EP1605281A1 - Logging tool with a parasitic radiation shield and method of logging with such a tool - Google Patents

Abstract

It is described a logging tool 20 for underground formations surrounding a borehole 14,comprising an elongated body 21 along a major axis; a collar 22 disposed peripherally aroundsaid body 21 having a collar wall defined by an inner and an outer surface; a radiation emittingsource 201 arranged to illuminate the earth formation 16 surrounding the borehole; at least oneradiation detector 211 arranged to detect radiation reflected by the earth formation resulting fromillumination by the source 201; at least one source collimation window 202 and one detectorcollimation window 212 through which the earth formation is illuminated and radiation isdetected; and characterized in that it further comprises at least one radiation shield 30 locatedbetween said inner collar surface and the outer surface of the tool, said radiation shieldpositioned so as to eliminate parasitic radiation that has not traversed the outer collar.

Description

Field of the invention
• This invention relates to logging of oil, water or gas well in underground formationssurrounding a borehole and more particularly to a logging tool with a parasitic radiation shieldsuch as a logging-while-drilling gamma ray density measurement tool.
Description of the Prior Art
• In hydrocarbon exploration and production, it is of prime importance to determine if agiven earth formation contains hydrocarbon, and the amount of hydrocarbon within theformation. Therefore, formation properties while drilling or in a freshly drilled hole are measuredto predict the presence of oil, gas and water in the formation. These formation properties may belogged with wireline tools, logging while drilling (LWD) tools, or measurement while drilling(MWD) tools. One method to predict formation properties is to measure the density of materialin earth formation using a source of nuclear radiation and a radiation detector. The density of amaterial can be determined either by a transmission or by a scattering measurement. In atransmission measurement the material, the density of which needs to be determined, is putbetween the radiation source and the detector. In a scattering measurement the intensity andenergy distribution of the radiation scattered back to a detector from the material underinvestigation is used to determine the density. Downhole measurements of formation density areof the scattering type since it is not usually possible to insert the formation material directlybetween source and detector, with the possible exception of rock samples removed from theformation.
• Gamma-ray scattering systems have been used for many years to measure the densityof a material penetrated by a borehole. Typically density is measured as a function of positionalong the borehole thereby yielding a "log" as a function of depth within the borehole. Themeasuring tool typically comprises a source of radiation and one or more radiation detectors,which are in the same plane as the source and typically, mounted within a pressure tightcontainer. Radiation impinges on and interacts with the material, and a fraction of the impinging radiation is scattered by the material and a fraction thereof will return to the detector. Afterappropriate system calibration, the intensity of the detected scattered radiation can be related tothe bulk density of the material.
• The radial sensitivity of the density measuring system is affected by several factorssuch as the energy of the gamma radiation emitted by the source, the axial spacing between thesource and the one or more gamma ray detectors, and the properties of the borehole and theformation. The formation in the immediate vicinity of the borehole is usually perturbed by thedrilling process, and more specifically by drilling fluid that invades the formation in the nearborehole region. Furthermore, particulates from the drilling fluid tend to buildup on the boreholewall. This buildup is commonly referred to as mudcake, and adversely affects the response of thesystem. In this way, intervening material between the tool and the borehole wall will adverselyaffect the tool response. Intervening material in the tool itself between the active elements of thetool and the outer radial surface of the tool will again adversely affect the tool response byproducing a background of scattered radiation which is independent of the presence of theborehole fluid, the mudcake or the formation. Typical sources are isotropic in that radiation isemitted with essentially radial symmetry. Flux per unit area decreases as the inverse square ofthe distance to the source. Radiation per unit area scattered by the formation and back intodetectors within the tool also decreases with increasing distance, but not necessarily as theinverse square of the distance. In order to maximize the statistical precision of the measurement,it is desirable to dispose the source and the detector as near as practical to the borehole environs,while still maintaining adequate shielding and collimation.
