Pulsed nuclear magnetism tool for formation evaluation while drilling

Abstract: The instruments of a pulsed nuclear magnetic resonance (NMR) device are included in a drill collar for evaluating earth formations. The resulting tool makes NMR measurements while the formation is being drilled.

Description: Field of the Invention The invention concerns a tool for logging while drilling. More specifically the invention concerns a tool for logging a formation using pulsed nuclear magnetic resonance (NMR) techniques while drilling a borehole into the formation. Background of the Invention Several pulsed NMR tools have been designed for borehole deployment via wireline and are described in U.S. Patent Nos: 4,350,955, issued 9/1982 to Jackson et al.; 4,629,986, issued 12/1986 to Clow et al.; 4,710,713, issued 12/1987 to Strikman; 4,717,876, issued 1/1988 to Masi et al.; 4,717,877, issued 1/1988 to Taicher et al.; 4,717,878, issued 1/1988 to Taicher et al.; 4,714,881, issued 12/1987 to Givens; 5,023,551, issued 6/1991 to Kleinberg et al.; 5,055,787, issued 10/1991 to Kleinberg et al.; and 5,055,788, issued 10/1991 to Kleinberg et al. All of these patents describe NMR tools which employ permanent magnets to polarize hydrogen nuclei, and RF coils to excite and detect nuclear magnetic resonance to determine porosity, free fluid ratio, or permeability of a formation, for example. The tools described in 4,717,877; 5,055,787; and 5,055,788 have been successfully tested in boreholes. The instrument concepts described in the related patents 4,710,713 and 4,717,877 have a rotationally invariant region of resonance, which is a thin cylindrical shell coaxial with the borehole. The electromagnetic fields have a simple azimuthal dependence: both the static and RF fields at any point in the vicinity of the instrument rotate at the same rate as the instrument rotates. The rotation does not disturb the NMR measurement because the nuclear magnetic moment adiabatically tracks the magnetic field. The radial thickness of the resonance region of these instruments is on the order of 1 mm. The instrument concepts described in the related patents 4,350,955, 4,629,986, and 4,717,876 have rotationally invariant static and RF fields and a relatively large resonant region (at least 10 mm thick). The resonant region is thicker because the magnitude of the magnetic field is stationary at the resonant region. These patents, however, do not describe rotating the instruments. Several logging while drilling (LWD) and measuring while drilling (MWD) tools have been designed for formation evaluation while drilling and drill string characterization while drilling, respectively. Logging or measuring instruments are placed in drill collars up to 100 ft behind the drill bit. An MWD tool is described in U.S. Patent Nos. 3,777,560 to Guignard and 4,479,564 to Tanguy. An LWD tool is described in U.S. Patent Nos. 4,899,112 to Clark et al. and 4,949,045 to Clark et al. Typically, these tools use electromagnetic techniques in evaluating resistivity of a formation while drilling. A nuclear magnetic resonance apparatus is disclosed in U.S. Pat. No. 4, 350, 955. The apparatus focuses a toroidal homogeneous magnetic field a distance into the formation around a borehole. The distance is controlled by adjusting the separation between two magnets which produce the magnetic field. A precession signal from the formation is detected by an RF coil. GB-A-2 056 082 describes a nuclear magnetic logging method wherein a small current is applied to a coil in order to provide a small residual field during the NMR signal decay period. This residual field, which is much smaller than the Earth's magnetic field and which decreases radially outwardly, introduces radial frequency dispersion in the NMR signal, thus permitting the removal of that part of the signal due to fluids in the borehole by passing the decay signal through band pass filters. WO-A-92 10768 discloses a nuclear magnetic resonance logging-while-drilling apparatus. The apparatus includes a tubular, permanent magnet for generating a static magnetic field in the bore hole region adjacent to the apparatus. Further, this apparatus includes an antenna mounted on the magnet for exciting nuclei of materials to be analyzed in the bore hole region. Summary of the Invention The invention concerns an apparatus comprising a drilling means for drilling a borehole in an earth formation; and a measuring means, connected to the drilling means, for making nuclear magnetic resonance (NMR) measurements while the borehole is being drilled. In a preferred embodiment, the invention concerns a drill string and bit for drilling a borehole in an earth formation and a pulsed NMR device which is housed by the drill string. The embodiment includes a channel within the drill string and pulsed NMR device through which drilling mud is pumped into the borehole. The pulsed NMR device comprises two tubular magnets which are mounted within the drill string to surround the channel. The pulsed NMR device also includes an antenna coil mounted in a recess in the exterior surface of the drill string and between the two tubular magnets, and associated driving circuitry located in a compartment of the drill string. Thus, the tool logs the earth formation using