Patent Publication Number: US-7897914-B2

Title: Downhole nuclear tool

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to downhole drilling, specifically downhole drilling for oil, gas, geothermal and horizontal drilling. More specifically, the invention relates to logging-while-drilling methods using a pulse neutron generator and detectors. Also, the invention relates to a method for a secondary nuclear measurement while drilling. 
     The prior art discloses several improvements for obtaining nuclear measurements downhole. U.S. Pat. No. 7,284,605, which is herein incorporated by reference for all that it contains, discloses a method for reducing stand-off effects of a downhole tool includes disposing the downhole tool in a borehole, wherein the downhole tool comprises at least one moveable section disposed between an energy source and a receiver on the downhole tool; and activating the at least one moveable section to reduce a thickness of at least one selected from a mud layer and a mudcake between the downhole tool and a wall of the borehole. 
     U.S. Pat. No. 6,666,285, which is herein incorporated by reference for all that it contains, discloses a logging-while-drilling gamma ray back scatter density system with elements configured to minimize material between sensor and the borehole environs, maximize shielding and collimation efficiency, and increase operational reliability and ruggedness. The system comprises a drill collar with a cavity in the outer wall, and an instrument package containing a sensor. The instrument package is disposed in the cavity and protrudes from the outer wall of the collar. Embodied as a density LWD system, the sensor consists of a gamma ray source and two detectors mounted within an instrument package framework made of high Z shielding material. A stabilized containing an alignment channel in the inner surface is disposed around the collar and receives the protrusion. 
     U.S. Pat. No. 5,250,806, which is herein incorporated by reference for all that it contains, discloses an apparatus and method for measuring density, porosity and other formation characteristics while drilling is disclosed. The apparatus, preferably housed in a drill collar and placed within a drill string, includes a source of neutrons and a source of gamma rays placed within a tubular body which is adapted to provide for the flow of drilling through it. Two sets of stabilizer blades are provided. One set, associated with the neutron source, includes secondary radiation detectors that are placed radially beyond the nominal outer radius of the body. Formation porosity measurement accuracy is substantially enhanced since the standoff of the detectors from the formation is substantially decreased. Another set, associated with the gamma ray source, includes one or more gamma ray detection assemblies in a single blade. Each of the gamma ray detector assemblies is also placed radially beyond the nominal outer radius of the tubular wall. 
     U.S. Pat. No. 5,242,020, which is herein incorporated by reference for all that it contains, discloses an extending arm is incorporated into a formation evaluation MWD collar or sub for extending outwardly from the tool and maintaining direct and continuous contact with the borehole wall (e.g., formation). In accordance with this invention, a method is presented for intermittently deploying the extendable arm and thereby decreasing drilling interference (caused by the arm) and avoiding the damage caused by accidents involving a nuclear source. 
     The prior art also discloses means for securing equipment in downhole tool string components, such as that disclosed in U.S. Pat. No. 7,299,867, which is herein incorporated by reference for all that it contains. This patent discloses a hanger mounted within a bore of a tubular string component has a split ring, a tapered key and a passageway formed in the hanger. The split ring has interfacial surfaces cooperating with interfacial surfaces of the tapered key. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect of the invention, a downhole tool string component comprises a tubular body with a first and a second tool joint adapted to connect to adjacent tool string components and a central bore adapted to pass drilling mud between the joints and a sleeve circumferentially disposed about an outer surface of the tubular body. The sleeve is rigidly attached to the outer surface at first and second sleeve ends and forms at least three stabilizer blades. A nuclear source and at least one nuclear detector are disposed within a gap formed between the inner surface of the sleeve and the outer surface of the tubular body. The tubular body may comprise a substantially uniform thickness between the bore and its outer surface along a length of the tubular body defined by the sleeve. 