• Prior art logging-while-drilling systems use a variety of source and detectorgeometries to minimize standoff, such as placing a gamma ray source and one or more gammaray detectors outside the tool body within a drill collar with a stabilizer disposed between sourceand detectors and the borehole and formation; or within stabilizer fins that radiate outward froma drill collar. This tends to minimize intervening material within the tool, and positions sourceand detectors near the borehole environs, but often at the expense of decreasing the efficiency ofshielding and collimation. The signal-to-noise ratio is often degraded by the detection of particlesthat have not probed the earth formation but instead have traveled trough low-density regions orvoids existing in the tool between source and detectors, and especially through collar andstabilizer.
• Shielding of source and detectors mounted in the tool body is well known in the priorart; chassis is shielded and detectors are mounted in a shielded holder with windows troughwhich radiation is detected. Other prior art patents focus on total radiation shielding of the tool tothe detriment of functionality: EP 0160351 describes a shielded tool casing with windows, whichreceives instrument package, US 6,666,285 describes an apparatus, which has a cavity to receivea solid shielded instrument package. Those apparatus, because they use a framework totallymade of high-density materials, are heavy and brittle, and in harsh drilling conditions, can bebroken resulting in the destruction and possibly the loss of the instrument package and morecritically the loss of the radioactive source. The problem of providing shielding in the collar andthe stabilizer has not been yet addressed successfully.
Summary of the Invention
• The present invention discloses a logging tool for underground formationssurrounding a borehole, comprising: an elongated body along a major axis; a collar disposedperipherally around said body having a collar wall defined by an inner and an outer surface.Further, the tool comprises a radiation emitting source arranged to illuminate the earth formationsurrounding the borehole; at least one radiation detector arranged to detect radiation reflected bythe earth formation resulting from illumination by the source; at least one source collimationwindow and one detector collimation window through which the earth formation is illuminatedand radiation is detected; and at least one radiation shield located between said inner collarsurface and the outer surface of the tool, said radiation shield positioned so as to eliminateparasitic radiation that has not traversed the outer collar.
• In a preferred embodiment, the tool further comprises a stabilizer located at theperiphery around the outer collar surface, wherein this stabilizer comprises a stabilizer walldefined by an inner stabilizer surface and an outer stabilizer surface, and wherein the radiationshield is located between this inner collar surface and this outer stabilizer surface. The stabilizerenhances the contact between the tool and the formation by reducing the space available for mudbetween the tool and the formation.
• The tool is designed so that the source and the detector are as near as practical to theborehole environs. The radiation shields increase the signal to noise ratio. And the inventionbelow proposes a robust, secure and functional configuration.
• In a preferred embodiment, the radiation shield is located between the emittingradiation source and the radiation detector and has a length along the axis, which is less than80% of the distance between the source and the detector. The radiation shield has a thickness inthe cross section perpendicular to the major axis, which is preferably less than 40% of the widthof the tool at the position of the radiation source. This makes it possible to eliminate a significantfraction of the radiation that are coming from source and that have not passed through theborehole fluid and the formation, but whose path was entirely inside the collar and the stabilizer.
• In a preferred embodiment, the radiation shield has an annular shape surrounding thedetector window and has a length along the axis, which is less than 40% of the distance betweenthe source and the detector. In a preferred embodiment, the radiation shield has a thickness in thecross section perpendicular to the major axis, which is less than 40% of the width of the tool atthe position of emitting radiation source. This enables eliminating a part of the radiations passingthrough the collar to the detecting window and not through the window in the collar to thedetector window.
