Abstract:
A coring apparatus permitting the taking of a non-rotating core sample and testing of same, as by NMR, prior to breakage and ejection from the apparatus. A core barrel is suspended from a rotating outer sleeve by one or more bearing assemblies which permit the core barrel to remain stationary during rotation of the sleeve with attached core bit for cutting the core. A core test device is fixed with respect to the core barrel on the outside thereof to test the core as it proceeds through the barrel. The apparatus optionally includes a directional detecting device such as an inclinometer and a compact set of circumferentially-spaced steering arms for changing the direction of the apparatus during coring.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 09/334,279, filed Jun. 16, 1999, now U.S. Pat. No. 6,148,933, issued Nov. 21, 2000, which is a divisional of application Ser. No. 08/805,492, filed Feb. 26, 1997, now U.S. Pat. No. 5,957,221, issued Sep. 28, 1999, which claims the benefit of U.S. Provisional Application No. 60/012,444, filed Feb. 28, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of this invention relates to sampling and downhole testing techniques for subterranean formation cores, particularly applications using continuous nuclear magnetic resonance analyses of formation cores in a measurement-while-drilling mode. 
     2. State of the Art 
     It is desirable for the well operator to test the properties of the formation adjacent the wellbore. Frequently, properties such as permeability and porosity are measured using techniques, including, but not limited to, nuclear magnetic resonance (NMR), X-ray, or ultrasonic imaging. 
     One way of using techniques for measurement of formation properties is to drill the hole to a predetermined depth, remove the drillstring, and insert the source and receivers in a separate trip in the hole and use NMR to obtain the requisite information regarding the formation. This technique involves sending out signals and capturing echoes as the signals are reflected from the formation. This technique involved a great deal of uncertainty as to the accuracy of the readings obtained, in that it was dependent on a variety of variables, not all of which could be controlled with precision downhole. 
     Coring has also been another technique used to determine formation properties. In one prior technique, a core is obtained in the wellbore and brought to the surface where it is subjected to a variety of tests. This technique also created concerns regarding alteration of the properties of the core involved in the handling of the core to take it and bring it to the surface prior to taking measurements. Of paramount concern was how the physical shocks delivered to the core would affect its ability to mimic true downhole conditions and, therefore, lead to erroneous results when tested at the surface. 
     Other techniques have attempted to take a core while drilling a hole and take measurements of the core as it is being captured. These techniques which have involved NMR are illustrated in U.S. Pat. Nos. 2,973,471 and 2,912,641. In both of these patents, an old-style bit has a core barrel in the middle, which rotates with the bit. As the core advances in the core barrel as a net result of forward progress of the bit, the core passes through the alternating current and direct current fields and is ultimately ejected into the annulus. 
     The techniques shown in the two described patents have not been commercially employed in the field. One of the problems with the techniques illustrated in these two patents is that the core integrity is destroyed due to the employment of a rotating core barrel. The rotating core barrel, which moves in tandem with the bit, breaks the core as it enters the core barrel and before it crosses the direct current and radio frequency fields used in NMR. The result was that unreliable data is gathered about the core, particularly as to the properties of permeability and porosity which are greatly affected by cracking of the core. Additionally, the physical cracking of the core also affected readings for bound water, which is water that is not separable from the core mass. 
     SUMMARY OF THE INVENTION 
     An apparatus is disclosed that allows the taking of cores during drilling into a nonrotating core barrel. NMR measurements and tests are conducted on the core in the nonrotating barrel and, thereafter, the core is broken and ejected from the barrel into the wellbore annulus around the tool. In conjunction with a nonrotating core barrel, a sub is included in the bottomhole assembly, preferably adjacent to the bit, which, in conjunction with an inclinometer of known design, allows for real-time ability to control the movement of the bit to maintain a requisite orientation in a given drilling program. The preferred embodiment involves the use of a segmented permanent magnet to create direct current field lines, which configuration facilitates the flow of drilling fluid within the tool around the outside of the core barrel down to the drill bit so that effective drilling can take place. 
     The apparatus of the present invention overcomes the sampling drawbacks of prior techniques by allowing a sample to be captured using the nonrotating core barrel and run past the NMR equipment. Various techniques are then disclosed to break the core after the readings have been taken so that it can be easily and efficiently ejected into the annular space. A steering mechanism is also provided, as close as practicable, to the drill bit to allow for orientation changes during the drilling process in order to facilitate corrections to the direction of drilling and to provide such corrections as closely as possible on a real-time basis while the bit advances. The specific technique illustrated is usable in combination with the disclosed nonrotating core barrel, which, due to the space occupied by the core barrel, does not leave much space on the outside of the core barrel to provide the necessary mechanisms conventionally used for steering or centralizing. 