     The nuclear source may be a pulse neutron generator in communication with a downhole generator driven by a drilling mud turbine. The thickness of the tubular body may be made of steel and a portion of the body proximate the neutron source comprises a thickness that inherently shields neutrons from penetrating into the bore. The neutron source may be at least partially disposed within a pocket formed in the inner surface of the sleeve and underneath one of the three stabilizer blades. The detectors may comprise the capability of distinguishing between neutrons and/or gamma rays of different magnitudes of energy. The nuclear source and detectors may be part of a downhole network incorporated within a tool string through a data coupler disposed within at least one of the tool joints of the tubular body. The nuclear source and the detectors may be synchronized with each other through the network. The tubular body may comprise a first modulus of elasticity and the sleeve may comprise a second modulus of elasticity, wherein the second modulus is 40 percent to 63 percent of the first modulus. The gap may comprise a near detector, a far detector, and an extra far detector axially aligned along the tubular body. At least one acoustic detector may also be disposed within the gap. 
     A method of making a secondary nuclear measurement while drilling may have the steps of providing a downhole tool string comprising a drill bit, a plurality of interconnected tool string components, a pulse neutron generator and a nuclear detector disposed within stabilizer assembly associated with one of the tool string components; providing a surface processing element capable of calculating downhole measurement; providing a network connecting the surface processing element to the pulse neutron generator and the detector; synchronizing the pulse neutron generator with the detectors over the network; emitting neutrons into the formation with the pulse neutron generator while drilling; measuring a formation response to the emitted neutrons through the detectors while drilling; and transmitting the measurements from the detectors to the surface processing element over the network while drilling. 
     Also, the pulse neutron generator may be powered by a downhole mud drive generator. The network may comprise a surface wireless connection, a satellite, a surface local area network, a surface wide area network, or combinations thereof. The network may comprise at least one data coupler disposed within shoulders of tool joints of the plurality of interconnected tool string components. The data coupler may be disposed within a recess formed in a shoulder of the tool joint and may comprise a coil disposed within a magnetically conductive, electrically insulating trough. The detectors may be turned on at the same time the pulse neutron generator emits the neutrons. The detectors may also be turned on at a pre-determined time after the pulse neutron generator emits the neutrons. The measurement may include a time lapse between the time of neutron emission and at least one measurement recorded by the detectors. The measurements may be analyzed in real-time while drilling and wherein the processing element may automatically make a drilling recommendation based off an analysis of the measurements. The processing element may automatically execute a command to drilling equipment to carry out the recommendation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective diagram of an embodiment of a tool string suspended in a borehole. 
         FIG. 2  is a perspective diagram of an embodiment of a tool string component. 
         FIG. 3  is a cross-sectional diagram of an embodiment of a tool string assembly. 
         FIG. 4  is a cross-sectional diagram of an embodiment of a tool string component. 
         FIG. 5   a  is a cross-sectional diagram of another embodiment of a tool string component in a borehole. 
         FIG. 5   b  is a perspective diagram of an embodiment of a tool string component. 
         FIG. 5   c  is a perspective diagram of another embodiment of a tool string component. 
         FIG. 5   d  is a perspective diagram of another embodiment of a tool string component. 
         FIG. 5   e  is a cross-sectional diagram of an embodiment of a tool string component. 
         FIG. 6  is a cross-sectional diagram of another embodiment of a tool string component in a borehole. 
         FIG. 7  is a cross-sectional diagram of another embodiment of a tool string component. 
         FIG. 8  is a cross-sectional diagram of another embodiment of a tool string component. 
         FIG. 9  is a diagram of an embodiment of drilling instrumentation 
         FIG. 10  is a perspective cross section of an embodiment of downhole components. 
         FIG. 11  is a perspective cut-away of an embodiment of a downhole component. 
         FIG. 12  is a flow diagram of an embodiment of a method of downhole logging while drilling. 
         FIG. 13  is a perspective diagram of an embodiment of a tool string in a borehole. 
