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 around said body  21  having a collar wall defined 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 the earth formation resulting from illumination by the source  201 ; at least one source collimation—window  202  and one detector collimation window  212  through which the earth formation is illuminated and radiation is detected; and characterized in that it further comprises at least one radiation shield  30  located between said inner collar surface and the outer surface of the tool, said radiation shield positioned so as to eliminate parasitic radiation that has not traversed the outer collar.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Patent Application EP04291261 .8 entitled “Tool Casing with Gamma Ray Window,” filed on Mar. 4, 2003 by Cheung et al., which claims priority from PCT Application PCT/EP01/03718 filed on Mar. 29, 2001 and claims priority from French Application No. FR20000004527 filed on Apr. 7, 2000. 
     FIELD OF THE INVENTION 
     This invention relates to logging of oil, water or gas well in underground formations surrounding a borehole and more particularly to a logging tool with a parasitic radiation shield such 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 a given earth formation contains hydrocarbon, and the amount of hydrocarbon within the formation. Therefore, formation properties while drilling or in a freshly drilled hole are measured to predict the presence of oil, gas and water in the formation. These formation properties may be logged 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 material in earth formation using a source of nuclear radiation and a radiation detector. The density of a material can be determined either by a transmission or by a scattering measurement. In a transmission measurement the material, the density of which needs to be determined, is put between the radiation source and the detector. In a scattering measurement the intensity and energy distribution of the radiation scattered back to a detector from the material under investigation is used to determine the density. Downhole measurements of formation density are of the scattering type since it is not usually possible to insert the formation material directly between source and detector, with the possible exception of rock samples removed from the formation. 
     Gamma-ray scattering systems have been used for many years to measure the density of a material penetrated by a borehole. Typically density is measured as a function of position  25  along the borehole thereby yielding a “log” as a function of depth within the borehole. The measuring 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 tight container. Radiation impinges on and interacts with the material, and a fraction of the impinging radiation is scattered by the material and a traction thereof will return to the detector. After appropriate system calibration, the intensity of the detected scattered radiation can be related to the bulk density of the material. 
     The radial sensitivity of the density measuring system is affected by several factors such as the energy of the gamma radiation emitted by the source, the axial spacing between the source and the one or more gamma ray detectors, and the properties of the borehole and the formation. The formation in the immediate vicinity of the borehole is usually perturbed by the drilling process, and more specifically by drilling fluid that invades the formation in the near borehole region. Furthermore, particulates from the drilling fluid tend to buildup on the borehole wall. This buildup is commonly referred to as mudcake, and adversely affects the response of the system. In this way, intervening material between the tool and the borehole wall will adversely affect the tool response. Intervening material in the tool itself between the active elements of the tool and the outer radial surface of the tool will again adversely affect the tool response by producing a background of scattered radiation which is independent of the presence of the borehole fluid, the mudcake or the formation. Typical sources are isotropic in that radiation is emitted with essentially radial symmetry. Flux per unit area decreases as the inverse square of the distance to the source. Radiation per unit area scattered by the formation and back into detectors within the tool also decreases with increasing distance, but not necessarily as the inverse 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-drilHng systems use a variety of source and detector geometries to minimize standoff, such as placing a gamma ray source and one or more gamma ray detectors outside the tool body within a drill collar with a stabilizer disposed between source and detectors and the borehole and formation; or within stabilizer fins that radiate outward from a drill collar. This tends to minimize intervening material within the tool, and positions source and detectors near the borehole environs, but often at the expense of decreasing the efficiency of shielding and collimation. The signal-to-noise ratio is often degraded by the detection of particles that have not probed the earth formation but instead have traveled trough low-density regions or voids existing in the tool between source and detectors, and especially through collar and stabilizer. Shielding of source and detectors mounted in the tool body is well known in the prior art; chassis is shielded and detectors are mounted in a shielded holder with windows trough which radiation is detected. Other prior art patents focus on total radiation shielding of the tool to the detriment of functionality: EP 0160351 describes a shielded tool casing with windows, which receives instrument package, U.S. Pat. No. 6,666,285 describes an apparatus, which has a cavity to receive a solid shielded instrument package. Those apparatus, because they use a framework totally made of high-density materials, are heavy and brittle, and in harsh drilling conditions, can be broken resulting in the destruction and possibly the loss of the instrument package and more critically the loss of the radioactive source. The problem of providing shielding in the collar and  10  the stabilizer has not been yet addressed successfully. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a logging tool for underground formations surrounding a borehole, comprising: an elongated body along a major axis; a collar disposed peripherally 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 formation surrounding the borehole; at least one radiation detector arranged to detect radiation reflected by the earth formation resulting from illumination by the source; at least one source collimation window and one detector collimation window through which the earth formation is illuminated and radiation is detected; and at least one radiation shield located between said inner collar surface and the outer surface of the tool, said radiation shield positioned so as to eliminate parasitic radiation that has not traversed the outer collar. 