     Another advantage of the present invention is the provision of components of the NMR measurement system in such a configuration as to minimize any substantial impediment to the circulating mud which flows externally to the core barrel and through the drill bit to facilitate the drilling operation. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 illustrates a sectional elevational view showing the nonrotating core barrel and one of the techniques to break the core after various measurements have taken place. 
     FIG. 2 is a sectional elevational view of the steering sub, with the arms in a retracted position. 
     FIG. 2 a  is the view in section through FIG. 2, showing the disposition of the arms about the steering sub. 
     FIG. 3 is a schematic illustration showing the use of a segmented permanent magnet as the source of the DC field lines in the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows the general layout of the components, illustrating, at the bottom end of the bottomhole assembly, a core bit  10 , which has a plurality of inserts  12 , usually polycrystalline diamond compact (PDC) cutting elements, which cut into the formation upon rotation and application of weight on bit (WOB) to the bottomhole assembly to create the wellbore W. The core bit  10  is attached at its upper end to tubular sleeve or housing  14  which rotates with the core bit  10 . Ultimately, the sleeve  14  is connected to the lower end of a pipe or tubing string (not shown) extending from the surface to the bottom hole assembly. Internal to the sleeve  14  is a core barrel  16  which is nonrotating with respect to the sleeve  14 . 
     The core barrel  16  is supported by lower bearing assembly  18 , which includes a seal assembly  20 , to prevent the circulating mud which is in the annulus  22 , formed between the core barrel  16  and the sleeve  14 , from getting into the lower bearing assembly  18  and precluding rotation of the core bit  10  and sleeve  14  with respect to the core barrel  16 . Lower bearing assembly  18  also includes longitudinal passages therethrough to allow the circulating mud to pass to core bit  10  on the exterior of core barrel  16  in annulus  22 . 
     The nonrotating core barrel  16  also has an upper bearing assembly  24 , which has a seal assembly  26 , again to keep out the circulating mud in the annulus  22  from entering the upper bearing assembly  24 . It should be noted that the seal assemblies  20  and  26  can be employed in upper and lower pairs, as required, to isolate the circulating mud in the annulus  22  from the contacting bearing surfaces of the stationary core barrel  16  and the rotating assembly of the sleeve  14 . Those skilled in the art will appreciate that a hub  28 , which is affixed to the rotating sleeve  14  and supports a part of the upper bearing assembly  24 , as well as seal assembly  26 , has longitudinal passages therethrough to allow the circulating mud to pass. 
     Outside of the stationary core barrel  16 , a permanent magnet  30  is disposed and can be seen better by looking at FIG.  3 . The transmitting coil  32  and receiving coil  34  are disposed as shown in FIG. 3 so that the direct current field lines  36  are transverse to the RF field lines  38 . The preferred embodiment illustrates the use of a permanent magnet  30 ; however, electromagnets can also be used without departing from the spirit of the invention. In the preferred embodiment, the magnet  30  has a C-shape, with an inwardly oriented DC field. This shape provides additional clearance in the annulus  22  to permit mud flow to the core bit  10 . Thus, one of the advantages of the apparatus of the present invention is the ability to provide a nonrotating core barrel  16 , while at the same time providing the necessary features for NMR measurement without materially restricting the mud flow in the annulus  22  to the core bit  10 . Alternative shapes which have an inwardly oriented DC field are within the scope of the invention. 
     Continuing to refer to FIG. 3, the balance of the components is shown in schematic representation. A surface-mounted power source, generally referred to as  40 , supplies power for the transmitter and receiver electronics, the power being communicated to a location below electronics  44  within sleeve  14  comprising a rotating joint such as a slip-ring connection or preferably an inductive coupling  42 . Thus, the transition between the downhole electronics  44  (see FIG. 1) which rotates with sleeve  14  and coils  32  and  34 , which are rotationally fixed with regard to core barrel  16 , occurs through the inductive coupling  42 . The inductive coupling  42  is the transition point between the end of the nonrotating core barrel  16  and the rotating ejection tube  45 . In essence, the inductive coupling  42  incorporates a ferrite band on the core barrel  16  and a pick-up wire involving one or more turns on the rotating ejection tube  45 . The rotating sleeve  14  supports the inductive coupling  42  with the transition between fixed and rotating components located within the inductive coupling  42 . 