pulsed NMR techniques while the tool drills a borehole into the formation. Brief Description of the Figures Figure 1 shows a cross section of a tool for pulsed NMR formation evaluation while drilling. Figure 2 shows a static field in a vertical plane of the tool of Figure 1. Figure 3 shows a static magnetic field of the tool of Figure 1. Figure 4 shows a gradient coil according to the invention of Figures 1-3. Detailed Description Because the resonance region of the instruments of U.S. Patent Nos. 4,710,713 and 4,717,877 is on the order of 1mm, the inventors expect that lateral vibrations of a drill collar which might be proposed for containing an NMR device would adversely affect an NMR measurement. For example, a lateral, 50 Hz vibration of 1 mm amplitude (10 g acceleration) would disable the measurement. Also, the inventors have determined that, since the drill string makes several revolutions in the duration of a pulsed NMR measurement (on the order of 1 second), an NMR instrument in a drilling string must be rotationally symmetric. Accordingly, the instrument of this invention provides greater immunity to lateral motion and is also rotationally symmetric about a longitudinal axis. A detailed description of the tool follows. Figure 1 shows a tool 10 according to this invention. The tool 10 includes a drill bit 12, drill string 14, and pulsed NMR device 16 (housed within a drill collar 17). The drill bit 12 and drill string 14 comprise a means for drilling a borehole 18 in an earth formation. A drive mechanism 15 rotates the drill string 14 and drill bit 12 as is known in the art. See U.S. Patent No. 4,949,045 to Clark et al., for example. The drill collar 17 is made of a nonmagnetic alloy. The tool 10 also comprises a means for making pulsed nuclear magnetic resonance (NMR) measurements while a borehole 18 is being drilled. The pulsed NMR device 16 is connected as part of the drill string 14 and is housed within the drill string 14. As seen in Figure 1, the pulsed NMR device 16 is rotationally symmetric about a longitudinal axis 20 of the drill string 14. The borehole 18 typically contains fluid, such as drilling muds which are pumped by pump 19 into the borehole 18 from the surface to carry away formation cuttings back to the surface. Such a pump is described in U.S. Patent No. 4,949,045 to Clark et al., for example. A channel 21 within the drill string 14 comprises a means for carrying the borehole fluid through the drill string 14. The channel 21 is cut within the drill collar 17 substantially parallel to the longitudinal axis 20. The channel 21 opens to the borehole 18 through ports 23 in the drill bit 12. The pulsed NMR device 16 comprises, in a preferred embodiment, a pair of tubular magnets 22 arranged and housed in the drill string 14 and within the pulsed NMR device 16 to surround the channel. The two sleeve shaped, tubular magnets 22 are permanently magnetized in the axial direction and are positioned in opposing directions. In Figures 1-3 the north magnetic poles of the two tubular magnets 22 face one another, for example. The magnets 22 are preferably made of Samarium-Cobalt, which has a high remanence, coercive force, and Curie temperature. The magnets 22 can be installed into a recess that is cut out either from the outside or from the inside of the drill collar 17. Cutting from the inside is preferred because the bending and torsional strengths of the drill collar 17 are proportional to the difference of the fourth powers of the drill collar's outer and inner radii. For example, in Figure 1 the inner D1 and outer D2 diameters of the drill collar 17 at the magnet-recess are 5.000" and 6.750", respectively. The inner D3 and outer D2 diameters of the magnets 22 are 3.000" and 5.000", respectively. These dimensional values, and those below, are only cited as examples. Numerous other values of the diameters, thickness, and depth are possible. The magnets 22 are protected from the pressure and abrasion of drilling mud by a 0.375" thick nonmagnetic steel barrel 24, for example. The barrel 24 defines the channel 21 and is positioned to isolate the tubular magnets 22 from the drilling mud comprising the borehole fluid. Had the recess for the magnets 22 been made from outside surface of the drilling collar 17, the recess would have to be at most 0.276" deep, for example, in order to achieve the same mechanical strength. This shallow recess must be considered in accommodating the magnets 22 and the protective barrel 24. The inventors have found it is preferable to cut the recess for the magnets 22 from the inside of the drill collar 17 to obtain greater mechanical strength and sufficient space to accommodate the magnets 22 and steel barrel 24. Figure 1 also shows an RF antenna 26 located in an antenna recess 28 between the two tubular magnets 22. The pulsed NMR portion 16 of this tool 10 comprises the antenna 26 mounted on the drill collar 17 and the two tubular magnets 22. The antenna 26 is mounted in an exterior recess 28 of an exterior surface of the drill collar 17. The antenna recess 28 is preferably on the outside of the drill collar 17, because at the frequency of operation (around 1.4 MHz in the example in Figure 1) the resulting RF magnetic field could not diffuse out through the drill collar 