/8 inches in width and 18 inches in length. Referring to Figs. 5a and 5b that represent this pad, the radioactive substance is dispersed within a tough, flexible sheet 142 of resilient material, such as an elastomer. Rubber is suitable, but polyurethane is more appropriate because of its easy fabrication and toughness.The sheet 142 is advantageously placed within a flexible protective sheath 144 made of nylon, for example. Straps 146 attached to the sheath 144 are used, with the help of press-studs 148, to hold the calibrator wrapped about the housing 12. Any other appropriate fastening means could also be used.

The radioactive substance dispersed within the flexible sheet 142 is preferably carnotite, whose chemical formula is K(UO2)2 (VO4)2, n H2O. However, several other source materials, such as uranium 238, radium 226, cobalt 60 or cesium 137, can also be used.

The amount of radioactive substance dispersed within the pad is determined, in each case, so as to produce the desired count rate reading on the meter 36. It must be mentioned that, as the calibrator is very close to the detector, a very small amount of radioactive substance is needed. As an example, an 18-inch long sleeve containing 3.5 grams of carnotite and having a strength of 0 2311Ci will provide approximately the same reading as a 100 IlCi pill of radium 226 located at 53 inches from the detector.

There is practically no health hazard from this amount of carnotite.

In operation, the flexible radioactive pad 40 is simply wrapped around the portion of the housing 12 containing the scintillator 16, and the gain control 39 is adjusted to obtain the desired reading on the meter 36. Marks 74 can be advantageously provided on the housing to show the exact position of the pad around the detector. Since the calibrator is as close as possible to the detector and surrounds it completely, the scattering effects from nearby objects and the errors due to the asymmetry of the detector are avoided.

An accurate calibration of the tool can thus be performed.

With regard to the versions shown in Figures 2 and 3, instead of being distributed axially or circularly, the radio-active material could be distributed both axially and circularly, thus forming a grid inside the sleeve, or a spiral or a matrix of dots or points.

WHAT WE CLAIM IS: 1. A method for field-calibrating a radioactivity logging instrument having a radiation detector and means connected to said detector for producing a reading of the amount of detected radiation, said method comprising: positioning a portable calibration member in sleeve-like disposition around said detector, said member having a radio-active material distributed therein; and adjusting said producing means to give said reading a predetermined value.

2. The method of claim 1, wherein the length of said member is about 1-5 to 2 times the length of the radiation detector.

3. The method of claim 1 or claim 2, wherein the member is a cylindrical sleeve made of metallic material.

4. The method of claim 3, wherein the internal diameter of said sleeve is substantially the same as the diameter of a housing enclosing the radiation detector.

5. The method of claim 3 or claim 4, wherein the thickness of said sleeve is about 1/20 to 1/10 of its internal diameter.

6. The method of claim 3, claim 4 or claim 5, wherein said sleeve is made of two hinge-connected substantially semicylindrical parts to allow clamping of the parts about the instrument.

7. The method of any one of claims 3 to 6, wherein said radioactive material is in the form of wires.

8. The method of claim 7 wherein said wires extend along generatrices of the sleeve.

9. The method of claim 7 wherein said wires extend along circumferences of the sleeve.

10. The method of claim 7 wherein said sleeve is made of two concentric tubes, one of which comprises grooves for containing the wires of radioactive material.

11. The method of any one of claims 7 to 10 wherein said wires are embedded in a protective substance.

12. The method of any one of claims 3 to 6, wherein radioactive material is uniformly distributed inside the sleeve.

13. The method of claim 12 wherein said radioactive material is in the form of a powder dispersed within a support medium.

14. The method of claim 12 wherein said radioactive material is in the form of a chemical product dissolved in a paint.

15. The method of claim 12 wherein said radioactive material is in the form of an electrolytic deposit.

16. The method of claim 12 wherein said sleeve is made of two concentric tubes, the uniformly distributed radioactive material being disposed between said tubes.

17. The method of any one of the preceding claims, wherein said radioactive material has an energy distribution that approximates the energy distribution of the radiation emitted by an earth formation.

18. The method of any one of the preceding claims, wherein said radioactive material is selected from the group of uranium 238, radium 226, cobalt 60 and cesium 137.

19. The method of any one of claims 3 to 16, wherein said sleeve is made of aluminium.

20. The method of any one of claims 3 to 16, wherein said sleeve is made of steel.

21. The method of claim 1 or claim 2, wherein said member is made of a flexible sheet.

22. The method of claim 21, including means for holding said sheet wrapped around the instrument.

23. The method of claim 21 of claim 22, wherein said sheet is made of a resilient material.

24. The method of claim 23, wherein said sheet is made of polyurethane.

25. The method of any one of claims 21 to 24 wherein said radioactive substance is carnotite.

26. The method of any one claims of 21 to 25 wherein said sheet is covered with a flexible sheath.

27. A method of claim 26 wherein said sheath is made of nylon.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. dispersed within the pad is determined, in each case, so as to produce the desired count rate reading on the meter 36. It must be mentioned that, as the calibrator is very close to the detector, a very small amount of radioactive substance is needed. As an example, an 18-inch long sleeve containing 3.5 grams of carnotite and having a strength of 0 2311Ci will provide approximately the same reading as a 100 IlCi pill of radium 226 located at 53 inches from the detector. There is practically no health hazard from this amount of carnotite. In operation, the flexible radioactive pad 40 is simply wrapped around the portion of the housing 12 containing the scintillator 16, and the gain control 39 is adjusted to obtain the desired reading on the meter 36. Marks 74 can be advantageously provided on the housing to show the exact position of the pad around the detector. Since the calibrator is as close as possible to the detector and surrounds it completely, the scattering effects from nearby objects and the errors due to the asymmetry of the detector are avoided. An accurate calibration of the tool can thus be performed. With regard to the versions shown in Figures 2 and 3, instead of being distributed axially or circularly, the radio-active material could be distributed both axially and circularly, thus forming a grid inside the sleeve, or a spiral or a matrix of dots or points. WHAT WE CLAIM IS:

1. A method for field-calibrating a radioactivity logging instrument having a radiation detector and means connected to said detector for producing a reading of the amount of detected radiation, said method comprising: positioning a portable calibration member in sleeve-like disposition around said detector, said member having a radio-active material distributed therein; and adjusting said producing means to give said reading a predetermined value.

24. The method of claim 23, wherein said sheet is made of polyurethane.

27. A method of claim 26 wherein said sheath is made of nylon.

28. A method for field-calibrating a

radioactivity logging instrument having a radiation detector and means connected to said detector for producing a reading of the amount of detected radiation, the method being substantially as herein described with reference to Figure 1 of the accompanying drawings.

29. A portable calibrator for radioactivity logging tools having a substantially cylindrical housing adapted to be moved in a borehole and a radiation detector carried by said housing, the calibrator being substantially as herein described with reference to Figures 2a and 2b of the accompanying drawings.

30. A portable calibrator for radioactivity logging tools having a substantially cylindrical housing adapted to be moved in a borehole and a radiation detector carried by said housing, the calibrator being substantially as herein described with reference to Figures 3a and 3b of the accompanying drawings.

31. A portable calibrator for radioactivity logging tools having a substantially cylindrical housing adapted to be moved in a borehole and a radiation detector carried by said housing, the calibrator being substantially as herein described with reference to Figures 4a and 4b of the accompanying drawings.

32. A portable calibrator for radioactivity logging tools having a substantially cylindrical housing adapted to be moved in a borehole and a radiation detector carried by said housing, the calibrator being substantially as herein described with reference to Figures 5a and 5b of the accompanying drawings.