     In a preferred embodiment, the tool further comprises a stabilizer located at the periphery around the outer collar surface, wherein this stabilizer comprises a stabilizer wall defined by an inner stabilizer surface and an outer stabilizer surface, and wherein the radiation shield is located between this inner collar surface and this outer stabilizer surface. The stabilizer enhances the contact between the tool and the formation by reducing the space available for mud between the tool and the formation. 
     The tool is designed so that the source and the detector are as near as practical to the borehole environs. The radiation shields increase the signal to noise ratio. And the invention below proposes a robust, secure and functional configuration. 
     In a preferred embodiment, the radiation shield is located between the emitting radiation source and the radiation detector and has a length along the axis, which is less than 80% of the distance between the source and the detector. The radiation shield has a thickness in the cross section perpendicular to the major axis, which is preferably less than 40% of the width of the tool at the position of the radiation source. This makes it possible to eliminate a significant fraction of the radiation that are coming from source and that have not passed through the borehole 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 the detector window and has a length along the axis, which is less than 40% of the distance between the source and the detector. In a preferred embodiment, the radiation shield has a thickness in the cross section perpendicular to the major axis, which is less than 40% of the width of the tool at the position of emitting radiation source. This enables eliminating a part of the radiations passing through the collar to the detecting window and not through the window in the collar to the detector window. 
     In a preferred embodiment, this invention is directed toward a radiation density measurement system in underground formations surrounding a borehole with a chemical radioactive source or an electronic radiation source emitting x-ray; or a chemical or electronic neutron source. 
     In a preferred embodiment, this invention is directed toward a gamma-ray logging-while-drilling density tool. The system comprises a source of gamma radiation and one or more gamma ray detectors. Multiple detectors (2 or more) provide better efficiency and allow compensation 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 types and classes of logging, logging-while-drilling or measurement-while-drilling systems. As an example, the invention can be used in a neutron porosity system for measuring formation porosity, 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-ray radiation, and permits the efficient shielding, collimation and optimum disposition of the source and detectors with respect to the borehole environs. 
     The present invention also discloses a method for logging a well utilizing a tool as mentioned above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further embodiments of the present invention can be understood with the appended drawings: 
         FIG. 1  illustrates a logging-while-drilling tool according to the invention. 
         FIG. 2   a  is a side view on the major axis of the tool of  FIG. 1  with the radiation shield localized between source and first detector. 
         FIG. 2   b  is a side view on the major axis of the tool of  FIG. 1  with the radiation shield localized closed to first detector. 
         FIG. 2   c  is a side view on the major axis of the tool of  FIG. 1  with both radiation shields. 
         FIG. 3  shows pulse-height spectra obtained by numerical modeling of the logging-while-drilling tool of  FIGS. 2   a  and  2   c  as well as a case in which neither of the shields  30  and  31  is present. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a logging-while-drilling tool, identified as a whole by the numeral  20 , disposed by means of a drill string within a well borehole  18  defined by a borehole wall  14  and penetrating an earth formation  16 . The upper end of the collar element  22  of the tool  20  is operationally attached to the lower end of a string of drill pipe  28 . The stabilizer element of the tool  20  is identified by the numeral  24 . A drill bit  26  terminates the lower end of logging tool  20 . It should be understood, however, that other elements can be disposed on either end of the tool  20  between the drill pipe  28  and the drill bit  26 . The upper end of the drill pipe  28  terminates at a rotary drilling rig  10  at the surface of the earth  12 . The drilling rig rotates the drill pipe  28  and cooperating tool  20  and drill bit  26  thereby advancing the borehole  18 . Drilling mud is circulated down the drill pipe  28 , through the axial passage in the collar  22 , and exits at the drill bit  26  for return to the surface  12  via the annulus defined by the outer surface of the drill string and the borehole wall  14 . 