17. The antenna 26 preferably comprises a coil which also surrounds the channel. The antenna recess 28 is preferably filled with a nonconducting material such as plastic or rubber. Alternatively, the antenna recess 28 can also be filled with a ferrite to improve the efficiency of the RF antenna 26. The RF antenna 26 can be protected by a sheet-metal shield with axial slots to let out the RF magnetic field. The slotted shield is mounted in the recess 28 and radially outside the antenna 26 coil. Examples of the construction of the antenna and the slotted shield have been described in the U.S. Patent No. 4,949,045 to Clark et al. issued August 1990 in the context of a Compensated Dual Resistivity Tool (mark of Schlumberger Technology Corporation), and U.S. Patent No. 4,536,714 to Clark issued August 1985. The pulsed NMR portion 16 of this tool 10 also includes driving circuitry. The drill collar 17 includes a compartment 30 in which the driving circuitry 32 is located. The small, atmospheric-pressure compartment 30 close to the RF antenna 26 houses the electronic, driving circuitry such as the Q-switch, duplexer, and preamplifier which drive the antenna and which have to be in close proximity of the RF antenna 26. This driving circuitry 32 is similar to that disclosed in the two U.S. patents to Kleinberg et al., 5,055,787 and 5,055,788. The NMR device 16 is pulsed and the formation is logged according to the technique described in U.S. Patent Nos. 5,055,787; 5,055,788; and 5,023,551 to Kleinberg et al. The information representing the evaluation of the formation is stored in a downhole memory (not shown) and is obtained from the pulsed NMR device 16 in a manner as described in U.S. Patent No. 4,949,045 to Clark et al. In Figure 2, a static magnetic field provided by the pulsed NMR device 16 is shown in a vertical plane that contains the borehole axis (not shown). To the right of the cross-section of the tool 10, the magnitude of the static field is illustrated by a contour map of 50 Gauss increments. The field is azimuthally symmetric. The pulsed NMR device 16 produces a field having a stationary point or saddle point, indicated by a (+) sign, in the formation. At this point, all spatial derivatives of the static magnetic field are zero. To the left of the cross-section of the tool 10, the magnitude and direction of the static field are illustrated by vectors. Figure 3 shows a resonance region 34 of the static magnetic field of Figure 2 in greater detail using a contour map of 10 Gauss increments. The pulsed NMR device 16 produces the static magnetic field having the resonance region 34 which extends from the tool 10 into the formation under evaluation. In this example of the invention, the stationary point of the resonance region 34 is 6.25" away from the borehole axis (not shown). If the drill bit 12 size is 8.5" (diameter), then the stationary point is 2.00" away from the surface of the borehole 18 wall. According to this invention, the static magnetic field is produced such that the stationary point occurs in the formation. In this manner, a large resonance region 34 is produced making the pulsed NMR device 16 relatively immune to tool motions. Figure 4 shows the resonance region 34 extending into the borehole 18. Because the resonance region 34 extends into the borehole 18, an electromagnetic signal is produced in the borehole fluid. The resulting electromagnetic signal of the borehole fluid must be cancelled because the pulsed NMR device 16 functions by detecting protons in fluids. Typically, the rock formation is 10% fluid by volume, but the borehole fluid contains more than 50% fluid and has a high density of protons. For this reason, the electromagnetic signal of the borehole fluid would dominate any formation signal detected by the pulsed NMR device 16. The electromagnetic signal of the borehole fluid can be cancelled by applying a pulsed magnetic field that is strong in the borehole 18 but weak at the stationary point (+) of the static magnetic field. Such a field for cancelling the electromagnetic signal of the borehole fluid can be produced by a gradient- coil 36. A pulsed current source 38 drives the gradient-coil 36, which produces magnetic field lines 40. Figure 4 shows the gradient-coil 36 is coiled around the longitudinal axis 20 of the drill collar 17, and, thus, around the pulsed NMR device 16. The gradient-coil 36 is coiled above and below the antenna 26, as viewed in Figure 1. The gradient-coil 36 may be placed in the antenna recess 28, or in a separate recess in the drill collar 17. Most importantly, the gradient-coil 36 is sited to most effectively eliminate the signal from the borehole, such as in the vicinity of the extensions 42 of the resonance region 34. In this manner, the magnetic field lines 40 of the gradient-coil 36 cancel the electromagnetic signal of the borehole fluid in the vicinity of the extensions 42. The location of the extensions 42 of the resonance region 34 is a function of the material, size, shape, and spacing of the permanent tubular magnets 22. The gradient-coil 36 may be unnecessary when the drill string 14 is in motion. Since the part of the resonant region in the borehole 18 is a thin shell, vertical and lateral vibrations may eliminate the borehole signal.