         FIG. 14  is a flow diagram of another embodiment of a method of downhole logging while drilling. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENT 
       FIG. 1  is a diagram of an embodiment of a tool string  140  suspended by a derrick  141 . A bottom- hole assembly  144  is located at the bottom of a bore hole  143  and comprises a drill bit  145 . As the drill bit  145  rotates downhole the tool string  140  advance further into the earth and formation  150 . The bottom- hole assembly  144  and/or downhole tools  30 , such as drill pipes, may comprise data acquisition devices which may gather data. The data may be sent to the surface via a transmission system to a data swivel  142 . The data swivel  142  may send the data to the surface equipment  146 . Further, the surface equipment  146  may send data and/or power to downhole tools  30  and/or the bottom- hole assembly  144 . In some embodiments of the invention, the downhole tool string does not incorporate a downhole telemetry system connecting the downhole tools to surface equipment. An enlarged view  1160  discloses a nuclear cloud  1120  in the formation  150  produced by pulse neutron generator a downhole tool. 
       FIG. 2  is a perspective diagram of an embodiment of a tool string component  50 . The tool string component  50  may comprise downhole logging-while-drilling (LWD) and/or measurement-while-drilling (MWD) tools such as nuclear tools, seismic tools, resistivity tools, and/or acoustic tools. The tool string component  50  may comprise stabilizer blades  60  disposed on its exterior surface. The stabilizer blades  60  may be adapted to centralize the tool string component  50  within the bore hole  143  ( FIG. 1 ) while drilling. The stabilizer blades  60  may house a nuclear tool adapted to take measurements of the formation  150 . The stabilizer assembly  135  may have an opening  160  separating the blade  60  into first and second portions. Often in downhole drilling applications subterranean formations  150  may dictate drilling along deviated paths to avoid hazards or to improve production in a pay zone. The opening  160  may reduce the stiffness of the stabilizer assembly  135  allowing it to more easily follow a deviated path through the formation  150 . 
       FIG. 3  is a cross-sectional diagram of an embodiment of a portion of a tool string  140 . The stabilizer assembly  135  may comprise a sleeve  311  slideably attached to a first tubular body  302  proximate its tool joints  303 . The sleeve  311  may have at least one stabilizer blade  60  that generally follows the length of the sleeve  311  and extends outward from the center axis of the first tubular body  302 . A gap  301  formed between the sleeve  311  and the first tubular body  302  may be adapted to accommodate a downhole tool such as nuclear sources, nuclear detectors, seismic sources, geophones, hydrophones,  10  piezoelectric stacks, and/or resistivity related devices. In the embodiment of  FIG. 3 , a nuclear tool  300  is disposed within the gap  301 , and a power generator  305  is located within the bore of the first tubular body  302 . The generator  305  may be driven by a positive displacement motor, a turbine, the mud flow, or combinations thereof. 
     A diameter formed by the distal ends of the stabilizer blades  60  may be slightly less than the diameter of the bore hole  143 , causing the distal surfaces of the stabilizer blades  60  to be substantially in continuous contact with the bore hole wall and minimizing the distance between the instrumentation and the formation. An arrangement that may be compatible with the present invention is disclosed in U.S. patent application Ser. No. 11/828,901, which is herein incorporated by reference for all that it contains. 
       FIG. 4  is a cross-sectional diagram of an embodiment of a tool string component  50 . The nuclear tool comprises a nuclear source  400 , such as a pulse neutron generator (PNG), which is adapted to send neutrons into a surrounding formation  150  ( FIG. 1 ), and a detector assembly  500 , which are adapted to sense subatomic particles that travel back to the tool string component  50 . The gap  301  that contains the PNG  400  may be adapted such that the emitted neutrons are inherently shielded from entering the bore of the tool string component  50 . This may be accomplished by providing the first tubular body  302  with a large enough thickness to deflect the emitted neutron. This may be advantageous because it may reduce the nuclear reactions that occur in the drilling mud, thereby, allowing the detectors to obtain a more accurate reading of the formation  150 . The sleeve  311  may be thin enough to allow the neutrons to travel though it and into the formation  150 . The sleeve  311  may also be secured to the tubular body  302  through a sleeve anchor  1150 . The sleeve anchor  1150  may connect to the tubular body  302  though a threadform  1151 . 
     In the preferred embodiment, the sleeve  311  is made of steel and comprises a similar modulus of elasticity as the tubular body  302 . 