• In a preferred embodiment, this invention is directed toward a radiation densitymeasurement system in underground formations surrounding a borehole with a chemicalradioactive source or an electronic radiation source emitting x-ray; or a chemical or electronicneutron source.
• In a preferred embodiment, this invention is directed toward a gamma-ray logging-while-drillingdensity tool. The system comprises a source of gamma radiation and one or moregamma ray detectors. Multiple detectors (2 or more) provide better efficiency and allowcompensation for the effect of mud and mudcake intervening between the tool and the formation.It is clear, however, that the basic concepts of the invention could be employed in other typesand classes of logging, logging-while-drilling or measurement-while-drilling systems. As anexample, the invention can be used in a neutron porosity system for measuring formationporosity, wherein the sensor comprises a neutron source and one or more neutron detectors.
• The gamma-ray radiation shield is fabricated from a high atomic number material,commonly referred to as "high Z" material. High Z material is an efficient attenuator of gamma-rayradiation, and permits the efficient shielding, collimation and optimum disposition of thesource and detectors with respect to the borehole environs.
• The present invention also discloses a method for logging a well utilizing a tool asmentioned above.
Brief description of the drawings
• Further embodiments of the present invention can be understood with the appendeddrawings:
• Figure 1 illustrates a logging-while-drilling tool according to the invention.
• Figure 2a is a side view on the major axis of the tool of figure 1 with the radiationshield localized between source and first detector.
• Figure 2b is a side view on the major axis of the tool of figure 1 with the radiationshield localized closed to first detector.
• Figure 2c is a side view on the major axis of the tool of figure 1 with both radiationshields.
• Figure 3 shows pulse-height spectra obtained by numerical modeling of the logging-while-drillingtool of figure 2a and 2c as well as a case in which neither of theshields30 and 31 is present.
Detailed description
• Figure 1 illustrates a logging-while-drilling tool, identified as a whole by thenumeral20, disposed by means of a drill string within awell borehole 18 defined by aborehole wall 14and penetrating anearth formation 16. The upper end of thecollar element 22 of thetool 20 isoperationally attached to the lower end of a string ofdrill pipe 28. The stabilizer element of thetool 20 is identified by the numeral 24. Adrill bit 26 terminates the lower end oflogging tool 20.It should be understood, however, that other elements can be disposed on either end of thetool 20 between thedrill pipe 28 and thedrill bit 26. The upper end of thedrill pipe 28 terminates at arotary drilling rig 10 at the surface of theearth 12. The drilling rig rotates thedrill pipe 28 andcooperatingtool 20 anddrill bit 26 thereby advancing theborehole 18. Drilling mud is circulateddown thedrill pipe 28, through the axial passage in thecollar 22, and exits at thedrill bit 26 forreturn to thesurface 12 via the annulus defined by the outer surface of the drill string and theborehole wall 14.
• Figures 2a, 2b and 2c illustrate conceptually radiation shields on thetool 20 of figure1 shown in side view on the major axis of the tool. In a first embodiment, the tool is a logging-while-drillinggamma-ray scattering tool with a chemical radioactive source. Thetool 20 is madeof anelongated tool body 21 and adrill collar 22 disposed peripherally around thetool body 21.In the illustrated tool, astabilizer 24 is disposed peripherally around thedrill collar 22; thestabilizer is optional and reduces the amount of mud between the tool and the formation wall andtherefore the influence of the borehole fluid on the measurement. Thetool 20 receives onesourcecollimation window 202 through which theearth formation 16 is illuminated by the radiationemitted from the radioactive source, and twodetector collimation windows 212 and 222 throughwhich the radiation coming from the outside of thetool 20 is detected. In the illustrated tool, asource ofgamma radiation 201 illuminating theearth formation 16 and affixed to asource holder200, is mounted in thecollar wall 22. Though this is the preferred way, other locations for thesource 201 are in thetool body 21 or in thestabilizer 24. Thesource 201 is preferably cesium-137(¹³⁷Cs) which emits gamma radiation with an energy of 0.66 million electron volts (MeV).Alternately, cobalt-60 (⁶⁰Co) emitting gamma radiation at 1.11 and 1.33 MeV can be used assource material. Thetool 20 receives a first or "short spaced"gamma ray detector 211 disposedat a first axial distance from thesource 201, and a second or "long spaced"gamma ray detector212 disposed at a second axial distance from the source, where the second spacing is greater thanthe first spacing. In the illustrated tool, the detectors are mounted in thetool body 21 in holders:210 for the first detector and 220 for the second detector. Though this is the preferred way, otherlocations for thedetectors 211, 221 are in thecollar wall 22 or in thestabilizer 24. The detectorsare preferably scintillation type such as sodium iodide (NaI) or Gadolinium-oxy-ortho-silicate(GSO) to maximize detector efficiency for a given detector size.