       FIGS. 2   a ,  2   b  and  2   c  illustrate conceptually radiation shields on the tool  20  of  FIG. 1  shown in side view on the major axis of the tool. In a first embodiment, the tool is a logging-while-drilling gamma-ray scattering tool with a chemical radioactive source. The tool  20  is made of an elongated tool body  21  and a drill collar  22  disposed peripherally around the tool body  21 . In the illustrated tool, a stabilizer  24  is disposed peripherally around the drill collar  22 ; the stabilizer is optional and reduces the amount of mud between the tool and the formation wall and therefore the influence of the borehole fluid on the measurement. The tool  20  receives one source collimation window  202  through which the earth formation  16  is illuminated by the radiation emitted from the radioactive source, and two detector collimation windows  212  and  222  through which the radiation coming from the outside of the tool  20  is detected. In the illustrated tool, a source of gamma radiation  201  illuminating the earth formation  16  and affixed to a source holder  200 , is mounted in the collar wall  22 . Though this is the preferred way, other locations for the source  201  are in the tool body  21  or in the stabilizer  24 . The source  201  is preferably cesium-137 ( 137 Cs) which emits gamma radiation with an energy of 0.66 million electron volts (MeV). Alternately, cobalt-60 ( 60 Co) emitting gamma radiation at 1.11 and 1.33 MeV can be used as source material. The tool  20  receives a first or “short spaced” gamma ray detector  211  disposed at a first axial distance from the source  201 , and a second or “long spaced” gamma ray detector  212  disposed at a second axial distance from the source, where the second spacing is greater than the first spacing. In the illustrated tool, the detectors are mounted in the tool body  21  in holders:  210  for the first detector and  220  for the second detector. Though this is the preferred way, other locations for the detectors  211 ,  221  are in the collar wall  22  or in the stabilizer  24 . The detectors are preferably scintillation type such as sodium iodide (Nal) 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 collar supports the stresses inherent to logging conditions, in  FIG. 2   a , a side view of the tool illustrates a radiation shield  30  located in the collar  22  whose shape is optimized to reduce leakage through the collar without affecting its mechanical strength. 
     The trajectories of gamma rays traveling from the source to the detector are like broken lines, on which each break corresponds to a collision with an electron within the surrounding material. Gamma radiations lose energy by means of the most pertinent reaction here: Compton scatter reaction. After undergoing one or more Compton scattering events, a small fraction of the emitted with reduced gamma-ray energy returns to the tool and is detected by the gamma radiation detector. The function of the radiation shield  30  is to intercept and attenuate by photoelectric absorption or by Compton scattering and subsequent photoelectric absorption, a significant fraction of those gamma rays that travel through the collar or/and stabilizer and that might otherwise go back to the detector after being scattered in the collar or/and stabilizer. 
       FIG. 2   b  illustrates a side view of the tool with a radiation shield  31  located on the inner collar surface in the collimation window  212  of the first detector  211 . The function of the radiation shield  31  is to intercept and attenuate gamma rays traversing the collar to the detecting window. 
       FIG. 2   c  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 the radiation shields, a Monte-Carlo N-Particle model is built based on the tool plan of  FIG. 2 . A compromise 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-137 radiation. Pulse-height spectra for energies between 0.1 and 0.5 MeV for the first Nal detector are computed for three different configurations: (1) tool without extra radiation shield, (2) tool with radiation shield  30  as in  FIG. 2   a , (3) tool with radiation shields  30  and  31  as in  FIG. 2   c.    
     One or more pieces of a high-density material, i.e. a material with a high atomic number (more than Z=70) and a high density (more than 15 g/cc) like tungsten, gold or depleted uranium, are inserted in the collar in a particular locations where their shielding efficiency will be maximal and their influence on the mechanical strength will be minimal. High Z materials are efficient attenuators of gamma radiation, and permit the efficient shielding, collimation and optimum disposition of the source and detectors with respect to the borehole environs. 