     Also illustrated in FIG. 1 is a kink or jog  46 , which acts to break the core after it passes through the measurement assembly shown in FIG.  3 . The breaking of the core can be accomplished by a variety of techniques not limited to putting a kink or jog  46  in the tube. Various other stationary objects located in the path of the advancing core within the nonrotating core barrel  16  can accomplish the breaking of the core. Accordingly, blades, grooves or knives can be used in lieu of the kink or jog  46 . The breaking of the core facilitates the ultimate ejection of the core from the exit port  48  of the ejection tube  45 . 
     With this layout, as illustrated, the driller can alter the weight on bit to meet the necessary conditions without affecting the integrity of the core. 
     One of the concerns in drilling is to maintain the appropriate orientation of the bit as the drilling progresses. The desirable coring technique, which is illustrated by use of the apparatus as previously described, can be further enhanced by providing steering capability as the core is being taken. An additional sub can be placed in the assembly shown in FIG. 1, preferably as close to the core bit  10  as possible. This assembly can be made a part of the rotating sleeve  14  and is illustrated in FIGS. 2 and 2 a.  It has a rotating inner body  49  on which an outer body  50  is mounted using bearings  52  and  54 . Seals  56  and  58  keep well fluids out of the bearings  52  and  54 . As a result, the outer body  50  does not rotate with respect to rotating inner body  49 . 
     The outer body  50  supports an inclinometer  60 , which is a device known in the art. Power and output signals from the inclinometer pass through a slip ring  62  for ultimate transmission between the nonrotating outer body  50  and the rotating inner body  49 . In the preferred embodiment, a plurality of arms  64  is oriented at  120  degrees, as shown in FIG. 2 a.  Each of the arms  64  is pivoted around a pin  66 . Electrical power is provided which passes through the slip ring  62  into the outer body  50  and to a thrust pad  68  associated with each arm  64 . Upon application of electrical power through wires such as wires  70  (see FIG. 2 a ), the thrust pad  68  expands, forcing out a particular arm  64 . The arms  64  can be operated in tandem as a centralizer, or individually for steering, with real-time feedback obtained through the inclinometer  60 . The closer the arms  64  are placed to the core bit  10 , the more impact they will have on altering the direction of the core bit  10  while the core is being taken. In the preferred embodiment, the thrust pad  68  can be made of a hydro-gel, which is a component whose expansion and contraction can be altered by electrical, heat, light, solvent concentration, ion composition, pH, or other input. Such gels are described in U.S. Pat. Nos. 5,274,018; 5,403,893; 5,242,491; 5,100,933; and 4,732,930. Alternatively, a metal compound, such as mercury, which responds to electrical impulse with a volume change may be employed. Accordingly, with the feedback being provided from the inclinometer  60 , electrical current or other triggering input can be controllably transmitted to the thrust pads  68  to obtain the desired change in orientation of the core bit  10  on the run while the core is being taken due to selective volume changes. 
     Those skilled in the art will appreciate with the disclosure of this invention that reliable coring while drilling techniques have been disclosed that give the ability, using NMR or other techniques, to obtain reliable readings of the core being taken as the drilling of the wellbore progresses. The apparatus reveals an ability to provide a nonrotating core barrel  16  without significantly impeding mud flow to the core bit  10  through an annulus  22 . Additionally, with the core barrel  16  taking up much of the room within the rotating sleeve  14 , the apparatus addresses another important feature of being able to steer the core bit  10 , using real-time feedback from an inclinometer  60 , all in an environment which does not lend itself to space for using more traditional actuation techniques for the arms  64 . In other words, because the stationary core barrel  16  takes up much of the space within the rotating sleeve  14 , traditional piston or camming devices for actuation of the arms  64  become impractical without dramatically increasing the outer diameter of the tool assembly. 
     The design using the bearing assemblies  18  and  24 , along with seal assemblies  20  and  26 , provides a mechanism for reliably taking a core and measuring its properties using known NMR techniques and other techniques without significant disturbance to the core after it is taken. Prior to ejecting the core and after testing the core, it is sufficiently disturbed and broken up to facilitate the smooth flow through the nonrotating core barrel  16  and ultimate ejection. 
     As an additional feature of the invention, effective steering is accomplished during the coring and measurement operation. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.