• Insertion of high-density materials in the collar is often undesirable since the collarsupports the stresses inherent to logging conditions, in figure 2a, a side view of the tool illustrates aradiation shield 30 located in thecollar 22 whose shape is optimized to reduceleakage through the collar without affecting its mechanical strength.
• The trajectories of gamma rays traveling from the source to the detector are likebroken lines, on which each break corresponds to a collision with an electron within thesurrounding material. Gamma radiations lose energy by means of the most pertinent reactionhere: Compton scatter reaction. After undergoing one or more Compton scattering events, asmall fraction of the emitted with reduced gamma-ray energy returns to the tool and is detectedby the gamma radiation detector. The function of theradiation shield 30 is to intercept andattenuate by photoelectric absorption or by Compton scattering and subsequent photoelectricabsorption, a significant fraction of those gamma rays that travel through the collar or/andstabilizer and that might otherwise go back to the detector after being scattered in the collaror/and stabilizer.
• Figure 2b illustrates a side view of the tool with aradiation shield 31 located on theinner collar surface in thecollimation window 212 of thefirst detector 211. The function of theradiation shield 31 is to intercept and attenuate gamma rays traversing the collar to the detectingwindow.
• Figure 2c illustrates a side view of the tool with both radiation shields 30 and 31.
• To estimate the amount of gamma ray leakage that is effectively removed by theradiation shields, a Monte-Carlo N-Particle model is built based on the tool plan of figures 2. Acompromise is found between the effective shielding and the mechanical strength of the tool.The model of source used is a mono-energetic 0.662 million electron volts (MeV) cesium-137radiation. Pulse-height spectra for energies between 0.1 and 0.5 MeV for the first NaI detectorare computed for three different configurations: (1) tool without extra radiation shield, (2) toolwithradiation shield 30 as in figure 2a, (3) tool withradiation shields 30 and 31 as in figure 2c.
• One or more pieces of a high-density material, i.e. a material with a high atomicnumber (more than Z=70) and a high density (more than 15 g/cc) like tungsten, gold or depleteduranium, are inserted in the collar in a particular locations where their shielding efficiency willbe maximal and their influence on the mechanical strength will be minimal. High Z materials areefficient attenuators of gamma radiation, and permit the efficient shielding, collimation andoptimum disposition of the source and detectors with respect to the borehole environs.
• Theradiation shield 30 of figure 2a is in a preferred embodiment, placed into a cavityin the outer surface of the collar, wrapped in a rubber envelope and then compressed underneatha cover plate screwed onto the collar between the source and the detector. In a preferredembodiment, better efficiency is obtained when length along the axis of this radiation shield isless than 80% of the first axial distance between source and detector; and when thickness of thisradiation shield in the cross section perpendicular to the major axis is less than 40% of the widthof the tool at the position of the source. In a second preferred embodiment, best efficiency isobtained when length along the axis of this radiation shield is less than 60% of the first axialdistance between source and detector; and when thickness of this radiation shield in the crosssection perpendicular to the major axis is less than 20% of the width of the tool at the position ofthe source. The radiation shield is disposed circumferentially around the collar outer surface, andpreferably covering less than 180° of this surface. The effectiveness of theradiation shield 30 ismaximized when its edge is brought closer to that of the collimation window of the first detector.The effectiveness is also increased when the thickness of the radiation shield is increased or anextension towards the source is made, but at the expense of a lower mechanical strength. As anexample of optimization, for a circular part of a tungsten patch, the length along the axis is 58mm whereas the first axial distance is 170 mm, and the thickness is 7 mm, and for the circularpart, the internal radius is 78 mm and the opening angle is 90°.