     In alternative embodiments, the sleeve  311  and first tubular body  302  may comprise different moduli of elasticity. The first tubular body  302  may comprise a first modulus of elasticity and the sleeve  311  may comprise a second modulus such that the second modulus is 40 percent to 63 percent of the first modulus. A lower modulus of elasticity may improve the downhole tool string component&#39;s  50  overall ability to bend, especially when deviating the trajectory of the borehole. The sleeve  311  may comprise titanium, carbon fiber, and/or copper. In some embodiments, the sleeve  311  may be hard-faced. The melting point of the sleeve  311  may be 1604 to 1660 degrees Celsius. The tensile strength of the sleeve  311  may be 897 mega-Pascals to 1000 mega-Pascals. The density of the sleeve  311  may be 0.14 lb/in 3  to 0.18 lb/in 3 . In embodiments where the density of the sleeve  311  is considerably lower than steel, a shield  1152  between the detector assembly  500  and the nuclear source  400  may prevent the neutrons from traveling directly to the detector assembly  500 . The shield  1152  may comprise a greater modulus of elasticity than the second modulus of elasticity. The shield  1152  may comprise carbide or steel and may be two to eight inches long. In some embodiments, the shield  1152  may comprise stress relief grooves to increase its flexibility and allow it to bend with the material of the sleeve  311 . 
       FIG. 5   a  discloses a nuclear cloud  1120  in the formation  150 . As the neutrons are emitted into the formation  150 , the neutrons collide with atoms and various nuclear interactions occur, such as: elastic and inelastic neutron scattering, neutron capture, and fast-neutron reactions. Generally these reactions will result in emitted neutrons bouncing through the formation  150  at reduced energy levels than when first emitted, and also gamma rays and other subatomic particles that are released during the nuclear interactions will bounce around within the formation. The neutrons and subatomic particles will travel in the various directions that they are deflected by the other atoms in the formation depending on the angle of their collisions and, thus, form a cloud  1120  of active subatomic particles. 
     The nuclear measurements may be performed while drilling mud is circulating through the bore hole as disclosed in  FIG. 5   a  with a downward symbol  1121  representing fluid traveling down the bore of the first tubular body  302 , and the upward symbols  1122  representing the drilling mud traveling up the bore hole  143  ( FIG. 1 ) in the annulus. Typically, some of the nuclear interactions occur within drilling mud disposed between the nuclear source  400  and the bore wall. Some of the drilling mud actually penetrates into cracks in the bore wall, requiring that the neutrons penetrate deeper into the formation to get a true measurement. A gap  301  formed underneath a stabilizer blade  60 , such as in the embodiment of  FIG. 5   a , is advantageous because it reduces the distance between the nuclear source  400  and the bore hole wall, thus, allowing more of the neutrons to travel deeper into the formation  150 . 
     A second tubular body  699  may be situated within the gap  301  between the sleeve  311  and the first tubular body  302 . The second tubular body  699  may support the sleeve  311  under downhole pressure, which has a propensity to collapse the sleeve  311  into the gap  301 , and may also house downhole instrumentation, such as the nuclear source  400  and detector assembly  500 , in pockets  1123  formed therein. The pocket  1123  may be aligned with a recess  399  formed in the inner diameter of the sleeve  311  and the instrumentation may be disposed within both the pocket  1123  and the recess  399 . In some embodiments, the instrumentation may reside within a pocket  1123  of the second tubular body  699 , a recess  399  of the sleeve  311 , or combinations thereof. 
       FIG. 5   b  discloses a second tubular body  699  disposed about the first tubular body  302  without the sleeve  311  for illustrative purposes. Pockets  1123  formed in the second tubular body  699  may go through the entire thickness of the second tubular body  699  or they may be formed only in a portion of the thickness. The second tubular body  699  may also be formed in axial segments  690 , one of the segments being a keystone segment  693 . In some embodiments of the present invention there may be only two segments  690 , one of which is the keystone segment  693 . 
     The segments  690  may interlock with each other, as disclosed in  FIG. 5   c , through a locking feature  695  along a length of a segment  690 . The pockets  1123  may extend along the axis of the second tubular body  699  a distance of five to seventy five percent of the length of the second tubular body. The second tubular body  699  may comprise more than one pocket  1123 , each of which may house different instrumentation. The electrical instrumentation housed within the pockets  1123  may be in electrical communication with each other. 