Claims: Logging-while-drilling apparatus comprising: (i) drilling means having: a) a drill bit for drilling a borehole in a formation; b) an exterior surface and a recess in the exterior surface; and c) a housing means; (ii) a pulsed nuclear magnetic resonance (NMR) means housed within the housing means for making pulsed NMR measurements in a region of the formation around the borehole while the borehole is being drilled, the pulsed NMR means having: a) two tubular magnets, each having magnetic poles, and arranged so one pair of like magnetic poles face one another; b) an antenna mounted between the two tubular magnets, and in the recess of the exterior surface of the drilling means; and c) driving circuitry means for driving the antenna, which circuitry is mounted in the housing means; and (iii) conduit means for carrying borehole fluid through the drilling means, which conduit means is surrounded by the two tubular magnets and the antenna, and which comprises a barrel situated to isolate the two tubular magnets from the borehole fluid; wherein the pulsed NMR means includes means for cancelling the component of the pulsed NMR measurements due to the borehole fluid by applying a pulsed magnetic field which is strong in the borehole fluid and weak in said region of the formation. The apparatus of claim 1, wherein the component cancelling means comprises coil means wound in the recess. The apparatus of claim 2, wherein the coil means is wound in the recess on each side of the antenna. The apparatus of claim 2 or claim 3, wherein the recess in which the antenna and coil means are mounted is filled with a nonconducting material. The apparatus of claim 2 or claim 3, wherein the recess in which the antenna and coil means are mounted is filled with a ferrite. The apparatus of any preceding claim, comprising a slotted shield in the recess and radially outside the antenna. A method of logging a borehole in an earth formation while drilling the borehole, the method comprising: using one portion of a logging-while-drilling tool to drill the borehole: using another portion of the tool to make pulsed nuclear magnetic resonance (NMR) measurements in a region of the formation around the borehole while the borehole is being drilled, by producing a rotationally symmetric magnetic field about the longitudinal axis of the borehole with two tubular magnets, each having magnetic poles, and arranged so one pair of like magnetic poles face one another, and using an antenna mounted between the two tubular magnets to excite and detect said NMR measurements; carrying borehole fluid through both portions of the tool with a barrel defining a channel and isolating the two tubular magnets from the borehole fluid; and cancelling the component of the NMR measurements due to the borehole fluid by applying a pulsed magnetic field which is strong in the borehole fluid but weak in said region of the formations. The method of by claim 7, wherein coil means which is wound longitudinally above and below the antenna and which is driven by a pulsed current source is used to cancel the borehole fluid component of the NMR measurements. The method of claim 8, wherein the magnetic field is produced through a slotted shield which is adjacent the antenna.

Inventors: KLEINBERG ROBERT L, SEZGINER ABDURRAHMAN, KLEINBERG, ROBERT L., SEZGINER, ABDURRAHMAN
Applicants: SCHLUMBERGER HOLDINGS, SCHLUMBERGER LTD, SCHLUMBERGER SERVICES PETROL, SCHLUMBERGER TECHNOLOGY BV, SCHLUMBERGER HOLDINGS LIMITED, SCHLUMBERGER LIMITED, SCHLUMBERGER TECHNOLOGY B.V., SERVICES PETROLIERS SCHLUMBERGER
Application year: 1993
Main classification: G01V3
IPC codes: G01R33, G01V3