     The radiation shield  30  of  FIG. 2   a  is in a preferred embodiment, placed into a cavity in the outer surface of the collar, wrapped in a rubber envelope and then compressed underneath a cover plate screwed onto the collar between the source and the detector. In a preferred embodiment, better efficiency is obtained when length along the axis of this radiation shield is less than 80% of the first axial distance between source and detector; and when thickness of this radiation shield in the cross section perpendicular to the major axis is less than 40% of the width of the tool at the position of the source. In a second preferred embodiment, best efficiency is obtained when length along the axis of this radiation shield is less than 60% of the first axial distance between source and detector; and when thickness of this radiation shield in the cross section perpendicular to the major axis is less than 20% of the width of the tool at the position of the source. The radiation shield is disposed circumferentially around the collar outer surface, and preferably covering less than 180° of this surface. The effectiveness of the radiation shield  30  is maximized 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 an extension towards the source is made, but at the expense of a lower mechanical strength. As an example of optimization, for a circular part of a tungsten patch, the length along the axis is 58 mm whereas the first axial distance is 170 mm, and the thickness is 7 mm. and for the circular part, the internal radius is 78 mm and the opening angle is 90°. 
     The radiation shield  30  of  FIG. 2   a  can be associated with another radiation shield  31  of  FIG. 2   b , located at the base and very close to the collimator window of the first detector, this radiation shield  31  has an annular shape surrounding this collimator window and with a trapezoidal section. Both radiation shields in this embodiment are illustrated on  FIG. 2   c . The efficiency is maximized with specific angular aperture of the trapezoidal section just as the dimension of the annular shield. Nevertheless, these dimensions of the annular shield are dictated by the requirements for mechanical strength. Therefore, in a preferred embodiment, better efficiency for the radiation shield  31  is obtained when this radiation shield is located between the first detector and the outer stabilizer surface facing the first detector, and when this radiation shield has an annular shape with a length along the axis or a diameter, which is less than 40% of the distance between source and first detector. In a second preferred embodiment, best efficiency for the radiation shield  31  is obtained when this radiation shield has an annular shape with a length along the axis or a diameter, which is less than 20% of the distance between source and first 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-drilling tool at the position of emitting radiation source. In a second preferred embodiment, this radiation shield 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. 
       FIG. 3  shows the pulse-height spectra obtained by numerical modeling of the tool with 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 is assumed to be very dense like tungsten (17.4 g/cm 3 ) so that practically no gamma-rays will return from the formation and the signal is entirely due to gamma-rays traveling through the collar 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 with a cesium-137 gamma ray source, the percentage of total gamma-ray leakage removed from the total signal by the radiation shields is evaluated. For a stabilizer diameter of 8% inches, the percentage of gamma-ray leakage removed Is of 45% with the radiation shield  30  alone and of 54% with both radiation shields  30  and  31 ; for a stabilizer diameter of 9⅜ Inches, this percentage is 43% and 51% respectively. 
     In a second model, the earth formation is assumed to be made of an aluminum alloy (2.805 g/cm 3 ) so gamma-rays will return in this model also from the formation. The percentage 20 of gamma-ray leakage removed from the signal by the radiation shields is evaluated in this model as well and the results are comparable to those obtained with the first model. For a stabilizer diameter of 8% inches, the percentage of gamma-ray leakage removed Is 43% with the radiation shield  30  alone and of 57% with both radiation shields  30  and  31 ; for a stabilizer diameter of 9⅜ inches, this percentage is 38% and 46% respectively. 
     The radiation shield  30  removes almost 50% of gamma-ray leakage and the radiation shield  31  removes an additional 10% of gamma-ray leakage. These radiation shields  30  and  31  mounted offer therefore various mechanical, operational and technical advantages. 
     Radiation shields between first and second detectors or in the collimation window of the long spaced detector are possible; nevertheless this second detector is less sensitive to gamma-ray leakage and a reduction of the leakage is less important. 
     In a second embodiment, the tool  20  is a logging-while-drilling density tool with an electronic radiation source. The source  201  is an x-rays generator. The shielding materials need to be inserted into the structural materials of the tool body, collar or stabilizer with the intent to optimize shielding with a minimal impact on the structural strength of the tool. Shielding  5  materials for lower energy gamma-rays or x-rays could be lighter materials. 
     In a third embodiment, the tool  20  is a logging-while-drilling neutron scattering tool with a chemical or electronic neutron source. The source  201  is a chemical source, as Radium-Beryllium source or an electronic source like pulsed neutron generator. The shielding materials need to be inserted into the structural materials of the tool body, collar or stabilizer with the  10  intent 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 neutron absorbing materials, like boron or cadmium for slow neutrons; and will typically be high atomic number materials like tungsten and/or hydrogenous materials for fast neutrons.