• Theradiation shield 30 of figure 2a can be associated with anotherradiation shield 31of figure 2b, located at the base and very close to the collimator window of the first detector, thisradiation shield 31 has an annular shape surrounding this collimator window and with atrapezoidal section. Both radiation shields in this embodiment are illustrated on figure 2c. Theefficiency is maximized with specific angular aperture of the trapezoidal section just as thedimension of the annular shield. Nevertheless, these dimensions of the annular shield are dictatedby the requirements for mechanical strength. Therefore, in a preferred embodiment, betterefficiency for theradiation shield 31 is obtained when this radiation shield is located between thefirst detector and the outer stabilizer surface facing the first detector, and when this radiationshield has an annular shape with a length along the axis or a diameter, which is less than 40% ofthe distance between source and first detector. In a second preferred embodiment, best efficiencyfor theradiation shield 31 is obtained when this radiation shield has an annular shape with alength along the axis or a diameter, which is less than 20% of the distance between source andfirst detector. In a preferred embodiment, this radiation shield has a thickness in the cross section perpendicular to the major axis, which is less than 40% of the width of the logging-while-drillingtool at the position of emitting radiation source. In a second preferred embodiment, this radiationshield has a thickness in the cross section perpendicular to the major axis, which is less than 20%of the width of the logging-while-drilling tool at the position of emitting radiation source.
• Figure 3 shows the pulse-height spectra obtained by numerical modeling of the toolwith optimized radiation shields 30 and 31 for the three configurations already described above.In order to determine the amount of gamma-radiation passing through the tool to the detectors,without interacting with the materials in the borehole or the formation, the earth formation isassumed to be very dense like tungsten (17.4 g/cm³) so that practically no gamma-rays willreturn from the formation and the signal is entirely due to gamma-rays traveling through thecollar and the stabilizer. From those data and for an energy range between 0.15 and 0.25 MeV,corresponding to the principal energy used for logging-while-drilling density measurements witha cesium-137 gamma ray source, the percentage of total gamma-ray leakage removed from thetotal signal by the radiation shields is evaluated. For a stabilizer diameter of 8¼ inches, thepercentage of gamma-ray leakage removed is of 45% with theradiation shield 30 alone and of54% with both radiation shields 30 and 31; for a stabilizer diameter of 9 3/8 inches, thispercentage is 43% and 51% respectively.
• In a second model, the earth formation is assumed to be made of an aluminum alloy(2.805 g/cm³) so gamma-rays will return in this model also from the formation. The percentageof gamma-ray leakage removed from the signal by the radiation shields is evaluated in thismodel as well and the results are comparable to those obtained with the first model. For astabilizer diameter of 8¼ inches, the percentage of gamma-ray leakage removed is 43% with theradiation shield 30 alone and of 57% with both radiation shields 30 and 31; for a stabilizerdiameter of 9 3/8 inches, this percentage is 38% and 46% respectively.
• Theradiation shield 30 removes almost 50% of gamma-ray leakage and theradiationshield 31 removes an additional 10% of gamma-ray leakage. These radiation shields 30 and 31mounted offer therefore various mechanical, operational and technical advantages.
• Radiation shields between first and second detectors or in the collimation window ofthe long spaced detector are possible; nevertheless this second detector is less sensitive togamma-ray leakage and a reduction of the leakage is less important.
• In a second embodiment, thetool 20 is a logging-while-drilling density tool with anelectronic radiation source. Thesource 201 is an x-rays generator. The shielding materials needto be inserted into the structural materials of the tool body, collar or stabilizer with the intent tooptimize shielding with a minimal impact on the structural strength of the tool. Shieldingmaterials for lower energy gamma-rays or x-rays could be lighter materials.
• In a third embodiment, thetool 20 is a logging-while-drilling neutron scattering toolwith a chemical or electronic neutron source. Thesource 201 is a chemical source, as Radium-Berylliumsource or an electronic source like pulsed neutron generator. The shielding materialsneed to be inserted into the structural materials of the tool body, collar or stabilizer with theintent to optimize shielding with a minimal impact on the structural strength of the tool.Shielding materials for neutrons will typically be hydrogenous materials and/or neutronabsorbing materials, like boron or cadmium for slow neutrons; and will typically be high atomicnumber materials like tungsten and/or hydrogenous materials for fast neutrons.