       FIG. 5   d  discloses second tubular body  699  held within the sleeve through a compression fit. During assembly, each segment  690  may be inserted into the sleeve  311  first. An expanding tool may be used to expand the inserted segments  690  for opening the space for the keystone segment  693 . Once the keystone segment  693  is inserted the expanding tool may be relaxed and removed, leaving the second tubular body  699  in compression. Testing reveals that a compression fit as described comprises lower stress concentrations in the downhole tool component over embodiments where the second tubular body  699  is not held in compression. Such an arrangement also allows less precision when making the various parts of the invention. 
       FIG. 5   e  discloses anti-rotation devices  694  adapted to restrict movement between combinations of the sleeve  311 , the second tubular body  699 , and the first tubular body  302 . The anti-rotation devices  694  may also comprise a tab, notch, protruding geometry, pins, inclined surfaces, wedges, or combinations thereof. 
     In  FIG. 6 , a nuclear source  400  and a detector assembly  500  are disclosed in a downhole tool string component  50 . The nuclear source  400  and the detector assembly  500  may be spaced under the same stabilizer blade  60  or under separate stabilizer blades  60 . In some embodiments, the detector assembly  500  and nuclear source  400  are housed by different portions of the same stabilizer blade  60 . 
       FIG. 7  discloses an embodiment of a detector assembly  500  disposed within the gap  301 . The detector assembly  500  may be used to detect the porosity and/or density of the formation  150  ( FIG. 1 ) by counting the gamma rays and/or neutrons returning to the tool at the detector assembly  500 . The sleeve  311  may comprise windows  1000  that are transparent to the subatomic particles allowing them to pass through to the detector assembly  500 . Typically the energy levels of the subatomic particles for measurements for porosity are substantially higher than the energy levels of the subatomic particles for density measurements, and the detector assembly  500  may be adapted to distinguish between the subatomic particles based on their energy levels. 
     The detector assembly  500  may comprise a near detector  520 , a far detector  521 , and an extra far detector  522 . The extra far detector  522  may be used to calibrate the measurements from the other detectors  520 ,  521 . Ratios calculated from the three detectors  520 ,  521 ,  522  may help estimate the true porosity and/or density of the formation  150 . Each detector  520 ,  521 ,  522  may comprise a scintillation material  1001 , such as a phosphor, that comprises a characteristic of generating an photoelectric signal upon contact with the subatomic particles. Typically, the collision with the subatomic particles do not have enough energy to produce a photoelectric signal large enough to be read by electronic devices, so a photomultiplier  1002  may be associated with at least one scintillation material  1001  to amplify the photoelectric signal. Wires  1003  may connect the photomultipliers  1002  to electronic equipment downhole that may process the photoelectric signals in real time. 
       FIG. 8  discloses a PNG nuclear source  400  in communication with a downhole network  1004 . The network and/or the electrical components may actuate a PNG nuclear source  400  on and off, control the sample rate, the duration of each sampling, and other parameters associated with the PNG nuclear source. 
       FIG. 9  discloses the PNG nuclear source  400  in communication with a downhole clock  900 . The PNG nuclear source  400  and the detector assembly  500  may be synchronized with each other through a network, a downhole processing element, a telemetry system, and/or a mud pulse system. The detector assembly  500  may also be turned on at the same time the PNG nuclear source  400  emits neutrons, or the detector assembly  500  may be turned on a pre-determined time after the neutrons are emitted. The measurement may include a time lapse between the time of neutron emission and at least one measurement recorded by the detector assembly  500 . Generally the detectors  520 ,  521 ,  522  are passive, but the downhole clock  900  allows the detectors  520 ,  521 ,  522  to receive improved accuracy in time stamping the emission and the detection of the subatomic particles. In some embodiments, the downhole clock  900  share the same source, which may be located downhole or up- hole over a telemetry system. Generally, nuclear reactions occur within milliseconds from actuating the PNG nuclear source  400 , thus, the electronics controlling the detectors  520 ,  521 ,  522  and the PNG nuclear source  400  must be precise. 