Claims (12)

1. A logging tool (20) for underground formations surrounding a borehole (14), comprising:

   an elongated body (21) along a major axis;

   a collar (22) disposed peripherally around said body (21) having a collar walldefined by an inner and an outer surface;

   a radiation emitting source (201) arranged to illuminate the earth formation (16)surrounding the borehole;

   at least one radiation detector (211) arranged to detect radiation reflected by theearth formation resulting from illumination by the source (201);

   at least one source collimation window (202) and one detector collimationwindow (212) through which the earth formation is illuminated and radiation isdetected; andcharacterized in that it further comprises

   at least one radiation shield (30) located between said inner collar surface and theouter surface of the tool, said radiation shield positioned so as to eliminateparasitic radiation that has not traversed the outer collar.
2. The tool of claim 1 further comprising a stabilizer (24) located at the periphery aroundthe outer collar surface, wherein said stabilizer comprises a stabilizer wall defined by aninner stabilizer surface and an outer stabilizer surface, and wherein the radiation shield islocated between said inner collar surface and said outer stabilizer surface.
3. The tool as claimed in any of the preceding claims, wherein the radiation shield (30) islocated between said emitting radiation source (201) and said radiation detector (211) andhas a length along the axis, which is less than 80% of the distance between said sourceand said detector.
4. The tool as claimed in any of the preceding claims, wherein the radiation shield (31) hasan annular shape surrounding the detector collimation window (212) and has a length along the axis, which is less than 40% of the distance between said source and saiddetector.
5. The tool of claim 3 or 4, wherein said radiation shield has a thickness in the cross sectionperpendicular to the major axis, which is less than 40% of the width of the said loggingtool at the position of said emitting radiation source (201).
6. The tool as claimed in any of the preceding claims, wherein the radiation emitting sourceis chosen in the list of: chemical radioactive source, an electronic radiation sourceemitting x-ray, a chemical neutron source or an electronic neutron source.
7. The tool of claim 1, wherein said logging tool (20) is a gamma-ray scattering densitymeasurement tool wherein:

   said emitting radiation source (201) is a gamma ray source or an x-ray source;

   a first radiation detector (211) is a short spaced gamma ray detector spaced axiallyat a first distance from said; and

   a second radiation detector (221) is a long spaced gamma ray detector spacedaxially at a second distance, greater than said first distance, from said emittingradiation source.
8. The tool of claim 1, wherein said logging tool (20) is a neutron scattering porositymeasurement tool wherein:

   said emitting radiation source is a neutron source;

   said radiation detector is a neutron detector spaced axially at a first distance fromsaid neutron source.
9. The tool as claimed in any of the preceding claims, wherein said radiation shield is madeof high atomic number material.
10. The radiation shield as claimed in claim 9, wherein the high atomic number materialincludes tungsten, depleted uranium, gold or an alloy thereof.
11. The tool as claimed in any of claims 1 to 8, wherein said radiation shield is made ofcomponents selected from the list consisting of: hydrogenous material, neutron absorbingmaterials, boron and cadmium.
12. A method for logging a well utilizing the tool (20) as claimed in claim 1.

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