       FIGS. 10 and 11  disclose parts of a downhole network that may be compatible with the present invention. One such network is described in U.S. Pat. No. 6,670,880, which is herein incorporated by reference for all that it teaches. The parts may include data transmission elements  38  located in a secondary shoulder  39  of a pin end  40  and in a secondary shoulder  41  of a box end  42  of tool string component. The data transmission elements  38  are connected by an electrical conductor  44  that runs through the central bore of the tool string component  50 . In the preferred embodiment, the electrical conductor  44  is a coaxial cable that is under tension. The network may extend from the tool string component  50  at the surface and incorporate a satellite, a surface local area network, a surface wide area network, wireless connections or combinations thereof. With such an expanded network, the data from the detector assembly  500  may be forwarded to equipment located on site or to any location around the world for analysis. 
       FIG. 11  discloses the transmission elements  38  formed in grooves in the secondary shoulders  39  of the tool joints. The transmission elements  38  may comprise a coil  1170  disposed in segmented circular trough  1171  of magnetically-conductive, electrically-insulating material. The coil  1170  is also in electrically communication with the electrical conductor  44  through a lead wire  59  coming off of the coil. The magnetizable element may be constructed out of a highly permeable and ductile material typically associated with the class of soft magnetic materials. 
       FIG. 12  is a block diagram of an embodiment of a method for measuring while drilling  1200 . The steps include: recording downhole drilling measurements starting with taking the measurements while drilling  1201 , analyzing the measurements  1202 , and then sending the signal automatically to data storage/processing elements  1203 . 
     In  FIG. 13 , a PNG nuclear source  400  and a detector assembly  500  (see  FIG. 8 ) are located underneath the sleeve  311  proximate the nuclear cloud  1120 . The measurements may be analyzed in real-time while drilling and a processing element may automatically make drilling recommendations based off the measurements. The processing element may also automatically execute a command to drilling equipment to carry out the recommendation. Raw data, modified data, filtered data, and/or compressed data obtained by the detectors may be sent up-hole by a telemetry system and the calculations may be made at the surface. The nuclear measurements may help identify boundaries  1132  between subterranean strata  1130 ,  1131 . Such identifications may be useful in geo-steering applications where it is desirable to stay within an oil bearing strata  1131 . The drilling equipment may control the trajectory of the tool string, mud flow, weight on bit, tool string RPM, firing rate of downhole tools, other LWD or MWD tools, and/or power generation. In some embodiments, the tool string&#39;s rate of penetration and RPM may be monitored, which data may be coordinated with nuclear tool&#39;s activity. In some embodiments, the life of the nuclear source may be prolonged by reducing the firing rate of the nuclear tool in appropriate situations. Such situations may include slower RPM and/or ROP, investigation of a formation of a lower interest, or combinations thereof. 
     A rotary steerable system may be in communication with the processing element, and may change the drilling trajectory based off of input from the tools. The rotary steerable system may comprise an indenter  1125  that protrudes beyond the working portion of the bit. The indenter  1125  may be adapted to lead the bit along the desired trajectory. A rotary steerable system that may be compatible with the present invention is disclosed in U.S. Pat. No. 7,360,610, which is herein incorporated by reference for all that it discloses. 
       FIG. 14  is a block diagram of an embodiment of a method for making secondary measurement while drilling. The method may include the steps of: providing  1401  a downhole tool string comprising a drill bit, a plurality of interconnected tool string components, a PNG, and a nuclear detector disposed within a stabilizer assembly associated with one of the tool string components; providing  1402  a surface processing element capable of calculating downhole measurements; providing  1403  a network connecting the surface processing element to the PNG and the detector; synchronizing  1404  the PNG with the detectors over the network; emitting  1405  neutrons into the formation with the PNG while drilling; measuring  1406  a formation response to the emitted neutrons through the detectors while drilling; and transmitting  1407  the measurements from the detectors to the surface processing element over the network while drilling. 
     Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.